User:Marshallsumter/Radiation astronomy/Comets

The image at the right is an optical astronomy image of the comet 67P/Churyumov-Gerasimenko. Rosetta's OSIRIS narrow-angle camera made the image on 3 August 2014 from a distance of 285 km. The image resolution is 5.3 metres/pixel.

The left image is rotated 90° from the right. The location of the right image is the front view of the left side just out of view in the left image. The object rotates by the right hand rule from the left image to the right.

Note that due to the evaporation of volatiles, the surface of the rocky object appears pitted or cratered.

Def. a "celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the Sun and having a "tail" of melted matter blown away [back]^[1] from it by the solar wind when [as]^[1] it is close to [approaches]^[1] the Sun"^[2] is called a comet.

Def. a "comet which orbits the Sun and which returns to the innermost point of its orbit at known, regular intervals"^[3] is called a periodic comet.

Def. "any periodic comet with an orbital period of less than 200 years"^[4] is called a short-period comet.

Def. any periodic comet with an orbital period from 200 to thousands of years is called a long-period comet.

Some wanderers are meteors.

A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km.^[5] Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble.

The Perseid meteor shower, usually the richest meteor shower of the year, peaks in August. Over the course of an hour, a person watching a clear sky from a dark location might see as many as 50-100 meteors. Most meteors are actually pieces of rock that have broken off a comet and continue to orbit the Sun. The Earth travels through the comet debris in its orbit. As the small pieces enter the Earth's atmosphere, friction causes them to burn up.

Def. "[a]ll other objects [not a planet or dwarf planet], except satellites, orbiting the Sun" are called collectively Small Solar-System Bodies.^[6]

"Coronal mass ejections (CMEs) are large‐scale expulsions of plasma and magnetic field from the solar corona to the interplanetary space. During a large CME event, ∼10¹⁶ g of coronal material with energies of ∼10³² ergs are ejected from the Sun [Hundhausen, 1997; Vourlidas et al., 2002]. While accelerating away from the Sun, CMEs present speeds between few tens up to ∼2500 km/s. CMEs with speeds exceeding the magnetosonic speed can drive fast shocks ahead of them. CME‐driven fast shocks are able to accelerate charged particles up to very high energies (∼GeV/nucleon) [Wang and Wang, 2006]."^[7]

Current "knowledge of the orbital structure of the outer solar system, [is] mostly slanted towards that information which has been learned from the Canada-France-Ecliptic Plane Survey (CFEPS: www.cfeps.net). Based on our current datasets (inside and outside CFEPS) outer solar system modeling is now entering the erra of precission cosmogony."^[8]

"Since the discovery of the first members of the Kuiper belt (Jewitt and Luu, 1993) the growth in knowledge of the outer solar system has been marked (perhaps driven) by the discovery of individual objects whose dynamics pointed at previously unknown reserviours; for example: 1993 RO and the plutinos, 1996 TL66 and the ‘scattering disk’, 2003 CR103 and the detectatch disk, 90377 Sedna and the Inner Oort Cloud."^[8]

The "‘main Kuiper belt’ is populated by dynamically ‘hot’ and ‘cold’ subcomponents (Brown 2001), the dyncamically ‘cold’ component is further sub-divided into a ‘stirred’ and ‘kernel’ component (Petit et al., 2011). The plane of the Ecliptic does not match the ecliptic or invariable planes of the solar sytem (Elliot et al., 2005). Collisional families exists, Haumea (Brown et al., 2007)."^[8]

Cosmic "ray protons at energies up to 10 GeV [may be] able to build-up large amount of organic refractory material at depth of several meters in a comet during [its] long life in the Oort cloud (~4.6 x 10⁷ yr). Ion bombardment might also lead to the formation of a substantial stable crust (Johnson et al., 1987)."^[9]

"Considering photon bombardment first, interstellar and solar ultraviolet (that is, h𝛎 > 3 eV) photons have copious fluxes in the Oort cloud and Kuiper belt, providing the energy necessary to break bonds and initiate substantial chemical change in cometary surfaces. Ultraviolet photosputtering is capable of eroding away the uppermost few micrometres of icy surfaces³⁵. But more importantly, in a classic series of laboratory experiments and theoretical studies, M. Greenberg showed that ultraviolet photons would produce significant alteration of the composition, colour, and volatility of the upper several to few tens of micrometres of cometary surfaces³⁶. Others^37,38 confirmed and extended these results, showing that ultraviolet photons promote surface darkening (to albedos of only a few per cent) and devolitalization that becomes progressively more severe with dosage, and therefore age. Because of their much closer proximity to the Sun, Kuiper belt comets experience a much (~10⁵ times) higher ultraviolet and solar cosmic ray (SCR) surface dose, greatly increasing the total deposited charged-particle energy incident on the surfaces of these bodies, relative to Oort cloud comets, but their ~10 times lower average surface age somewhat mitigates this effect."^[10]

"The abundance ratios of stable isotopes of the light elements in comets may provide clues of cosmogonical significance."^[11]

"In 1997 we observed comet Hale-Bopp with the 2.6 m Nordic Optical Telescope on La Palma, Canary Islands, with a view to estimating the ¹²C/¹³C abundance ratio. About twenty high-resolution (λ /Δ λ ~ 70000) spectra of the strong CN Violet (0,0) band were secured with the SOFIN spectrograph from 7 to 13 April. The heliocentric and geocentric distances of the comet were then close to 0.9 AU and 1.4 AU, respectively. While the data do show the expected lines of the ¹³C¹⁴N isotopic molecule, we have been surprised to find in addition a number of very weak features, which are real and turn out to be positioned very near to the theoretical wavelengths of lines pertaining to the R branch of ¹²C¹⁵N."^[11]

Perhaps the most prominent cyan planetary source is Uranus, which has only been visited by the space probe Voyager 2. More recent images come from the Hubble Space Telescope in orbit around Earth.

Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.^[12]

“During the Halley Monitoring Program at La Silla from Feb.17 to Apr.17,1986 ... In the light of the neutral CN-radical a continuous formation and expansion of [cyan] gas-shells could be observed.”^[13] “The gas-expansion velocity decreases with increasing heliocentric distance from 1 km/s in early March to 0.8 km/s in April.”^[13]

Shown at right, "Lulin's green color comes from the gases that make up its Jupiter-sized atmosphere. Jets spewing from the comet's nucleus contain cyanogen (CN: a poisonous gas found in many comets) and diatomic carbon (C₂). Both substances glow green when illuminated by sunlight"^[14]

The electric blue glow of electricity results from the spectral emission of the excited ionized atoms (or excited molecules) of air (mostly oxygen and nitrogen) falling back to unexcited states, which happens to produce an abundance of electric blue light. This is the reason electrical sparks in air, including lightning, appear electric blue. It is a coincidence that the color of Cherenkov radiation and light emitted by ionized air are a very similar blue despite their very different methods of production.

On the right is a visual image of comet PanSTARRS C/2012 K1.

"Now within the inner solar system, the icy body from the Oort cloud sports two tails, a lighter broad dust tail and crooked ion tail extending below and right. The comet's condensed greenish coma makes a nice contrast with the spiky yellowish background star above. NGC 3319 appears at the upper left of the frame that spans almost twice the apparent diameter of the full Moon."^[15]

"NASA's new Spitzer Space Telescope has captured [the image right] of an unusual comet that experiences frequent outbursts, which produce abrupt changes in brightness. Periodic comet Schwassmann-Wachmann I (P/SW-1) has a nearly circular orbit just outside that of Jupiter, with an orbital period of 14.9 years. It is thought that the outbursts arise from the build-up of internal gas pressure as the heat of the Sun slowly evaporates frozen carbon dioxide and carbon monoxide beneath the blackened crust of the comet nucleus. When the internal pressure exceeds the strength of the overlying crust, a rupture occurs, and a burst of gas and dust fragments is ejected into space at speeds of 450 miles per hour (200 meters per second)."^[16]

"This 24-micron image of P/SW-1 was obtained with Spitzer's multiband imaging photometer. The image shows thermal infrared emission from the dusty coma and tail of the comet. The nucleus of the comet is about 18 miles (30 kilometers) in diameter and is too small to be resolved by Spitzer. The micron-sized dust grains in the coma and tail stream out away from the Sun. The dust and gas comprising the comet's nucleus is part of the same primordial materials from which the Sun and planets were formed billions of years ago. The complex carbon-rich molecules they contain may have provided some of the raw materials from which life originated on Earth."^[16]

"Schwassmann-Wachmann 1 is thought to be a member of a relatively new class of objects called "Centaurs," of which 45 objects are known. These are small icy bodies with orbits between those of Jupiter and Neptune. Astronomers believe that Centaurs are recent escapees from the Kuiper Belt, a zone of small bodies orbiting in a cloud at the distant reaches of the solar system."^[16]

"Two asteroids, 1996 GM36 (left) and 5238 Naozane (right) were serendipitously captured in the comet image. Because they are closer to us than the comet and have faster orbital velocities, they appear to move relative to the comet and background stars, thereby producing a slight elongated appearance. The Spitzer data have allowed astronomers to use thermal measurements, which reduce the uncertainties of visible-light albedo (reflectivity) measurements, to determine their size. With radii of 1.4 and 3.0 kilometers, these are the smallest main-belt asteroids yet measured by infrared means."^[16]

In the second image pair, "NASA's Spitzer Space Telescope captured the picture on the left of comet Holmes in March 2008, five months after the comet suddenly erupted and brightened a millionfold overnight. The contrast of the picture has been enhanced on the right to show the anatomy of the comet."^[17]

"Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, traveling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes mysteriously exploded as it approached the asteroid belt. Astronomers still do not know the cause of these eruptions."^[17]

"Spitzer's infrared picture at left reveals fine dust particles that make up the outer shell, or coma, of the comet. The nucleus of the comet is within the bright whitish spot in the center, while the yellow area shows solid particles that were blown from the comet in the explosion. The comet is headed away from the sun, which lies beyond the right-hand side of the picture."^[17]

"The contrast-enhanced picture on the right shows the comet's outer shell, and strange filaments, or streamers, of dust. The streamers and shell are a yet another mystery surrounding comet Holmes. Scientists had initially suspected that the streamers were small dust particles ejected from fragments of the nucleus, or from hyperactive jets on the nucleus, during the October 2007 explosion. If so, both the streamers and the shell should have shifted their orientation as the comet followed its orbit around the sun. Radiation pressure from the sun should have swept the material back and away from it. But pictures of comet Holmes taken by Spitzer over time show the streamers and shell in the same configuration, and not pointing away from the sun. The observations have left astronomers stumped."^[17]

"The horizontal line seen in the contrast-enhanced picture is a trail of debris that travels along with the comet in its orbit."^[17]

"The Spitzer picture was taken with the spacecraft's multiband imaging photometer at an infrared wavelength of 24 microns."^[17]

"The deuterium enrichment of cometary water is one of the most important cosmogonic indicators in comets. The (D/H)_H2O ratio preserves information about the conditions under which comet material formed, and tests the possible contribution of comets in delivering water for Earth's oceans. Water (H₂O) and HDO were sampled in comet 8P/Tuttle from 2008 January 27 to 2008 February 3 using the new IR spectrometer (Cryogenic Infrared Echelle Spectrograph) at the 8.2 m Antu telescope of the Very Large Telescope Observatory atop Cerro Paranal, Chile."^[18]

"The submillimeter emission from [a cometary] nucleus can be estimated under the assumption of thermal equilibrium."^[19]

"[V]isible meteors consist of 0.1- to 1-mm-sized debris from active comets (Williams 1990)."^[19]

The "effective opacity decreases as a+ [the maximum grain radius] increases in [the] radius range [1 to 100 mm], apparently because the larger particles become individually optically thick and so contribute to the mass [the total grain mass of the cometary coma] faster than they contribute to the radiating cross section."^[19]

"Calculations were made using the wavelength-dependent complex refractive indices of silicate (Draine 1985), glassy carbon (Edoh 1983), and Tholin (Khare et al. 1984). [...] these materials were chosen as broadly representative of the types of matter thought to be present in comet dust."^[19]

"Comet [Okazaki-Levy-Rudenko] was [observed November 18-20 and 22-24, 1989 UTC and] found to be a weak but persistent source at 800 μm".^[19]

No comets were successfully detected in the radio band until the 1973 passage of comet Kohoutek.^[20]

Interplanetary scintillation refers to random fluctuations in the intensity of radio waves of celestial origin, on the timescale of a few seconds. It is analogous to the twinkling one sees looking at stars in the sky at night, but in the radio part of the electromagnetic spectrum rather than the visible one. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron and protons that make up the solar wind.

Scintillation occurs as a result of variations in the refractive index of the medium through which waves are traveling. The solar wind is a plasma, composed primarily of electrons and lone protons, and the variations in the index of refraction are caused by variations in the density of the plasma.^[21] Different indices of refraction result in phase changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency of the wave and its angular size are broadened, and the intensity varies.^[22]

"Comets provide important clues to the physical and chemical processes that occurred during the formation and early evolution of the Solar System [...] Comparing abundances and cosmogonic values (isotope and ortho:para (o/p) ratios) of cometary parent volatiles to those found in the interstellar medium, in disks around young stars, and between cometary families, is vital to understanding planetary system formation and the processing history experienced by organic matter in the so-called interstellar-comet connection [2]. [...] ground-based radio observations towards comets C/2009 P1 (Garradd) and C/2012 F6 (Lemmon) [...] constrain the chemical history of these bodies."^[23]

"Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed."^[24] "[N]uclei are ... destroyed by ablation or explosion ... in the chromosphere, producing flare-like events with cometary abundance spectra."^[24]

"The death of a comet at r ~ R_ʘ has been seen directly only very recently (Schrijver et al 2011) using the SDO AIA XUV instrument. This recorded sublimative destruction of Comet C/2011 N3 as it crossed the solar disk very near periheloin q = 1.139R_ʘ."^[24]

"The phenomenon of flare induced sunquakes - waves in the photosphere - discovered by Kosovichev and Zharkova (1998) and now widely studied (e.g. Kosovichev 2006) should also result from the momentum impulse delivered by a cometary impact."^[24]

"The Leonid meteor shower peaked early Saturday (Nov. 17 [2012]), and some night sky watchers caught a great view. The Leonids are a yearly meteor display of shooting stars that appear to radiate out of the constellation Leo. They are created when Earth crosses the path of debris from the comet Tempel-Tuttle, which swings through the inner solar system every 33 years."^[25]

"The Orionid meteor shower [leftover bits of Halley's Comet] is scheduled to reach its maximum before sunrise on Sunday morning (Oct. 21 [2012]). This will be an excellent year to look for the Orionids, since the moon will set around 11 p.m. local time on Saturday night (Oct. 20) and will not be a hindrance at all ... The orbit of Halley's Comet closely approaches the Earth's orbit at two places. One point is in the early part of May producing a meteor display known as the Eta Aquarids. The other point comes in the middle to latter part of October, producing the Orionids."^[26]

The Perseid meteor shower, usually the richest meteor shower of the year, peaks in August. Over the course of an hour, a person watching a clear sky from a dark location might see as many as 50-100 meteors. Most meteors are actually pieces of rock that have broken off a comet and continue to orbit the Sun. The Earth travels through the comet debris in its orbit. As the small pieces enter the Earth's atmosphere, friction causes them to burn up.

The Quadrantids (QUA) are a January meteor shower, with the zenithal hourly rate (ZHR) of this shower as high as that of two other reliably rich meteor showers, the Perseids in August and the Geminids in December.^[27]

The meteor rates exceed one-half of their highest value for only about eight hours (compared to two days for the August Perseids), which means that the stream of particles that produces this shower is narrow, and apparently deriving within the last 500 years from some orbiting body.^[28] The parent body of the Quadrantids was tentatively identified in 2003^[29] as the minor planet (196256) 2003 EH1, which in turn may be related to the comet C/1490 Y1^[30] that was observed by Chinese, Japanese and Korean astronomers some 500 years ago.

Def. a "naturally occurring solid object, [which is] smaller than a planet^[31] and is not a comet,^[32] that orbits a star"^[33] is called an asteroid.

Usage notes

"The term "asteroid" has never been precisely defined. It was coined for objects which looked like stars in a telescope but moved like planets. These were known from the asteroid belt between Mars and Jupiter, and were later found co-orbiting with Jupiter (Trojan asteroids) and within the orbit of Mars. They were naturally distinguished from comets, which did not look at all starlike. Starting in the 1970s, small non-cometary bodies were found outside the orbit of Jupiter, and usage became divided as to whether to call these "asteroids" as well. Some astronomers restrict the term "asteroid" to rocky or rocky-icy bodies with orbits up to Jupiter. They may retain the term planetoid for all small bodies, and thus tend to use it for icy or rocky-icy bodies beyond Jupiter, or may use dedicated words such as centaurs, Kuiper belt objects, transneptunian objects, etc. for the latter. Other astronomers use "asteroid" for all non-cometary bodies smaller than a planet, even large ones such as Sedna and (occasionally) Pluto. However, the distinction between asteroid and comet is an artificial one; many outer "asteroids" would become comets if they ventured nearer the Sun. The official terminology since 2006 has been small Solar System body for any body that orbits the Sun directly and whose shape is not dominated by gravity."^[31]

"Two comets observed at low activity (visible nuclei) also have properties more consistent with D asteroids than any other class (very low reported geometric albedos of 0.02 and red colors)."^[34]

The possible importance of the Yarkovsky effect is the movement of meteoroids about the Solar System.^[35]

The diurnal effect is the dominant component for bodies with diameter greater than about 100 m.^[36]

On very long timescales over which the spin axis of the body may be repeatedly changed due to collisions (and hence also the direction of the diurnal effect changes), the seasonal effect will also tend to dominate.^[36]

The effect was first measured in 1991–2003 on the asteroid 6489 Golevka which drifted 15 km from its predicted position over twelve years (the orbit was established with great precision by a series of radar observations in 1991, 1995 and 1999 from the Arecibo Observatory radio telescope).^[37]

The "population of asteroids in comet-like orbits using available asteroid size and albedo catalogs of data taken with the Infrared Astronomical Satellite [I], AKARI [A], and the Wide-field Infrared Survey Explorer [W] on the basis of their orbital properties (i.e., the Tisserand parameter with respect to Jupiter, T_J, and the aphelion distance, Q, [is] 123 asteroids in comet-like orbits [with] Q < 4.5 AU and T_J < 3, [including] a considerable number (i.e., 25 by our criteria) of asteroids in comet-like orbits have high albedo, pv > 0.1. [As] such high-albedo objects mostly consist of small (D < 3 km) bodies distributed in near-Earth space (with perihelion distance of q < 1.3 AU) [may be] susceptible to the Yarkovsky effect and drifted into comet-like orbits via chaotic resonances with planets."^[38]

"There are 138,285 asteroids whose albedos and sizes are given in the I–A–W catalog. [...] nearly all high-albedo [asteroids in comet-like orbits] ACOs consist of small asteroids at q < 1.3 AU. This trend cannot be explained by the observational bias. Because the result is obtained based on the mid-infrared data, which, unlike optical observations, are less sensitive to albedo values, it provides reliable sets of asteroid albedo information. If there are big ACOs with high albedo beyond q = 1.3 AU, they would be detected easily. Although further dynamical study is essential to evaluate the population quantitatively, we propose that such ACOs with high albedos were injected from the domain of T_J > 3 via the Yarkovsky effect, because small objects with higher surface temperature are susceptible to the thermal drag force and gradually change their orbital elements to be observed as ACOs in our list."^[38]

"Although there are uncertainties in the dynamical simulation such as the value of the Yarkovsky force and the rocket force (for active comets), we conservatively consider that these three objects (2000 SU236, 2008 UM7, and 2009 SC298) are ACOs and PDCs. ["potential dormant comet" (PDC) is one having a low albedo (pv < 0.1) among ACOs. The second term is a paronomasia associating the spectra of potential dormant comets with spectra similar to P-type, D-type, or C-type asteroids (Licandro et al. 2008; DeMeo & Binzel 2008).]"^[38]

"Let us consider how the Yarkovsky effect moves an asteroid into a comet-like orbit. As shown [...], high-albedo ACOs concentrate in a range of 2 < a < 3.5 AU, similar to main-belt asteroids and [Jupiter-family comets] JFCs. The Tisserand parameter is a function of a, e, and i, [the semimajor axis, eccentricity, and inclination, respectively] while the Yarkovsky effect changes a. Due to the similarity in a between high-albedo ACOs and main-belt asteroids, we conjecture that subsequent dynamical effects may change e and i. Widely known as a standard model for orbital evolution of near-Earth asteroids, the Yarkovsky effect could move small main-belt asteroids' orbits until they are close to resonances with planets, and subsequently, these resonances can push them into terrestrial planet crossing orbits (see, e.g., Morbidelli et al. 2002). Numerical simulations demonstrated that chaotic resonances cause a significant increase in the e and i of test particles in the resonance regions (Gladman et al. 1997). Bottke et al. (2002) suggested that some objects on T_J < 3 (or even T_J < 2) can result from chaotic resonances. [...] Although there are a couple of ACOs close to resonances, their semimajor axes are not related to these major resonances. Therefore, it may be reasonable to think that encounters with terrestrial planets as well as chaotic resonances with massive planets can drift main-belt asteroids into comet-like orbits."^[38]

"In particular, we stress again the significance of high-albedo ACOs. As we discussed through our ground-based observation with the Subaru Telescope, high-albedo ACOs, which may have composition similar to silicaceous asteroids, definitively exist in the I–A–W database. Considering the very low T_J as well as the small size and perihelion distance, we suggest that such high-albedo ACOs have been injected via nongravitational forces, most likely the Yarkovsky effect."^[38]

Def. that part of outer space between the planets of a solar system and its star is called interplanetary space.

Def. the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move is called an interplanetary medium.

"It is found that near 1 AU, the dominant group of the local geometrical cross section changes."^[39] Approximately 80 % of interplanetary dust is cometary at R ~ 0.8 AU.

A typical comet nucleus has an albedo of 0.04.^[40]

At left is an image of Comet West. "Comet West was a stunning sight in the predawn sky of March, 1976, bright with a tall and broad dust tail. ... [T]he comet [was] discovered on photographs taken in August 1975 by Richard West of the European Southern Observatory ... Comet West passed perihelion on February 25, 1976, at a distance of 0.20 a.u. [and] had reached about magnitude -3 at perihelion. Several observers saw it telescopically in daylight, and John Bortle observed it with the naked eye shortly before sunset. ... The following morning, March 7, ... It was brilliant, with a head as bright as Vega (which was nearly overhead) and a huge tail, about 20 degrees tall, straight near the bottom and bending to the left in its upper reaches. The comet quickly faded during March".^[41]

Although many comets are photographed in black and white, not that many are actually only white but have colors. The image at right of McNaught Comet shows white and other colors, as does Comet West at left.

The velocities of the cyan molecule as produced in the head of comet Bennett 1970 II have been measured.^[42]

"A typical comet nucleus has an albedo of 0.04.^[40]

"This image, taken by Deep Space 1 on September 22, 2001, has been enhanced to reveal dust being ejected from the nucleus of comet Borrelly. As a result, the nucleus, which is about eight kilometers (about five miles) long, is bright white in the image. The main dust jet is directed towards the bottom left of the frame, around 35 degrees away from the comet-Sun line. The jet emerges as actually comprised of at least three smaller features. This active region as a whole is at least three kilometers (less than two miles) long."^[43]

"Another, smaller, jet feature is seen on the tip of the nucleus on the lower right-hand limb. Dust also seems to be ejected from there into the night-side hemisphere, probably from the dayside hemisphere. The expansion of the gas and dust mixture into the vacuum of space has swept some material around the body of the nucleus so that it appears above the night-side hemisphere. The night-side of the nucleus could not be seen, of course."^[43]

"The line between day and night on the comet is towards the upper right. This representation shows a faint ring of brightness separated from the terminator by a dark, unlit area. It is possible that this is a crater rim, seen in grazing illumination, which is just about to cross into darkness as the comet rotates. The direction to the Sun is directly downwards."^[43]

On the left is a close-up picture of comet Borelly. The right portion is a topographic relief map of the cometary nucleus.

"Comets are sometimes described as "dirty snowballs," but a close flyby of one by NASA's Deep Space 1 spacecraft last fall detected no frozen water on its surface."^[44]

"The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice."^[45]

"We know the ice is there. It's just well-hidden. Either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice."^[45]

"The Deep Space 1 science team released pictures and other initial findings days after the spacecraft flew within 2,171 kilometers (1,349 miles) of the comet's solid nucleus on September 22, 2001."^[44]

"Comet Borrelly is in the inner solar system right now, and it's hot, between 26 and 71 degrees Celsius (80 and 161 degrees Fahrenheit), so any water ice on the surface would change quickly to a gas. As the components evaporate, they leave behind a crust, like the crust left behind by dirty snow."^[46]

"It seems to be covered in this dark material, which has been loosely connected with biological material. This suggests that comets might be a transport mechanism for bringing the building blocks of life to Earth."^[46]

"It's remarkable how much information Deep Space 1 was able to gather at the comet, particularly given that this was a bonus assignment for the probe."^[47]

"The short period comets have orbital periods <20 years and low inclination. Their orbits are controlled by Jupiter and thus they are also called Jupiter Family comets. [...] Because the orbit crosses that of Jupiter, the comet will have gravitational interactions with this massive planet. The objects orbit will gradually change from these interactions and eventually the object will either be thrown out of the Solar System or collide with a planet or the Sun."^[48]

Perihleion distance in AU = 1.243, eccentricity = 0.641, inclination = 7.0, and orbital period in years = 2.745.^[49]

“During the Halley Monitoring Program at La Silla from Feb.17 to Apr.17,1986 ... In the light of the neutral CN-radical a continuous formation and expansion of [cyan] gas-shells could be observed.”^[13]

“The gas-expansion velocity decreases with increasing heliocentric distance from 1 km/s in early March to 0.8 km/s in April.”^[13]

The 1910 approach, which came into naked-eye view around 10 April^[50] and came to perihelion on 20 April,^[50] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.^[51] Furthermore, the comet made a relatively close approach of 0.15AU,^[50] making it a spectacular sight. Indeed, on 19 May, the Earth actually passed through the tail of the comet.^[52][53] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,^[54] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet."^[55] His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.^[56] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.^[55]

"It is quite possible that [faint streamers preceding the main tail and lying nearly in the prolonged radius vector] may have touched the Earth, probably between May 19.0 and May 19.5, [1910,] but the Earth must have passed considerably to the south of the main portion of the tail [of Halley's comet]."^[57]

A magnetohydrodynamics (MHD) and chemical comet-coma model is applied to describe and analyze the plasma flow, magnetic field, and ion abundances in Comet Halley.^[58] A comparison of model results is made with the data from the Giotto mission.^[58]

In November 2007 the JPL team targeted Deep Impact toward Comet 103P/Hartley (Hartley 2); however, this would require an extra two years of travel for Deep Impact (including earth gravity assists in December 2007 and December 2008).^[59] On May 28, 2010, a burn of 11.3 seconds was conducted, to enable the June 27 Earth fly-by to be optimized for the transit to Hartley 2 and fly-by on November 4. The velocity change was 0.1 m/s (0.33 ft/s).^[60]

On November 4, 2010, the Deep Impact extended mission (EPOXI) returned images from comet Hartley 2.^[61] EPOXI came within 700 kilometers (430 mi) of the comet, returning detailed photographs of the "peanut" shaped cometary nucleus and several bright jets. The probe's medium-resolution instrument captured the photographs.^[61]

In the second image pair, "NASA's Spitzer Space Telescope captured the picture on the left of comet Holmes in March 2008, five months after the comet suddenly erupted and brightened a millionfold overnight. The contrast of the picture has been enhanced on the right to show the anatomy of the comet."^[17]

"Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, traveling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes mysteriously exploded as it approached the asteroid belt. Astronomers still do not know the cause of these eruptions."^[17]

"Spitzer's infrared picture at left reveals fine dust particles that make up the outer shell, or coma, of the comet. The nucleus of the comet is within the bright whitish spot in the center, while the yellow area shows solid particles that were blown from the comet in the explosion. The comet is headed away from the sun, which lies beyond the right-hand side of the picture."^[17]

"The contrast-enhanced picture on the right shows the comet's outer shell, and strange filaments, or streamers, of dust. The streamers and shell are a yet another mystery surrounding comet Holmes. Scientists had initially suspected that the streamers were small dust particles ejected from fragments of the nucleus, or from hyperactive jets on the nucleus, during the October 2007 explosion. If so, both the streamers and the shell should have shifted their orientation as the comet followed its orbit around the sun. Radiation pressure from the sun should have swept the material back and away from it. But pictures of comet Holmes taken by Spitzer over time show the streamers and shell in the same configuration, and not pointing away from the sun. The observations have left astronomers stumped."^[17]

"The horizontal line seen in the contrast-enhanced picture is a trail of debris that travels along with the comet in its orbit."^[17]

"The Spitzer picture was taken with the spacecraft's multiband imaging photometer at an infrared wavelength of 24 microns."^[17]

"A beautiful Valentine's Day comet [sped] past Earth [last night]. Known as the Valentine's Day comet C/2018Y1 Iwamoto, it's the first binocular comet of 2019, which means its green glow will be visible to the human eye through a pair of binoculars."^[62]

"Travelling at roughly 238,000 kilometres per hour (or 148,000 miles per hour), the comet has just passed the sun and will be heading closer to Earth throughout Thursday 14th February."^[62]

"It will be visible throughout the day but the best views will occur after dark. You can track exactly where the comet is in the sky using this online tool."^[62]

"This particular bright green comet was only discovered recently by astronomer Masayuki Iwamoto".^[62]

The neutral cyan coma of comet Kohoutek 1973 XII is measured.^[63]

At right is Comet Lovejoy as detected in STEREO/SECCHI's EUVI-A imager's 17.1-nm wavelength. "The comet is clearly visible racing away from the Sun, leaving a wiggly-tail in its wake! Why the wiggles? We're not sure -- we need to start studying that when we get all of the spacecraft data from STEREO-B this weekend. However, we think there may some kind of helical motion going on, or perhaps there's a projection affect and we're seeing tail material magnetically "clinging" to coronal loops and moving with them. There are other possibilities too, though, and we will certainly investigate those! We should have equivalent images from the STEREO-A spacecraft which we will also get this weekend. When we pair these together, and throw in the SDO images too, we should be able to get an incredibly unique 3-D picture of how this comet is reacting the intense coronal heat and magnetic loops."^[64]

Shown at the right "Lulin's green color comes from the gases that make up its Jupiter-sized atmosphere. Jets spewing from the comet's nucleus contain cyanogen (CN: a poisonous gas found in many comets) and diatomic carbon (C₂). Both substances glow green when illuminated by sunlight".^[14]

Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on 7 August 2006 by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope.^[65] It was the brightest comet in over 40 years, and was easily visible to the naked eye for observers in the Southern Hemisphere in January and February 2007.

On the right is a visual image of comet PanSTARRS C/2012 K1.

"Now within the inner solar system, the icy body from the Oort cloud sports two tails, a lighter broad dust tail and crooked ion tail extending below and right. The comet's condensed greenish coma makes a nice contrast with the spiky yellowish background star above. NGC 3319 appears at the upper left of the frame that spans almost twice the apparent diameter of the full Moon."^[15]

"NASA's new Spitzer Space Telescope has captured [the image right] of an unusual comet that experiences frequent outbursts, which produce abrupt changes in brightness. Periodic comet Schwassmann-Wachmann I (P/SW-1) has a nearly circular orbit just outside that of Jupiter, with an orbital period of 14.9 years. It is thought that the outbursts arise from the build-up of internal gas pressure as the heat of the Sun slowly evaporates frozen carbon dioxide and carbon monoxide beneath the blackened crust of the comet nucleus. When the internal pressure exceeds the strength of the overlying crust, a rupture occurs, and a burst of gas and dust fragments is ejected into space at speeds of 450 miles per hour (200 meters per second)."^[16]

"This 24-micron image of P/SW-1 was obtained with Spitzer's multiband imaging photometer. The image shows thermal infrared emission from the dusty coma and tail of the comet. The nucleus of the comet is about 18 miles (30 kilometers) in diameter and is too small to be resolved by Spitzer. The micron-sized dust grains in the coma and tail stream out away from the Sun. The dust and gas comprising the comet's nucleus is part of the same primordial materials from which the Sun and planets were formed billions of years ago. The complex carbon-rich molecules they contain may have provided some of the raw materials from which life originated on Earth."^[16]

"Schwassmann-Wachmann 1 is thought to be a member of a relatively new class of objects called "Centaurs," of which 45 objects are known. These are small icy bodies with orbits between those of Jupiter and Neptune. Astronomers believe that Centaurs are recent escapees from the Kuiper Belt, a zone of small bodies orbiting in a cloud at the distant reaches of the solar system."^[16]

"A comet that flew close to Mars showered the red planet with fine cometary dust, according to observations by a trio of spacecraft."^[66]

"Comet C/2013 A1 Siding Spring passed within 139,500 kilometres of the red planet on 19 October, the closest a comet has ever been seen to come to a planet without actually colliding with it. To avoid being damaged by the comet dust, all spacecraft orbiting Mars moved to the far side of the planet for 20 minutes while the comet dust was at its most intense, but this did not prevent them from studying the effects it had on Mars’ atmosphere."^[66]

“They call this comet encounter a once-in-a-lifetime event, but it’s more like once in a million years.”^[67]

"The European Space Agency’s Mars Express spacecraft detected an increase in electrons in Mars’ upper atmosphere, partly ionising it. This was attributed to fine cometary dust penetrating the atmosphere, which led to a meteor storm of thousands of meteors per hour. The increase in electrons led to the creation of a temporary new layer of charged particles in the ionosphere, which runs from an altitude of 120 kilometres to several hundred kilometres above. This is the first time such an event has been seen, even on Earth the extra density of electrons was measured to be five to ten times higher than normal by NASA’s Mars Reconnaissance Orbiter. Another NASA spacecraft, MAVEN, which also observed the new layer in the ionosphere, will monitor for any long-term events as it goes about its regular duties of studying Mars’ atmosphere."^[66]

"MAVEN’s Imaging Ultraviolet Spectrograph was able to ascertain the species of ions that flooded into the ionosphere from the comet, the first time a comet that has come direct from the distant Oort Cloud has been sampled in this way. It detected the signal of magnesium, iron and sodium ions following the meteor shower, a signal that dominated Mars’ ultraviolet spectrum for hours afterwards, taking two days to dissipate."^[66]

"The results show that dust from the comet, which has a nucleus two kilometres across, according to high resolution images from the Mars Reconnaissance Orbiter, had a dramatic effect on Mars’ atmosphere."^[66]

“Observing the effects on Mars of the comet’s dust slamming into the upper atmosphere makes me very happy that we decided to put our spacecraft on the other side of Mars at the peak of the dust tail passage and out of harm’s way.”^[68]

"Comet Swan recently made a swing through the inner solar and emerged in the evening sky. Astronomy enthusiast Ginger Mayfield recorded the blue-green color of the comet's nucleus and a tenuous tail in this composite created from multiple images taken on October 26 from Divide, Colorado."^[69]

The physical parameters of the neutral cyan coma of comet West (1975n) have been measured.^[70]

The Sun-Jupiter binary may serve to establish an upper limit for interstellar cometary capture when three bodies are extremely unequal in mass, such as the Sun, Jupiter, and a third body (potential comet) at a large distance from the binary.^[71] The basic problem with a capture scenario even from passage through “a cloud of some 10 million years, or from a medium enveloping the solar system, is the low relative velocity [~0.5 km s^-1] required between the solar system and the cometary medium.”^[72] The capture of interstellar comets by Saturn, Uranus, and Neptune together cause about as many captures as Jupiter alone.^[72]

"Deep Space 1 was launched on 24 October 1998 as part of NASA's New Millennium Program, which is managed by JPL for NASA's Office of Space Science, Washington, D.C. The California Institute of Technology manages JPL for NASA."^[43]

"Deep Space 1 completed its primary mission testing ion propulsion and 11 other advanced, high-risk technologies in September 1999. NASA extended the mission, taking advantage of the ion propulsion and other systems to undertake this chancy but exciting, and ultimately successful, encounter with the comet."^[43]

The Deep Space 1 spacecraft carried out a flyby of asteroid 9969 Braille, which was its primary science target. The mission was extended twice to include an encounter with comet 19P/Borrelly.

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.^[73] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.^[74]

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories^[73] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.^[75] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).^[76] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion^[77] that using a heliocentric unperturbed two-body curve fitting, best-fit suggests they may escape the Solar System.

As of 2018, 1I/ʻOumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While ʻOumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet.^[78] Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.^[79]

If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.^[80]

An interstellar object is an astronomical object that is located in interstellar space including objects that are on an interstellar trajectory but are temporarily passing close to a star, such as certain asteroids and comets (including exocomets^[81][82])

An interstellar comet can probably, on rare occasions, be captured into a heliocentric orbit while passing through the Solar System. Computer simulations show that Jupiter is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years.^[83]

Current models of Oort cloud formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many.^[82] Other simulations suggest that 90–99% of comets are ejected.^[84] There is no reason to believe comets formed in other star systems would not be similarly scattered.^[81]

A more recent estimate, following the detection of 'Oumuamua, predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1×10⁴, each with a residence time of ~10 years."^[85]

There should be hundreds of 'Oumuamua-size interstellar objects in the Solar System, based on calculated orbital characteristics, with known examples: 2011 SP25, 2017 RR2, 2017 SV13, and 2018 TL6.^[86] These are all orbiting the sun, but with unusual orbits, and are assumed to have been trapped at some occasion.

"An object that seems to be a comet from around another star is speeding through the solar system. This comet, dubbed C/2019 Q4 (Borisov), marks the second time that astronomers have seen an interstellar visitor on its way past the sun."^[87]

"Ukrainian amateur astronomer Gennady Borisov spotted the comet on August 30."^[87]

"In the days that followed, other astronomers took enough observations of the comet to show that its orbit is not bound to the sun, a telltale sign that the object is from interstellar space."^[88]

The "International Astronomical Union’s Minor Planet Center in Cambridge, Mass., [...] reported C/2019 Q4’s trajectory September 11 in a public bulletin."^[87]

"There have been three or four different groups that have checked the orbit and poked at it from different directions, and everybody reaches the same conclusion."^[88]

"The comet is close to the sun and low on the horizon, which make it difficult to observe."^[88]

“This is an unusual thing. You always want to be extra careful when you get an answer that’s not typical. That said, I think it will hold up."^[88]

"C/2019 Q4, on the other hand, is just past the solar system’s doorstep, on a trajectory that will take it between the orbits of Jupiter and Mars. It will make its closest approach to Earth (at about twice the distance between Earth and the sun) on December 29."^[87]

"A newly discovered comet is screaming away from Earth, and based on its weird orbital trajectory might be the first comet ever observed to come from interstellar space. A sky-surveying telescope in Hawaii spotted the fast-moving object, now called C/2017 U1, on 18 October, after its closest approach to the sun. The following week, astronomers made 34 separate observations of the object and found it has a strange trajectory that doesn't appear to circle the sun."^[89]

The image on the right shows 'Oumuamua's hyperbolic trajectory across the full solar system, with annual markers, and planet positions on 1/1/2018.

ʻOumuamua showed no signs of a cometary coma despite its close approach to the Sun, but underwent non-gravitational acceleration which is seen in many icy comets,^[90][91] although other reasons have been suggested.^[92][93][94]

The object could be a remnant of a disintegrated interstellar comet (or exocomet).^[95][96]

Comets Machholz 1 is a possible example of such comets, as it has an atypical chemical makeup for comets in the Solar System.^[97]

Comet Hyakutake C/1996 B2 is a possible example of such comets, as it has an atypical chemical makeup for comets in the Solar System.^[98]

It is possible for objects orbiting a star to be ejected due to interaction with a third massive body, such a process was initiated in early 1980s when C/1980 E1, initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System, changing its orbit from elliptical to hyperbolic and making it the most eccentric known object at the time, with an eccentricity of 1.057.^[99] It is headed for interstellar space.

Before entering the inner Solar System for a 1982 perihelion passage, C/1980 E1 had a barycentric (epoch 1950-Jan-01) orbit with an aphelion of 75,000 astronomical units (1.2 ly), and a period of approximately 7.1 million years.^[100]

As the comet was approaching on 9 December 1980, it passed within 0.228 AU of Jupiter,^[101] which accelerated the comet briefly giving an (epoch 1981-Jan-09) eccentricity of 1.066.^[100] The comet came to perihelion on 12 March 1982,^[102] when it had a velocity of 23.3 kilometres per second (52,000 mph) with respect to the Sun. Since the epoch of 1977-Mar-04, C/1980 E1 has had a barycentric eccentricity greater than 1,^[100] keeping it on a hyperbolic trajectory that will eject it from the Solar System. Objects in hyperbolic orbits have a negative semimajor axis, giving them a positive orbital energy. After leaving the Solar System, C/1980 E1 will have an interstellar velocity (${\displaystyle v_{\infty }}$) of 3.77 km/s.^[103] The Minor Planet Center does not directly list a semimajor axis for this comet.^[104]

The escape velocity from the Sun at Neptune's orbit is 7.7 km/s. By June 1995, the comet was passing Neptune's orbit at 30.1 AU from the Sun continuing its ejection trajectory at 8.6 km/s.^[105] Since February 2008, the comet has been more than 50 AU from the Sun.^[106]

Asteroid (514107) 2015 BZ509 may be a former interstellar object, captured some 4.5 billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun.^[107]

514107 Kaʻepaokaʻāwela, provisionally designated 2015 BZ
₅₀₉ and nicknamed Bee-Zed,^[108] is a small asteroid, approximately 3 km (2 mi) in diameter,^[109] in a resonant, co-orbital motion with Jupiter.^[110] It is an unusual minor planet in that its orbit is retrograde, which is opposite to the direction of most other bodies in the Solar System.^[111] It was discovered on 26 November 2014, by astronomers of the Pan-STARRS survey at Haleakala Observatory on the island of Maui, United States.^[112] Kaʻepaokaʻāwela is the first example of an asteroid in a 1:–1 resonance with any of the planets.^[113]

The Hills cloud (also called the inner Oort cloud and inner cloud^[114]) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 AU from the Sun, and whose inner border, less well-defined, is hypothetically located at 250-1500 AU, well beyond planetary and Kuiper Belt object orbits - but distances might be much greater. If it exists, the Hills cloud contains roughly 5 times as many comets as the Oort cloud.^[115]

Objects ejected from the Hills cloud are likely to end up in the classical Oort cloud region, maintaining the Oort cloud.^[116]

The existence of the Hills cloud is plausible, since many bodies have been found already. It would be denser than the Oort cloud.^[117][118]

Comets may be rooted in a cloud orbiting the outer boundary of the Solar System.^[119]

Comets are usually destroyed after several passes through the inner Solar System, so if any had existed for several billion years (since the beginning of the Solar System), no more could be observed now.^[120] The distribution of the inverse of the semi-major axes showed a maximum frequency which suggested the existence of a reservoir of comets between 40,000 and 150,000 AU (0.6 and 2.4 ly) away.^[120] This reservoir, located at the limits of the Sun's sphere of astrodynamic influence, would be subject to stellar disturbances, likely to expel cloud comets outwards or inwards.^[120]

Most estimates place the population of the Hills cloud at about 20 trillion (about five to ten times that of the outer cloud), although the number could be ten times greater than that.^[121] The orbits of most cloud comets have a semi-major axis of 10,000 AU, much closer to the Sun than the proposed distance of the Oort cloud.^[117] Moreover, the influence of the surrounding stars and that of the galactic tide should have sent the Oort cloud comets either closer to the Sun or outside of the Solar System. The presence of an inner cloud, which would have tens or hundreds of times as many cometary nuclei as the outer halo was proposed.^[117]

The majority of comets in the Solar System were located not in the Oort cloud area, but closer and in an internal cloud, with an orbit with a semi-major axis of 5,000 AU.^[122]

It is likely that the Hills cloud is the largest concentration of comets across the Solar System.^[123] The Hills cloud is much denser than the outer Oort cloud; it is somewhere between 5,000 and 20,000 AU in size. In contrast, the Oort cloud is between 20,000 and 50,000 AU (0.3 and 0.8 ly) in size.^[124]

The mass of the Hills cloud may be five times more massive than the Oort cloud.^[125] Or, the mass of the Hills cloud to be 13.8 Earth masses, if the majority of the bodies are located at 10,000 AU.^[122]

The vast majority of Hills cloud objects consists of various ices, such as water, methane, ethane, carbon monoxide and hydrogen cyanide.^[126] However, the discovery of the object 1996 PW, an asteroid on a typical orbit of a long-period comet, suggests that the cloud may also contain rocky objects.^[127]

The carbon analysis and isotopic ratios of nitrogen firstly in the comets of the families of the Oort cloud and the other in the body of the Jupiter area shows little difference between the two, despite their distinctly remote areas, which suggests that both come from a protoplanetary disk,^[128] a conclusion also supported by studies of comet cloud sizes and the recent impact study of Comet Tempel 1.^[129]

This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units (AU). The scale is logarithmic, with each specified distance ten times further out than the previous one. Credit: NASA / JPL-Caltech.{{free media}}

An artist's rendering is of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility. Credit: NASA.{{free media}}

The Oort cloud or the Öpik–Oort cloud^[131] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.^[132] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.^[133]

The Oort cloud is divided into two regions: a circumstellar disc-shaped inner Oort cloud (or Hills cloud) and a circumstellar envelope, spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space.^[134][135]

Voyager 1, the fastest^[136] and farthest^[137][138] of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years^[135][139] and would take about 30,000 years to pass through it.^[140][141] However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1.

Def. a "roughly spherical region of space composed of comet-like bodies and other minor planets and asteroids that orbit distantly in planetary systems"^[142] is called an Oort cloud.

Def. a "roughly spherical region of space from 50,000 to 100,000 astronomical units (approximately 1 light year) from the sun; supposedly the source of most comets around the Solar System"^[143] is called an Oort Cloud.

The submillimeter "wavelength view [at right] of a ring of dust particles around Epsilon Eridani, taken with the SCUBA camera at the James Clerk Maxwell Telescope. The false-colour scale is brightest where there is more dust. Epsilon Eridani is marked by the star symbol, although the star itself is not seen at submillimetre wavelengths. Pluto's orbit (marking the edge of our Solar System) is shown at the same scale."^[144]

"The ring is "strikingly similar" to the outer comet zone in our Solar System, and shows an intriguing bright region that may be particles trapped around a young planet."^[144]

"What we see looks just like the comet belt on the outskirts of our Solar System, only younger, [...] It's the first time we've seen anything like this around a star similar to our Sun. In addition, we were amazed to see a bright spot in the ring, which may be dust trapped in orbit around a planet."^[144]

"Epsilon Eridani is far more similar to our Sun than either Vega or Fomalhaut."^[144]

"This star system is a strong candidate for planets, but if there are planets, it's unlikely there could be life yet. When the Earth was this young, it was still being very heavily bombarded by comets and other debris."^[144]

"It is also a star in our local neighbourhood, being only about 10 light years away, which is why we can see so much detail in the new image."^[144]

"If an astronomer could have seen what our Solar System looked like four billion years ago, it would have been very much as Epsilon Eridani looks today, [...] This is a star system very like our own, and the first time anyone has found something that truly resembles our Solar System; it's one thing to suspect that it exists, but another to actually see it, and this is the first observational evidence."^[145]

"Beyond Pluto in our Solar System is a region containing more than 70,000 large comets, and hundreds of millions of smaller ones, called the "Kuiper belt". The image [...] shows dust particles that the astronomers believe are analogous to our Kuiper belt at the same distance from Epsilon Eridani as the Kuiper belt is from our Sun. Although the image cannot reveal comets directly, the dust that is revealed is believed to be debris from comets."^[144]

"Epsilon Eridani's inner region contains about 1,000 times more dust than our Solar System's inner region, which may mean it has about 1,000 times more comets [...]. Epsilon Eridani is believed to be only 500 million years to 1 billion years old; our Sun is estimated to be 4.5 billion years old, and its inner region is believed to have looked very similar at that age."^[144]

"The new image -- which is from short-radio wavelengths, and is not an optical picture -- was obtained using the 15-meter James Clerk Maxwell Telescope [JCMT] at the Mauna Kea Observatory in Hilo, Hawaii. The JCMT is the world's largest telescope dedicated to the study of light at "submillimeter" wavelengths. The [...] camera called SCUBA (Submillimeter Common User Bolometer Array), which was built by the Royal Observatory in Edinburgh (which is now the UK Astronomical Technology Centre). SCUBA uses detectors cooled to a tenth of a degree above absolute zero (-273 degrees Celsius) to measure the tiny amounts of heat emission from small dust particles at a wavelength close to one-millimeter."^[144]

"The implication is that if there is one system similar to ours at such a close star, presumably there are many others, [...] In the search for life elsewhere in the universe, we have never known where to look before. Now, we are closing in on the right candidates in the search for life."^[145]

"A region near the star that is partially evacuated indicates that planets may have formed, [...] the presence of planets is the most likely explanation for the absence of dust in this region because planets absorb the dust when they form."^[144]

"There may be a planet stirring up the dust in the ring and causing the bright spot, or it could be the remnants of a massive collision between comets."^[146]

The first exocomets were detected in 1987^[147][148] around Beta Pictoris, a very young A-type main-sequence star. There are now a total of 11 stars around which exocomets have been observed or suspected.^{[149][150][151][152]}

All discovered exocometary systems (Beta Pictoris, HR 10,^[149] 51 Ophiuchi, HR 2174,^[150] 49 Ceti, 5 Vulpeculae, 2 Andromedae, HD 21620, HD 42111, HD 110411,^[151][153] and more recently HD 172555^[152]) are around very young A-type stars.

A gaseous cloud around 49 Ceti has been attributed to the collisions of comets in that planetary system.^[154]

The International Cometary Explorer (ICE) spacecraft was originally known as [the] International Sun/Earth Explorer 3 (ISEE-3) satellite.

ISEE-3 was launched on August 12, 1978. It was inserted into a "halo" orbit about the libration point some 240 Earth radii upstream between the Earth and Sun. ISEE-3 was renamed ICE (International Cometary Explorer) when, after completing its original mission in 1982, it was gravitationally maneuvered to intercept the comet P/Giacobini-Zinner. On September 11, 1985, the veteran NASA spacecraft flew through the tail of the comet. The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV.

The instruments aboard ISEE-3 are designed to detect

1. protons in the energy range 150 eV - 7 keV and electrons in the 10 eV - 1 keV range (Solar wind plasma experiment),
2. Low, Medium and High-Energy Cosmic Rays (1-500 MeV/n, Z = 1-28, electrons 2-10 MeV, for Medium Energy; H to Ni, 20-500 MeV/n for High-energy),
3. H-Fe 30 MeV/n - 15 GeV/n and electrons 5-400 MeV for the Cosmic-Ray Energy Spectrum experiment,
4. 17 Hz - 100 kHz magnetic and electric field wave levels (Plasma Waves Spectrum Analyzer),
5. low-energy solar proton acceleration and propagation processes in interplanetary space, Energetic Particle Anisotropy Spectrometer (EPAS),
6. 2 keV to > 1 MeV interplanetary and solar electrons,
7. radio mapping of solar wind disturbances (type III bursts) in 3-D, 30 kHz - 2 MHz,
8. solar wind ion composition, 300-600 km/s, 840 eV/Q to 11.7 keV/Q, M/Q = 1.5 to 5.6,
9. cosmic ray isotope spectrometer 5-250 MeV/n, Z=3-28, A=6-64 (Li-Ni),
10. ground based solar studies with the Stanford ground-based solar telescope, and the comparison of these measurements with measurements of the interplanetary magnetic field and solar wind made by other experiments on this spacecraft,
11. X- and gamma-ray bursts, 5-228 keV, and
12. Gamma-ray bursts, 0.05-6.5 MeV direction, profile, spectrum.^[155]

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