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{ Practical astronomy | Astronomy | Minor planets and comets }

Minor planets and comets

Minor planets and comets inhabit the space between (and beyond) the major planets. Minor planets are rocks and large rocky bodies that might have become part of a rocky planet. They may have failed to collide with others to grow into a planet. Some smaller bodies may also result from the breakup of a larger minor planet. The orbits around the Sun of minor planets are similar to major planets. They usually are nearly circular and stay reasonably close to the plane of the ecliptic. Minor planets are often called asteroids.

C/1995 O1 Hale-Bopp
C/1995 O1 Hale-Bopp on 1997-03-28.

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C/1995 O1 (Hale-Bopp) was an exceptionally bright comet; this did not come as a surprise but had been predicted about two years in advance. In the image, the comet has passed through the constellation Cassiopeia (top of the image). Both the blue straight ion tail and the curved white dust tail are visible. The image is affected by sodium-illuminated cloud at the bottom right and by a general background gradient.

Comets are different. Their orbits tend to be excentric, often to the point that they cannot be distinguished from a parabolic or hyperbolic orbit. They show little tendency toward the plane of the ecliptic. Far from the Sun these bodies look like minor planets, but under the influence of solar radiation their composition of ice as well as dust reveals them as quite different from the minor planet rocks. When comets approach the Sun, they warm up, the ice starts to evaporate (sublimate), dust grains are released. Molecules are released, dissociated and ionised. Comets then develop in general two tails. Sunlight acts upon the dust grains and drives them away from the comet's head in a direction pointing away from the Sun; this forms the curved dust tail. The solar wind of ionised particles acts upon the gas to form the more straight ion tail. Although sunlight moves much faster than the solar wind, its impact on the dust grains is much less. Combined with the comet head's movement around the Sun, this leads to a curvature in the dust tail. The ions from comet and Sun are of similar mass, so that the ion tail is quite fast and hence appears straight.

There is also a general population of dust grains in the ecliptic that can rarely be seen as the zodiacal light, the reflection and scattering of sunlight by the dust grains. This dust may stem from fragmentation of minor planets or comets, or from comet tails.

When dust particles from interplanetary space enter the Earth's atmosphere they cause a brief trail of light, a meteor. When larger bodies collide with the Earth, they form a fireball and a solid remnant meteorite may later be found.

A recent, extreme case is the 1908 Tunguska event, when an interplanetry object exploded a few km above the Siberian forest and felled the trees within a 25 km radius. About 50,000 years ago an asteroid of 50 m diameter dug out the 1 km wide Meteor Crater in Arizona. About 15 million years ago a 1.5 km asteroid created the Nördlinger Ries of 24 km diameter.

Top of the list must be 66 million years ago, when an asteroid of 10 km diameter hit the coast of Yucatán to form a crater of 200 km diameter and 30 km depth. The debris spread across the globe and caused the extinciton of many species, most famously all non-avian dinosaurs.

Minor planets

One major population of minor planets orbits the Sun in the Titius-Bode gap where a planet is missing between Mars and Jupiter. At the start of the 19th century, a group of German astronomers around von Zach, Olbers and Schroeter set out to discover the missing planet. With proverbial German efficiency, they divided the ecliptic into longitude intervals and assigned these to different eminent astronomers from all over Europe. The term "Himmelspolizey" was coined, each "bobby on his beat" looking for the planet in their sector.

Literally during the first day of the century, 1801-01-01, the first such body was discovered by Giuseppe Piazzi. However, he claims to have done so independently and before receiving instructions from the German group. This object, (1) Ceres, is rather faint for a planet. A group around Olbers and Schroeter continued the search, discovering three further objects by 1807. Olbers found (2) Pallas and (4) Vesta and Schroeter's assistant Harding found (3) Juno. Thus the pan-European effort came to more or less nothing and all four minor planets were discovered either independently or by the core group of investigators. Many more discoveries were made around the middle of the 19th century. 100 objects were known by 1868.

(134340) Pluto
(134340) Pluto on 2007-08-01 and 2007-08-09.

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The dwarf planet Pluto, quite far from Earth as minor planets go, betrays its non-stellar nature by moving nearly 10' over the course of nine days.

The earliest discoveries were called "planets", but with more and more, fainter and smaller objects being discovered, all were demoted to be called asteroids or minor planets. A more famous demotion occurred in 2006 when (134340) Pluto was demoted from planet to the new category of dwarf planet. Pluto had been discovered in 1930 as the ninth planet, but does not really fit the sequence of the eight planets in terms of distance, orbit and size. Along with Pluto's demotion, (1) Ceres was promoted to dwarf planet, as it stands out among minor planets in terms of size and governance over its orbit.

Minor planets are also classified by different distances from the Sun. The early discoveries are all of minor planets in the asteroid belt between Mars and Jupiter, while there are also a number of trans-Neptunian objects or Kuiper-belt objects. Other classifications go by the orbit of the minor planet. Trojans are located in the orbit of Jupiter at the Lagrange points 60° heliocentric longitude ahead and behind Jupiter.

Important for the defence of the planet Earth are the near-Earth objects or NEOs. They are simply defined as having a perihelion distance of less than 1.3 au. This does not necessarily place them near the Earth. But the criterion is both simple and includes all dangerous minor planets (except for those still to be discovered).

Minor planets move in elliptic Kepler orbits around the Sun. The orbit can change during a close encounter with a planet, or during collision with another minor planet. The approach to describing the orbits is rather more pragmatic than it is for the major planets. Frequently, updated Kepler elements of the current elliptic orbits of all minor planets are published. They are good for perhaps weeks and months, but probably not years.

Minor planets are referred to by their permanent ID, which is a number that orders the objects approximately by time of discovery or time of ID assignment. This number is usually put in parentheses, like (134340) for Pluto. Many minor planets are also given names, which will be based on a suggestion by the discoverer. Number and name are then combined in the form of "(2100) Ra-Shalom".

(2100) Ra-Shalom
(2100) Ra-Shalom on 2025-08-22.

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The near-earth object (2100) Ra-Shalom is here quite close to Earth as minor planets go. Its non-stellar nature is betrayed by its movement during the five or ten minutes ove which the frames are taken. The insert shows part of the full image at the same scale. In the larger version, the frames are aligned on the stars and then stacked. In the smaller version the minor planet has been measured in the frames and the frames have been aligned so that the minor planet coincides. It is the small, circular dot in the centre of the insert. When aligned on stars, the minor planet turns into a trail, when aligned on the minor planet, the stars turn into trails.

Before the permanent ID is assigned, which is usually a considerable time span, the minor planet is known by its provisional designation. This is based on the time of discovery. Four digits specify the year and a letter out of [A-HJ-Y] specifies the half-month. This is followed by a counter. Initially, counting to 25 by means of the letters [A-HJ-Z] was deemed sufficient. However, with increased awareness of planet safety, telescopic surveys discover objects in much larger numbers, and the counter has to be indexed by a number telling how often the count of 25 has overrun. Remember, this is a counter within a half month period of time, so this means a lot of discoveries. A recent example is "2025 QR41", which was dicovered during the second half of 2025-08. The discovery counter amounts to 17+41·25 = 1042.

If, after being assigned the permanent ID (the minor planet number), no common name is given to the object, then the provisional ID is continued to be used as the common name, e.g. "(4596) 1981 QB" as opposed to "(4597) Consolmagno".

Comets

Comets must have been observed since prehistoric times. The Bayeux Tapestry, a contemporary commemoration of the Norman conquest of England in 1066, depicts 1P/Halley. The 1301 appearance of 1P/Halley is thought to have inspired Giotto di Bondone to paint it as the Star of Bethlehem. (In return, the 1986 space probe exploring the comet was named Giotto.) Halley observed the comet in 1682. Inspired by Newton's new gravity, and helped by Flamsteed's observations, he identified it as the comet of 1531 and 1607. He later predicted the return in 1758.

Comets were discovered so much earlier than minor planets, because near the Sun their brightness and appearance is so much more prominent.

153P/Ikeya-Zhang
153P/Ikeya-Zhang in 2002-03 and 2002-04.

This is a montage of seven images of this bright comet of spring 2002. All images are adjusted to the same brightness representation, all are adjusted to the same scale (~106 pix/° in the full version), and all are rotated to be oriented in equatorial coordinates. Observe how the direction of the tail changes as the comet moves across the sky from the evening West horizon to high declination low above the North horizon, the tail always pointing away from the Sun. Also observe how the balance between the broad, curved dust tail and the thin, straight ion tail changes over time.

The images were taken (left to right) on the evenings of 2002-03-13, 2002-03-25, 2002-03-29, 2002-04-01, 2002-04-06, 2002-04-07, 2002-04-16. Black corresponds to a surface brightness of −1 MJy/sr, pink to +5 MJy/sr, and white to 50 MJy/sr.

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Comets move in elliptic Kepler orbits around the Sun. However, the mathematical fit to limited astrometric observations often leads to a nominally parabolic or mildly hyperbolic orbit. The orbit can change during a close encounter with a planet, and it is thought that all short-period comets were converted by planets like Jupiter from their former long-period orbits with aphelion in the Oort cloud beyond the Kuiper belt. The approach to describing the orbits is pragmatic and similar to minor planets. Frequently, updated Kepler elements of the current orbits of comets are published. They are good for perhaps weeks and months, but probably not years. For short-period comets they suffice to re-discover them on their return, but a new orbit determination is then urgently required.

17P/Holmes
17P/Holmes on 2007-11-11.

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In late October 2007, comet Holmes brightens about 500,000-fold from being beyond the reach of amateur astronomers' telescopes to competing with the brighter stars of the night sky – easily visible to the naked eye. From day to day its coma grows, while its total brightness remains more or less constant. Here it is almost the apparent size of the Sun, and larger than the real size of the Sun.

The naming of comets is similar to, but different from that of minor planets. The common name has more relevance; this being the name of the discoverer(s), it is immediately available. There is also a provisional ID based on the year and half-month of dicovery. Initially, the provisional ID and the common name are combined with the latter in parentheses, e.g. "C/2024 E1 (Wierzchos)". Note the "C/" prefix for "comet". The counter here is one or more digits. Comet discoveries are relatively rare, only sometimes is a second digit required. The likely reason is that at discovery a comet tends to have no tail or coma, so that its nucleus is mistaken for an asteroid and given a minor planet provisional ID. When the mistake is discovered, the provisional ID is given the "C/" prefix, but the counter remains unchanged. An example is "C/2013 US10 (Catalina).

When the opposite happens, what was thought to be a comet turns out to be an asteroid, then the ID also remains the same, but the prefix is changed to "A/".

A permanent ID, a comet number, is given to short-period comets. These then also have the prefix changed to "P/". Thus "C/2002 C1 (Ikeya-Zhang) eventually became "153P/Ikeya-Zhang". It also has the retrofitted provisional ID "C/1661 C1", but is not also known by its then discoverer Hevelius. This comet has the longest "short period" of 365 years. Before, 200 years was considered the limit, based on the 188 year 273P/Pons–Gambart.

In recent years, a few objects have been discovered that have highly excentric orbits and too large velocities to be gravitationally bound to the Sun. Such interstellar objects are to some degree treated like comets. Their orbits are obviously not asteroidal and they also tend to develop a coma. The provisional IDs are then cometary like "C/2025 N1 (ATLAS)", but they very soon obtain a permanent ID, in this case "3I/ATLAS".

Comets are notoriously unpredictable in terms of how bright they will be, but also whether they might break up into pieces close to the Sun. This is because so much depends on the unknown detailed structure of the comet and how it will respond to the increased influence from the Sun. So it is possible that a predicted "comet of the century" becomes the flop of the century, while some average comet might develop a sudden burst of activity and brightness.

Although the tails are the hallmark of the comets, the tails exist only while the comet is amongst the inner planets, relatively close to the Sun, and even then it is fainter and harder to photograph than the head.

12P/Pons-Brooks C/2023 A3 (Tsuchinshan-ATLAS)
12P/Pons-Brooks on 2024-03-16 and
C/2023 A3 (Tsuchinshan-ATLAS) on 2024-10-14.

C/2014 Q2 (Lovejoy) C/2025 A6 (Lemmon)
C/2014 Q2 (Lovejoy) on 2015-01-18 and
C/2025 A6 (Lemmon) on 2025-10-27.

C/2025 R3 (PANSTARRS) 3I/ATLAS
C/2025 R3 (PANSTARRS) on 2026-04-11 and
3I/ATLAS on 2025-11-20.

C/2025 K1 (ATLAS) 29P/Schwassmann-Wachmann
C/2025 K1 (ATLAS) on 2025-12-11 and
29P/Schwassmann-Wachmann on 2026-03-18.

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Usually, the stacks of frames are aligned on the comet and the stars turn into trails. An exception here is C/2023 A3, where only one frame with short exposure is used. For 3I/ATLAS, both ways of stacking are shown. Sometimes a grey negative image is preferred, as it can better show faint detail, in particular structure in the tail. For C/2025 K1 a grey image with false colour encoding of brightness is shown alongside the regular image.

While most comets here have impressive tails, if you put your mind and equipment to imaging fainter comets, they usually only show a coma and no tail. 29P/Schwassmann-Wachmann is without tail most of the time.

3I is a peculiar object, the third so far that came from interstellar space and that is on its way back, perhaps to visit yet another solar system out there.

It does happen that comets break up, such as C/2025 K1, here split into three fragments separated along the tail. Comets are icy dirtballs that spend most of their time far from the Sun. When they venture into the inner solar system, the sunlight sublimates the ice and the nucleus then sometimes breaks into pieces.



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Attribution-Noncommercial-No Derivative Works 3.0 Unported

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