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Binary asteroid is a term referring to a system of two asteroids orbiting their common centre of gravity. If their sizes do not differ significantly, they are also called double asteroids. If one of them is notably smaller, it is called a satellite (or a moon) of the asteroid. Then the centre of gravity lies inside its large partner. There is no strict border between those two categories determined.

By January 2006, 86 asteroids and trans-Neptunian objects have been discovered or are strongly suspected to be binary. Researchers have even estimated that up to 16 per cent of all asteroids larger than 200 metres could be binary asteroids. Besides that two triple systems are known.

1 Formation
2 Ida and Dactyl
3 Near Earth objects
4 Other asteroids
5 Trans-Neptunian objects
6 Multiple systems
7 Doublet craters
8 External links
9 London in foreign languages
10 a pattern
11 list

[edit] Formation

Many asteroids consist of several pieces of rock, which are held together only by their gravity. Thus, when they pass by a planet, they can be easily torn up by its tidal force into several smaller pieces, which later recompose, forming two bodies instead of one. Such splitting could occur also when an asteroid is forced to rotate too quickly. Asymetric receiving of solar energy and radiating it away in the form of heat could be the mechanism causing the rotation. Another theory suggests that asteroid companions can be formed from debris knocked off the primary asteroid by an impact. The theory of catching one asteroid by another, similarly as some planets might have caught some of their moons, has been rejected, because gravity of asteroids is not strong enough to slow down other fast-moving bodies.

[edit] Ida and Dactyl

Dactyl, the first confirmed satellite of an asteroid, is visible to the right of Ida.

Although astronomers had suspected asteroids of having satellites a long time ago, the first clear evidence was obtained in 1994 when photographs of the asteroid (243) Ida arrived from the space probe Galileo, which was on the way to explore Jupiter and its moons. The photographs were taken on 28 August 1993 but they were transmitted back to the Earth somewhat later. The small moonlet orbiting Ida was recognized on 17 February 1994 and reported on 12 March 1994 by M. Belton and R. Carlson on behalf of the Galileo SSI and NIMS teams. It was named Dactyl after the mythical creatures who used to live on the Mount Ida according to the Greek mythology.

[edit] Near Earth objects

The first discovery of a binary asteroid using just Earth-bound instruments was made in January 1994 by two cooperating teams of astronomers from ESO and Ondřejov Observatory (Czech Republic). When studying the light curve of the newly discovered near-Earth asteroid 1994 AW₁, they noticed distinctive periodic declines in its brightness and came to the conclusion that they are caused by eclipses of its companion. Soon, many more discoveries based on this method followed.

Picture of the binary asteroid 1994 KW₄ taken by astronomers using NASA's Goldstone radar telescope. Images show a trail of a smaller component orbiting a larger object. This system was the third binary near-Earth asteroid pair revealed by radar, but it was the first time when scientists captured the complete orbit of one component around the other.

On 3 October 2000 a team of astronomers from Arecibo Observatory announced the discovery of a companion of the asteroid 2000 DP₁₀₇ from their radar observations. Since that time radar became another common technique of exploring near-Earth binaries and sometimes is also used for confirming the results from photometric observations.

24 near-Earth asteroids were known or strongly suspected to be binary by the end of January 2006. The Arecibo team discovered or confirmed discoveries of 15 binaries, the team from Ondřejov Observatory discovered or co-discovered the binary nature of 13 near-Earth asteroids.

[edit] Other asteroids

Besides Ida with Dactyl there are 34 more known or suspected Main Belt binary asteroids (in January 2006). Twenty of them were discovered by the photometric technique of light curve observations, most of them on or with participation of Ondřejov Observatory.

However, since 1 November 1998, when Petit-Prince, a satellite of (45) Eugenia, was discovered on photographs made on Canada-France-Hawaii Telescope at Mauna Kea, Hawaii, the method of direct imaging by Earth-based telescopes using adaptive optics technology became also quite common. Besides CFHT many discoveries were made thanks to Keck II and the Very Large Telescope. Several binary asteroids have been discovered also by the Hubble Space Telescope.

Astronomers also know one binary Trojan ((617) Patroclus, the companion of which was discovered in 2001 on Gemini Observatory, Hawaii) and five Mars-crossers (the first of them (5407) 1992 AX, recognized on Ondřejov, Kharkiv and Table Mountain observatories in 1997).

[edit] Trans-Neptunian objects

NASA's Hubble Space Telescope snapped pictures of the binary TNO 1998 WW₃₁. This composite picture shows the apparent orbit of one icy body aroung the other. The blue oval represents the orbital path.

The first TNO discovered to be binary was 1998 WW₃₁, announced by astronomers from CFHT on 16 April 2001. From then to January 2006 twenty more were discovered, mostly by direct imaging method with telescopes HST, CFHT, VLT, Keck II and Clay (Massachussets Institute of Technology), but lightcurve observations are used as well.

The most prominent member of this class is 2003 UB₃₁₃, which received an unofficial nickname Xena after the main character from a popular TV series. This asteroid (with its size 3000 km) proved to be about 50% larger than the planet Pluto. Its moon was discovered in 2005 and similarly nicknamed Gabrielle, after Xena’s TV companion.

The discovery of this asteroid increased the discussion, whether Pluto should still be considered a planet. Both its size and eccentric orbit, resembling other TNOs more than planets, serve as arguments. If Pluto is declassed from the family of planets in future, it will result also in changes in all historical tables of binary discoveries. Pluto, with its satellite Charon discovered in 1978, would become not only the first known binary TNO, but the first known binary minor planet system at all. And because two more satellites were discovered in 2005, it would become also the second known multiple system of this kind and the first one with more than 2 satellites.

[edit] Multiple systems

Except Pluto, only two multiple systems are known. The first was the Main Belt asteroid (87) Sylvia with its satellites Romulus and Remus (discovered in 2001 and 2004), named after the mythical mother of founders of Rome and after the mythical founders themselves. Two years later the discovery of the trans-Neptunian object 2003 EL₆₁ with its satellites S/2005 (2003 EL₆₁)1 and S/2005 (2003 EL₆₁)2 followed.

The orbiting moons enabled to calculate Sylvia's mass and density, which is only about 20% greater than that of water. Scientists have therefore come to the conclusion that it is made up of a mixture of water ice nad rock and even contains between 25 to 60% empty space. It is probably nothing more than a loose rubble pile made up of debris that fell together after an asteroid collision. The two moons, orbiting in the same plane and direction, have probably originated in the same collision. Because a lot of Main Belt asteroids seem to have a similar composition, astronomers expect more multiple systems discoveries.

[edit] Doublet craters

Doublet craters are slightly separated pairs of craters formed near-simultaneously. They are considered to be remnants of binary asteroid impacts. They comprise about 2% of craters over 10 metres in diameter on Mars, 10-15% on the Earth, and 2-14% on Venus, excluding craters known to have been formed by a grazing impact of a single object which "skips" after the first hit, such as a series of craters in Argentina. The biggest of doublet craters found on the Earth are West Clearwater Lake (32 km in diameter) and East Clearwater Lake (22 km) in Quebec, Canada, separated by 28.5 km and formed by impacts of asteroids of 3.3 and 2.1 km in diameter (respectively). Other big doublet craters are Ries - Steinheim in Germany and Kamensk - Gusev in Russia.