Contact binary (small Solar System body)

A contact binary is a small Solar System body such as a minor planet or a comet that is composed of two bodies that have gravitated toward each other until they touch, resulting in a bilobated, peanut-like overall shape. Contact binaries are often rubble piles but distinct from real binary systems such as binary asteroids. The term is also used for stellar contact binaries.

Contact binaries of varying probability among the small Solar System bodies: Left: Comet 67P/Churyumov–Gerasimenko, with two distinct lobes connected by a "neck" as seen by the Rosetta probe Right: Kuiper belt object 486958 Arrokoth as observed by the New Horizons probe Bottom: Diagram of asteroid 25143 Itokawa with its two lobes having a different density suggesting that they were once separate bodies

An example of a what is thought to be a contact binary is the Kuiper belt object 486958 Arrokoth, which was imaged by the New Horizons spacecraft during its flyby in January 2019.[1]

Description

Comet Churyumov–Gerasimenko and Comet Tuttle are most likely contact binaries,[2][3] while asteroids suspected of being contact binaries include the unusually elongated 624 Hektor and the bilobated 216 Kleopatra and 4769 Castalia. 25143 Itokawa, which was photographed by the Hayabusa probe, also appears to be a contact binary which has resulted in an elongated, bent body. Asteroid 4179 Toutatis with its elongated shape, as photographed by Chang'e-2, is a contact binary candidate as well.[4] Among the distant minor planets, the icy Kuiper belt object Arrokoth was confirmed to be a contact binary when the New Horizons spacecraft flew past in 2019.[1]

Candidates

The table contains near-Earth objects observed by radar, considered to be contact binaries (candidate objects with a darker background).[5] LCDB = Lightcurve Database.

Object	Mean-diameter or Dimension (km)	Rotation period (hours)	LCDB	Refs
2063 Bacchus	2.6×1.1×1.1	14.9	LCDB	catalog MPC JPL
4450 Pan	1.0	60	LCDB	catalog MPC JPL
4486 Mithra	1.6	67.5	LCDB	catalog MPC JPL
4769 Castalia	0.6	4	LCDB	catalog MPC JPL
11066 Sigurd	3.0	8.5	LCDB	catalog MPC JPL
(179806) 2002 TD66	0.3	9.5	LCDB	catalog MPC JPL
2005 TF49	—	—	LCDB	(candidate) MPC · JPL
2007 TU24	0.3	36	LCDB	MPC · JPL
8P/Tuttle	4.5	—	LCDB	MPC · JPL
2013 US3	0.163	450	LCDB	(candidate) MPC · JPL
3752 Camillo	2.33	37.846	LCDB	catalog MPC JPL
1981 Midas	1.95	5.220	LCDB	catalog MPC JPL
(496817) 1989 VB	0.310	16	LCDB	catalog MPC JPL
2014 JO25	0.818	4.531	LCDB	MPC · JPL
2014 HQ124	0.4	16+	LCDB	MPC · JPL
2015 JD1	—	—	LCDB	MPC · JPL
(413260) 2003 TL4	0.392	27.2	LCDB	(candidate) catalog MPC JPL
(462959) 2011 DU	0.188	10.290	LCDB	(candidate) catalog MPC JPL

See also

• Binary system (astronomy)

References

1. "Ultima Thule is first contact binary to be explored by a spacecraft". UPI. Retrieved 21 September 2019.
2. Quick Rosetta update: Churyumov-Gerasimenko is a contact binary!
3. Success! A final flawless burn. Rosetta now in tandem with its comet
4. The formation mechanism of 4179 Toutatis' elongated bi-lobed structure in a close Earth encounter scenario
5. Dr. Lance A. M. Benner (18 November 2013). "Binary and Ternary near-Earth Asteroids detected by radar". NASA/JPL Asteroid Radar Research. Retrieved 1 March 2014.