| Minorplanet: | yes |
| 1866 Sisyphus | |
| Background: |
|
| Discovered: | 5 December 1972 |
| Mpc Name: | (1866) Sisyphus |
| Alt Names: | 1972 XA |
| Adjectives: | Sysiphean, Sisyphian |
| Named After: | Σίσυφος Sīsyphos |
| Epoch: | 4 September 2017 (JD 2458000.5) |
| Uncertainty: | 0 |
| Observation Arc: | 61.55 yr (22,482 days) |
| Earliest Precovery Date: | 26 January 1955 |
| Aphelion: | 2.913AU |
| Perihelion: | 0.8747312AU |
| Semimajor: | 1.8936992AU |
| Eccentricity: | 0.5386 |
| Period: | 2.61 yr (952 days) |
| Mean Motion: | / day |
| Inclination: | 41.202° |
| Asc Node: | 63.498° |
| Arg Peri: | 293.09° |
| Satellites: | 1 |
| Moid: | 0.1045AU |
| Mars Moid: | 0.1291AU |
1866 Sisyphus is a binary stony asteroid, near-Earth object and the largest member of the Apollo group, approximately 7 kilometers in diameter.
It was discovered on 5 December 1972, by Swiss astronomer Paul Wild at Zimmerwald Observatory near Bern, Switzerland, and given the provisional designation . It was named after Sisyphus from Greek mythology.
This S-type asteroid (composed of rocky silicates) orbits the Sun in the inner main-belt at a distance of 0.9–2.9 AU once every 2 years and 7 months (952 days). Its orbit has an eccentricity of 0.54 and an inclination of 41° with respect to the ecliptic.
The Apollo asteroid has an Earth minimum orbit intersection distance of 0.1037AU, which corresponds to 40.4 lunar distances. It will pass 0.11581abbr=onNaNabbr=on from Earth on 24 November 2071, and will peak at roughly apparent magnitude 9.3 on 26 November 2071. When it was discovered it peaked at magnitude 9.0 on 25 November 1972. It is one of the brightest near-Earth asteroids.
In the SMASS classification, Sisyphus is a common stony S-type asteroid.
In 1985, this object was detected with radar from the Arecibo Observatory at a distance of 0.25 AU. The measured radar cross-section was 8 square kilometers. During the radar observations, a small minor-planet moon was detected around Sisyphus, although its existence was not reported until December 2007. Robert Stephens confirmed that it is a suspected binary, and Brian Warner added additional weight to this conclusion, giving hours as the satellite's orbital period, longer than the 25 hours previously reported by Stephens.
With a measured mean diameter in the range of 5.7–8.9 kilometers, it is the largest of the Earth-crossing asteroids, comparable in size to the Chicxulub object whose impact contributed to the extinction of the dinosaurs. Larger near-Earth asteroids which are neither classified as Apollos nor Earth-crossers include 1036 Ganymed (32 km), 3552 Don Quixote (19 km), 433 Eros (17 km), and 4954 Eric (10.8 km).
This minor planet is named after Sisyphus from Greek mythology and refers to the cruel king of Ephyra, punished by being given the task of rolling a large stone up to a hill in the underworld, only to have it roll down again each time he neared the top. The official was published by the Minor Planet Center on 20 December 1974 .