1995 SN55, is a minor planet and likely centaur that orbits in the outer Solar System beyond the orbit of Jupiter. With an estimated diameter of approximately 290 kilometers, it would be one of the largest centaurs. First observed by Spacewatch in 1995, it became a lost minor planet with an insufficiently defined orbit after only 7 weeks of observations, and has not been observed since.
A. Gleason 
|Discovery site||Kitt Peak Obs.|
|Discovery date||20 September 1995|
(discovery: first observed only)
|MPC designation||1995 SN55|
|centaur  · distant |
|Orbital characteristics |
|Epoch 6 October 1995 (JD 2449996.5)|
|Uncertainty parameter 9 · E|
|Observation arc||36 days|
|114.39 yr (41,782 days)|
|0° 0m 30.96s / day|
300 km (est. at 6.0; 0.08)
|6.0 · 6.2|
First observation and lossEdit
1995 SN55 was about 39 AU from the Sun when it was first observed in 1995, by astronomer Arianna Gleason of the Spacewatch survey at Kitt Peak Observatory in Arizona, United States. It was only observed 14 times over 36 days from 20 September 1995, until 26 October the same year.
There have been numerous attempts to recover 1995 SN55, as recently as 2018. So far, it has still not been positively detected, indicating it is either dimmer than expected, or on a different orbit than calculated. As of 2018[update], the uncertainty in the heliocentric distance has increased to ±2.7 billion km.
distance from the Sun
|1995-Sep||±350 million km|
|1999-Jul||±500 million km|
|2004-Aug||±1 billion km|
|2013-Mar||±2 billion km|
|2020-Aug||±3 billion km|
|2027-Jan||±4 billion km|
|2032-May||±5 billion km|
Classification and orbitEdit
Centaurs have a perihelion greater than Jupiter and a semi-major axis less than that of Neptune. 1995 SN55 orbits the Sun at a distance of 7.9–39.2 AU once every 114 years and 5 months (41,782 days). Its orbit has an eccentricity of 0.66 and an inclination of 5° with respect to the ecliptic. Due to this short observation arc , the object has a very poorly known orbit with the highest possible uncertainty parameter value of 9 and is considered a lost minor planet.
JPL's small body data base shows this object having an aphelion distance of 39.2 AU, whereas the Deep Ecliptic Survey (DES) finds an aphelion distance of 91 AU, which would make it a trans-Neptunian object by JPL's orbital classification (hence the uncertainty whether 1995 SN55 is a centaur at all).
Diameter and albedoEdit
If confirmed to be a centaur, 1995 SN55 would be one of the largest centaurs known with an diameter estimate of 280 and 290 kilometers. Based on a generic magnitude-to-diameter conversion, 1995 SN55 could measure 300 kilometers, using an observed absolute magnitude of 6.0, and an albedo of 0.08, which is typically assumed for centaurs.
Numbering and namingEdit
Due to its uncertain orbit, this minor planet has not been numbered. A numbering and subsequent naming will only be considered upon its possible "rediscovery".
- "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center. Retrieved 21 October 2008.
- "1995 SN55". Minor Planet Center. Retrieved 30 July 2017.
- "JPL Small-Body Database Browser: (1995 SN55)" (1995-10-26 last obs.). Jet Propulsion Laboratory. Archived from the original on 12 December 2012. Retrieved 30 July 2017.
- "List of Known Trans-Neptunian Objects (incl. centaurs)". Johnston's Archive. 22 July 2017. Retrieved 7 September 2017.
- Michael E. Brown. "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 7 September 2017.
- "Asteroid Size Estimator". CNEOS/JPL. Retrieved 7 September 2017.
- "TNO/Centaur diameters and albedos". Johnston's Archive. 5 September 2016. Retrieved 7 September 2017.
- Bauer, James M.; Grav, T.; Mainzer, A. K.; Masiero, J. R.; Blauvelt, E.; Stevenson, R.; et al. (October 2013). "Centaurs and Scattered Disk Objects in the Thermal Infrared: Analysis of WISE/NEOWISE Observations" (PDF). American Astronomical Society. 773 (1). arXiv:1306.1862. Bibcode:2013DPS....4550806B. doi:10.1088/0004-637X/773/1/22. Retrieved 7 September 2017.
- Marc W. Buie (26 October 1995). "Orbit Fit and Astrometric record for 95SN55". SwRI (Space Science Department). Retrieved 21 October 2008.
- "LCDB Data for (1995)". Asteroid Lightcurve Database (LCDB). Retrieved 30 July 2017.