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2020 SO
	The orbit of 2020 SO around Earth and Sun from Nov. 2020 to Mar. 2021
Discovery
Discovered by	Pan-STARRS 1
Discovery site	Haleakalā Obs.
Discovery date	17 September 2020
Designations
MPC designation	2020 SO
Alternative designations	P116rK2
Minor planet category	NEO · Apollo (May 2020); Atira (Dec 2020)
Orbital characteristics
	Epoch 17 December 2020 (JD 2459200.5)
	Uncertainty parameter 2
Observation arc	167 days
Earliest precovery date	19 August 2020
Aphelion	0.988 AU
Perihelion	0.985 AU
Semi-major axis	0.986 AU
Eccentricity	0.00181
Orbital period (sidereal)	0.98 yr (357.9 d)
Mean anomaly	276.388°
Mean motion	1° 0m 21.877s / day
Inclination	0.1389°
Longitude of ascending node	216.656°
Argument of perihelion	311.989°
Earth MOID	0.01628 AU (May 2020); 0.00106 AU (Dec 2020)
Physical characteristics
Mean diameter	6–12 m (assumed)
Synodic rotation period	0.0026080±0.0000001 h; or 9.39 s
Apparent magnitude	22.4 (at discovery); 14.1 (1 Dec 2020)
Absolute magnitude (H)	27.66±0.34; 28.43

2020 SO^[a] is a near-Earth object identified to be the Centaur upper stage used on 20 September 1966 to launch the Surveyor 2 spacecraft. The object was discovered by the Pan-STARRS 1 survey at the Haleakala Observatory on 17 September 2020. It was initially suspected to be an artificial object due to its low velocity relative to Earth and later on the noticeable effects of solar radiation pressure on its orbit. Spectroscopic observations by NASA's Infrared Telescope Facility in December 2020 found that the object's spectrum is similar to that of stainless steel, confirming the object's artificial nature.^[8] Following the object's confirmation as space debris, the object was removed from the Minor Planet Center's database on 19 February 2021.^[9]

Overview

As it approached Earth, the trajectory indicated the geocentric orbital eccentricity was less than 1 by 15 October 2020,^[10]^[b] and the object became temporarily captured on 8 November when it entered Earth's Hill sphere.^[11] It entered via the outer Lagrange point L₂ and will exit via Lagrange point L₁. During its geocentric orbit around Earth, 2020 SO made a close approach to Earth on 1 December 2020 at a perigee distance of approximately 0.13 lunar distances (50,000 km; 31,000 mi).^[4] It also made another close approach on 2 February 2021, at a perigee distance of approximately 0.58 LD (220,000 km; 140,000 mi).^[4] Since discovery the time of uncertainty for February 2021 closest approach to Earth was reduced from ±3 days to less than 1 minute.^[4] It left Earth's Hill sphere at around 8 March 2021.^[12]^[b]

Paul Chodas of the Jet Propulsion Laboratory suspects 2020 SO of being the Surveyor 2 Centaur rocket booster, launched on 20 September 1966.^[11]^[12]^[13] The Earth-like orbit and low relative velocity suggest a possible artificial object. Spectroscopy may help determine if it is covered in white titanium dioxide paint.^[14] Goldstone radar will make^[when?] bistatic observations transmitting from the 70-meter DSS-14 and receiving at the 34-meter DSS-13.^[15] As a result of the bistatic DSS-14/RT-32 radar observations, a rotation period of about 9.5 seconds was obtained,^[16] which corresponds to the photometric observations.^[6] Obtained range-Doppler radar images^[16] confirm that the object has an elongated shape with a length of about 10 meters and a width of about 3 meters.

Around the time of closest approach on 1 December 2020, the object was only brightened to about apparent magnitude 14.1,^[7] and required a telescope with roughly a 150mm (6") objective lens to be seen visually.^[17] It displays a large light curve amplitude of 2.5 magnitudes, signifying a highly elongated shape or albedo variations on its surface. It has a rotation period of approximately 9 seconds.^[18]

At the time of its discovery, 2020 SO had unremarkable motion typical of a main-belt asteroid.^{[citation needed]} However, the four observations that Pan-STARRS obtained over the course of 1.4 hours showed non-linear motion due to the rotation of the observer around Earth's axis, which is a signature of a nearby object.^[1]^[10]

Orbital Elements for May and December 2020
Parameter	Epoch	Orbit type	Period (p)	Aphelion (Q)	Perihelion (q)	Semi-major axis (a)	Inclination (i)	Heliocentric eccentricity (e)	Geocentric eccentricity (e)^[10]^[c]
Units			(years)	AU			(°)
	2020-May-31^[4]	Apollo	1.056	1.0722	1.0020	1.0371	0.14061°	0.03389	737
	2020-Dec-17^[2]	Atira	0.980	0.9882	0.9847	0.9865	0.13842°	0.00180	0.89934

Animation of 2020 SO's orbit

Around the Sun

Around the Earth

Sun · Earth · 2020 SO · Moon

In January and February 2036, it will again approach Earth with a geocentric eccentricity less than 1 since the relative velocities will be small,^[10] but will not be within Earth's Hill sphere of 0.01 AU (1.5 million km).^[4]^[d]

Notes

^ This was the 14th object ("O") discovered in the first half (period "S") of September 2020. See Provisional designation in astronomy § New-style provisional designation.
^ ^a ^b The JPL Horizons On-Line Ephemeris System shows the geocentric orbital eccentricity dropping below 1 by 15 October 2020. But a second condition for capture is that the object is within Earth's Hill sphere which has a radius of roughly 0.01 AU (1.5 million km). Meeting both conditions is when the object is in a temporary satellite capture around Earth.
^ Orbital eccentricity must be below 1 to be orbiting the central body.
^ An object needs to be within Earth's Hill sphere to truly be in orbit. An object 1AU from Earth could have a geocentric e<1 if the relative velocities are small, but we would not say it is orbiting Earth.

References

^ ^a ^b ^c "MPEC 2020-S78 : 2020 SO". Minor Planet Electronic Circular. Minor Planet Center. 19 September 2020. Retrieved 20 September 2020.
^ ^a ^b ^c ^d ^e "2020 SO". Minor Planet Center. International Astronomical Union. Archived from the original on 11 February 2021. Retrieved 20 September 2020.
^ "2020 SO". NEO Exchange. Las Cumbres Observatory. 18 September 2020. Retrieved 20 September 2020.
^ ^a ^b ^c ^d ^e ^f ^g ^h "JPL Small-Body Database Browser: 2020 SO" (2020-12-01 last obs.). Jet Propulsion Laboratory. Archived from the original on 26 January 2021. Retrieved 20 September 2020.
^ "NEO Earth Close Approaches". Center for Near Earth Object Studies. Jet Propulsion Laboratory. Retrieved 10 October 2020.
^ ^a ^b Peter Birtwhistle (Great Shefford Observatory). "Light curve".
^ ^a ^b "2020SO Ephemerides for 1 December 2020". NEODyS (Near Earth Objects – Dynamic Site). Archived from the original on 26 September 2020. Retrieved 26 September 2020.
^ Talbert, Tricia (2 December 2020). "New Data Confirm 2020 SO to be the Upper Centaur Rocket Booster from the 1960's". NASA. Retrieved 2 December 2020.
^ "MPEC 2021-D62 : DELETION OF 2020 SO". Minor Planet Electronic Circular. Minor Planet Center. 19 February 2020. Retrieved 19 February 2020.
^ ^a ^b ^c ^d Horizons output. "Geocentric Orbital Elements for Asteroid (2020 SO)". Retrieved 27 September 2020. ("Ephemeris Type" select "Orbital Elements" · "Center" select 500 for Geocentric. Output lists Eccentricity as "EC".)
^ ^a ^b Greicius, Tony (12 November 2020). "Earth May Have Recaptured a 1960s-Era Rocket Booster". NASA. Retrieved 12 November 2020.
^ ^a ^b Dunn, Marcia (11 October 2020). "Fake asteroid? NASA expert IDs mystery object as old rocket". phys.org. Retrieved 12 October 2020.
^ Harris, Alan (20 September 2020). "Re: another natural satellite of Earth... again". groups.io. Retrieved 20 September 2020.
^ B., Mark (12 October 2020). "NASA Expert Believes New "Asteroid" is a Discarded Rocket Part". The Science Times. Retrieved 3 November 2020. Spectroscopy on the surface of 2020 SO can also determine whether it has titanium dioxide - the paint material used on space rockets.
^ "Goldstone Radar Observations Planning: 2020 SO". Asteroid Radar Research. Retrieved 30 November 2020.
^ ^a ^b "2020 SO · IAA RAS". iaaras.ru. Retrieved 10 December 2020.
^ "Limiting Magnitude". The Wilderness Center Astronomy Club. Archived from the original on 17 November 2020. Retrieved 26 September 2020.
^ Masi, Gianluca (2 December 2020). "Near-Earth object 2020 SO: rotation and time-lapse – 01 Dec. 2020". Virtual Telescope Project. Retrieved 2 December 2020.