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2000 SG₃₄₄
	2000 SG344 seen on 29 September 2000, as a dim streak in the center of the image, moving against the background field of stars
Discovery
Discovered by	D. J. Tholen; R. J. Whiteley
Discovery site	Mauna Kea Obs.—UH88
Discovery date	29 September 2000; (first observation only)
Designations
Minor planet category	NEO · Aten
Orbital characteristics
	Epoch 2020-May-31 (JD 2457400.5)
	Uncertainty parameter 3
Observation arc	507 days (1.39 yr)
Aphelion	1.0429 AU (156.02 Gm)
Perihelion	0.91199 AU (136.432 Gm)
Semi-major axis	0.97744 AU (146.223 Gm)
Eccentricity	0.06696
Orbital period (sidereal)	0.97 yr (352.96 d)
Mean anomaly	35.680°
Mean motion	1.0198°/day
Inclination	0.112202°
Longitude of ascending node	191.91°
Argument of perihelion	275.35°
Earth MOID	0.0008 AU (120,000 km)
Physical characteristics
Mean diameter	37 m (assumed); 15–70 meters
Mass	7.1×107 kg (assumed)
Absolute magnitude (H)	24.7

2000 SG₃₄₄ is a small Aten asteroid first observed in 2000. It is assumed to have a diameter of approximately 37 meters (120 feet) – or roughly twice that of the Chelyabinsk meteor – and an assumed mass of 7.1×10⁷ kg (71,000 tonnes),^[3] but the size is only known within about a factor 2. It is the largest object known to have a better than 1/1000 chance (0.1%) of impacting Earth and has the third highest cumulative Palermo rating at −2.77. The next good chance to observe the object will be in May 2028 when it passes 0.02 AU (3,000,000 km; 1,900,000 mi) from Earth.^[2]

Because of its very Earth-like orbit and because it would have been near the Earth in 1971 (coinciding with the Apollo program), there was speculation that 2000 SG₃₄₄ might not be an asteroid but a man-made object such as an S-IVB booster stage from a Saturn V rocket which would make it about 15 meters in diameter and much less massive.^[4]^[5] (cf. J002E3, the S-IVB booster of Apollo 12 which was mistaken for an asteroid.)

Upcoming Earth approaches
Date & time	Nominal distance	uncertainty region (3-sigma)
2028-May-07 03:32 ± 4 minutes	2931689 km^[2]	± 46000 km^[6]
2030-Sep-22 22:36 ± 10 hours	5121080 km^[2]	± 380000 km^[7]

Possible impacts with Earth

Until December 2004, it was considered to have the highest (though still very low) likelihood of any near-Earth object to impact Earth in the next 100 years. It is ranked a zero on the Torino scale of impact risk because of its small size (the scale is 0–10) and is listed on Sentry Risk Table.^[3]^[8] It was briefly surpassed in December 2004 by 99942 Apophis (which at the time was known only by its provisional designation 2004 MN₄). Smaller asteroids such as 2006 JY26 and 2006 RH120 have a greater chance of impacting Earth.

Based on 31 observations of 2000 SG₃₄₄ made from May 1999 to October 2000, there is about a 1 in 360 chance that it will collide with Earth between 2069 and 2121.^[3] The greatest chance of impact is on 16 September 2071 with a 1 in 1000 chance of impact. Assuming the object is a rocky asteroid and that it reaches Earth's surface without exploding in the atmosphere, the impact energy released would be an estimated 1.0 megatons of TNT, comparable to the Tunguska and Chelyabinsk events, which could create an impact crater approximately 100 feet (30 m) wide.

Virtual impactors^[3]
Date	Impact probability (1 in)	JPL Horizons nominal geocentric distance (AU)	NEODyS nominal geocentric distance (AU)	uncertainty region (3-sigma)
2069-09-18	5 million	0.07 AU (10 million km)	0.08 AU (12 million km)^[9]	± 32 million km^[10]
2070-09-17	4300	0.18 AU (27 million km)	0.14 AU (21 million km)	± 275 million km
2071-09-10	7100	0.43 AU (64 million km)	0.35 AU (52 million km)	± 462 million km
2071-09-16	1000	0.44 AU (66 million km)	0.36 AU (54 million km)	± 475 million km

Proposed NASA mission

In 2008, NASA considered this asteroid as a possible target for a crewed mission (Artemis 2) using the Orion spacecraft, prior to a projected 2030 push to Mars.^[11]^[12] Those plans were since abandoned. 2000 SG₃₄₄ will be observable in May 2028 at an apparent magnitude of 19.^[13]