2000 SG344

is a small Aten asteroid first observed in 2000. It is assumed to have a diameter of approximately – or roughly twice that of the Chelyabinsk meteor – and an assumed mass of 7.1 kg, 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 of impacting Earth and has the fifth highest cumulative Palermo scale rating at −2.77. The next good chance to observe the object will be in May 2028 when it passes from Earth.
Because of its very Earth-like orbit and because it would have been near the Earth in 1971, there was speculation that 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.

Date & time	Nominal distance	uncertainty region
2028-May-07 03:32 ± 4 minutes
2030-Sep-22 22:36 ± 10 hours

Possible impacts with Earth

Until December 2004, it was considered to have the highest 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 and is listed on Sentry Risk Table. It was briefly surpassed in December 2004 by 99942 Apophis. Smaller asteroids such as and have a greater chance of impacting Earth.
Based on 31 observations of 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. 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 wide.

Date	Impact probability	JPL Horizons nominal geocentric distance	NEODyS nominal geocentric distance	uncertainty region
2069-09-18	5 million
2070-09-17	4300
2071-09-10	7100
2071-09-16	1000

Proposed NASA mission

In 2008, NASA considered this asteroid as a possible target for a crewed mission using the Orion spacecraft, prior to a projected 2030 push to Mars. Those plans were since abandoned. will be observable in May 2028 at an apparent magnitude of 19.