Kosmos 2421 (Cosmos 2421) was a Russian spy satellite launched in 2006, but began fragmenting in early 2008. It also had the Konus-A gamma-ray burst experiment by the Yoffe FizTekh Institute. Three separate fragmentation events produced about 500 pieces of trackable debris, but about half of those had already re-entered by the fall of 2008.
The International Space Station adjusted its orbit to avoid debris fragment 33246 from the Kosmos 2421 breakup. That piece was predicted to have a 1 in 72 chance of hitting the station without a change. Kosmos 2421 was in a higher orbit than ISS, so when ISS's apogee (high point of orbit) surpassed the debris field's perigee (low point of orbit), many fragments would cross ISS's orbit.
Kosmos 2421 was launched on June 25, 2006 on a Tsyklon-2 from LC90 at Baykonur. Other designations are 2006-026A and NORAD 29247. It is a US-PU/Legenda type satellite, and was in a 65 degree, 93 minute circular orbit 410–430 km up.
A fragment re-entered in August 2010.
There have been 190 known satellite breakups between 1961 and 2006. Kosmos 2421 was one of the top ten space junk producing events up to 2012. There was estimated to be 500,000 pieces of debris in orbit at that time.