The U.S. Joint Space Operations Center (JspOC), which tracks space debris, has reported that a Russian rocket ullage engine exploded in space around 5:20 a.m. EDT (9:20 GMT) on Wednesday, June 1. According to a statement issued by JspOC, the motor was orbiting the Earth in a highly-elliptical orbit and suffered a break-up – disintegrating into at least 20 pieces.
The destroyed ullage motor appears to be a part of a Blok DM-2 upper stage used on a Proton-M rocket that was launched in December of 2008. The mission took to the skies from the Baikonur Cosmodrome in Kazakhstan and successfully delivered a trio of GLONASS navigation satellites into orbit.
Spaceflight101.com noted that the engine was tracked for the last time in an orbit of approximately 435 by 11,680 miles (700 by 18,800 kilometers) and was inclined an estimated 65.3 degrees. The site reports that this notification was released late on Tuesday, May 31.
The cause of the on-orbit explosion is still unknown, but most probably, it was triggered by the mixing of hypergolic propellants. It could also be due to a collision in space, however JspOC, relying on initial estimates, has excluded that as a possibility.
“There is no indication this breakup was caused by a collision,” JspOC said in a statement.
The pieces of debris will be further monitored by the center to collect post-event tracking data in order to asses the trajectory of possible hazardous space junk. The number of pieces is probably higher than current calculations as many of them are too small to be detected from the ground.
Ullage motors, like the one that exploded, are used for flight stabilization, altitude control and propellant settling activities. During a typical Proton flight, a pair of these motors remain attached to the Blok DM-2, and separates when the upper stage’s main engine reaches full thrust.
Each ullage motor that is installed on a Blok DM-2 upper stage is filled with nitrogen tetroxide (N2O4) and unsymmetrical dimethylhydrazine (UDMH) propellants. This combination causes spontaneous explosions when propellant tanks corrode and these two fuels mix with each other. Under these conditions such an explosive event could occur even years after the rocket was launched, when the corrosion process is well underway.
Explosions of ullage motors are relatively common. According to Spaceflight101.com, the first such event took place in 1984. This latest break-up is the 45th known explosion of this motor so far.
Developed by RKK Energia, the Blok DM-2 is a Russian upper stage used in Proton-K and Proton-M launchers. It measures an estimated 20.6 feet (6.28 meters) tall, and some 12.1 feet (3.7 meters) in diameter and has an empty mass of about 2.1 metric tons. The stage burns for about ten and a half minutes during the flight.
Out of all variants of the Blok D upper stage, DM-2 is the most often employed configuration. It has flown some 115 times between 1982 and 2012.