UARS Re-enters Earth’s Atmosphere
Written by Faith Anderson on September 26, 2011
Space Surveillance Network Misses UARS
NASA and the U.S. Joint Space Operations Center at Vandenberg Air Force Base in California depend on ground-based radars and optical sensors at twenty-five sites around the world to gather data about where defunct satellites like the UARS are located. In some cases, the spacecraft may not pass over any of the sensors, which make up the Space Surveillance Network. One of the ways that NASA determines that a satellite is no longer in orbit is to have sensors in the Space Surveillance Network look for it, according to Nick Johnson, chief orbital debris scientist at NASA’s Johnson Space Center in Houston. If they don’t find the satellite, then it has re-entered Earth’s atmosphere. To ensure that a spacecraft has fallen back to Earth, it must have missed three opportunities to have been spotted by sensors. “In this particular case,” Johnson explained, “it took a while for the satellite track to go over three sensors.” For that reason, it took NASA several hours to confirm that UARS had re-entered the atmosphere, and the agency has yet to receive any credible reports of people on the ground observing the spacecraft’s plunge.
Satellite Landing Still a Mystery
According to Johnson, the UARS satellite would have been clearly visible whether it landed during the day or night. “If we continue to have a lack of reports of seeing something that looks like a re-entering UARS, that would give further credence to the fact that it was probably over water,” Johnson reports. Many people around the world have exchanged photos and videos of what they think might have been the satellite plunging back to Earth. However, almost all of these reports came from parts of the world significantly removed from where the satellite was passing over at the time. NASA and the Joint Space Operations Center will continue to investigate the fall of the UARS, but officials estimate being left with some mysteries. “This is not an unusual event,” Johnson comments. “We do not always know precisely where these re-entries occur and this is not a unique situation to UARS.”