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Most satellites share the same fate at the end of their lives. Their orbits decay, and eventually, they plunge through the atmosphere toward Earth. Most satellites are destroyed during their rapid descent, but not always<\/p>\n
Heavy pieces of the satellite, like reaction wheels, can survive and strike the Earth. Engineers are trying to change that.<\/p>\n
<\/p>\n Satellite debris can strike Earth and is a potential hazard, though the chances of debris striking anything other than ocean or barren land are low. Expired satellites usually just re-enter the atmosphere and burn up. But there are a lot of satellites, and their number keeps growing. <\/p>\n In February 2024, the ESA\u2019s European Remote Sensing 2 (ERS2) satellite fell to Earth. The ESA tracked the satellite and concluded that it posed no problem. \u201cThe odds of a piece of satellite falling on someone\u2019s head is estimated at one in a billion,\u201d ESA space debris system engineer Benjamin Bastida Virgili said. <\/p>\n That would be fine if ERS 2 was an isolated incident. But, according to the ESA, an object about as massive as ERS 2 reenters Earth\u2019s atmosphere every one to two weeks. The statistics may show there\u2019s no threat to people, but statistics are great until you\u2019re one of them. <\/p>\n The risk of being struck by chunks of a satellite isn\u2019t zero. In 1997, a piece of mesh from a Delta II rocket struck someone\u2019s shoulder in Oklahoma. It was a light piece of debris, so the person was okay. But it was an instructive event. <\/p>\n The heaviest parts of satellites, like reaction wheels, can be hazardous because they may not be destroyed during re-entry. Reaction wheels provide three-axis control for satellites without the need for rockets. They give satellites fine pointing accuracy and are useful for rotating satellites in very small amounts. <\/p>\n Reaction wheels can be quite massive. The Hubble Space Telescope has four reaction wheels weighing 45 kg (100 lbs) each. Other satellites don\u2019t have such massive wheels, but the Hubble\u2019s hefty wheels indicate the extent of the hazard. ESA engineers are designing reaction wheels that will break up during re-entry to reduce the hazard of one striking Earth.<\/p>\n \u201c\u2026 the need is becoming urgent as more and more satellites are placed in space.\u201d<\/p>\n Koby\u00e9 Bodjona, Mechanisms Engineer at the ESA<\/cite><\/p><\/blockquote>\n<\/figure>\n As part of the design process, they\u2019re testing their wheels in a plasma wind tunnel at the University of Stuttgart Institute of Space Systems. The heated plasma in the tunnel moves at several km\/sec, mimicking the friction a satellite is exposed to when it plunges through Earth\u2019s atmosphere. The wheel is rotated inside the tunnel as if tumbling through the atmosphere.<\/p>\n At a recent Space Mechanisms Workshop at ESA\u2019s ESTEC technical center in the Netherlands, engineers showed a clip of the blow-torch effect that the atmosphere has on falling debris. <\/p>\n\n