v. To force a satellite or space vehicle out of its orbit and into the atmosphere for reentry to Earth.
Space station Mir's expected incineration in a 'controlled' de-orbit over the South Pacific on Friday is a fitting demise for an aged pioneer crippled by the passage of time.
—“Mir's pioneering legacy,” USA Today, March 22, 2001
1981 (earliest)
Over the Indian Ocean at an altitude of 173 miles, the Columbia's two orbital maneuvering-system rockets, in the aft section, will fire for two and a half minutes in a 'de-orbit burn.' By blasting against the direction of flight, the rockets will slow the spaceship from its orbital speed of 17,500 miles per hour so it starts dropping out of orbit. The centrifugal force of its orbital speed will give way to the pull of earth's gravity.
—John Noble Witford, “Bringing the Craft Back Down: A Time of Risk,” The New York Times, April 13, 1981
This term is in the news because of the Mir space station's fiery return home this morning. However, it has been used in space circles (I'm sure there's a pun in there somewhere) for many years. Its first appearance in the mainstream media (as an adjective, in this case) appears to have been in a New York Times story from 1981, which also provides a decent explanation of the de-orbit process.

After that, de-orbit would show up occasionally, averaging about eight stories a year through 1996. Then things started picking up:

1997 - 49 citations
1998 - 87
1999 - 132
2000 - 210
2001 - 126 (as of March 22)

Of these stories, just under half discussed the Mir space station, so it's clear that the ultimate fate of this ill-starred craft was on a lot of peoples' minds over the past four years.
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