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Space.com's 56-year-old NASA satellite expected to fall to Earth this weekend says:

"OGO-1 is predicted to re-enter on one of its next three perigees, the points in the spacecraft's orbit closest to our plant, and current estimates have OGO-1 re-entering Earth's atmosphere on Saturday, Aug. 29, 2020, at about 5:10 p.m. EDT [2110 GMT], over the South Pacific approximately halfway between Tahiti and the Cook Islands," NASA officials wrote in an update Thursday (Aug. 27).

"The spacecraft will break up in the atmosphere and poses no threat to our planet — or anyone on it — and this is a normal final operational occurrence for retired spacecraft," they added.

The new observations come courtesy of the University of Arizona's Catalina Sky Survey (CSS) and the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS), both of which independently detected a small object on an apparent impact trajectory.

Analyses by researchers at the CSS, the Center for Near-Earth Object (NEO) Studies at NASA's Jet Propulsion Laboratory in Southern California and the European Space Agency's NEO Coordination Center revealed that the object in question was not an asteroid but rather OGO-1, NASA officials said.

Why does OGO-1's trajectory information come from "the University of Arizona's Catalina Sky Survey (CSS) and the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS), both of which independently detected a small object on an apparent impact trajectory" rather than from normal satellite tracking systems? It's perigee is only about 350 km and it's a well-known US-launched spacecraft.

It's not small, and its orbit should have been quite predictable, was it dropped from tracking or somehow lost for years?

For more information see:


OGO-1 spotted Images of OGO-1 captured during asteroid survey operations on Tuesday, August 25 by University of Arizona’s Catalina Sky Survey, funded by NASA’s Planetary Defense Coordination Office. Credits: Catalina Sky Survey/University of Arizona/NASA

Images of OGO-1 captured during asteroid survey operations on Tuesday, August 25 by University of Arizona’s Catalina Sky Survey, funded by NASA’s Planetary Defense Coordination Office. Credits: Catalina Sky Survey/University of Arizona/NASA

Source

OGO-1 sketch

Source

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Space-Track has only tracked the object intermittently. They are not, in general, a reliable source for high-flying objects with orbital periods of about two days or more. I am not entirely sure why. Radar is poorly suited to such tracking (effectiveness drops off as the fourth power of distance, instead of inverse-square as it does for optical observations), but they do have telescopes.

This has been an annoyance for asteroid observers, who routinely pick up bits of junk with orbital periods of two days to two months. To identify such junk, I've computed TLEs based on observations from NEO folk for this object (and for other high-flying artsats) for some years now. As a result, the NEO community can usually identify and ignore artsats.

In this case, my TLEs were based on data that ended on August 18, a bit before the object went through some perigee passes and resultant drag. I had a nominal drag term included, but didn't really expect that it would be very precise. Indeed, Catalina found the object about two degrees off prediction.

Once we had their data, I could update the TLEs and we got quite a bit of tracking data, enough to determine that it would come in over Tahiti at about 20:42 UTC on the 29th. And an observer on the island was able to get video of the re-entry.


Two students from University of Hawai'i spotted the modified orbit of the satellite OGO-1 launched back in 1964 and based on the calculations, it was scheduled to re-enter Earth's atmosphere on august 29th around 10:45am (08:45pm UTC) over the Pacific Ocean.

Thanks to the the precious informations provided by Peter Jenniskens, Jean-Pierre Bonnefoy and Régis Plichart, I was able to film it from my island home, Tahiti !!

Filmed using a BlackMagic Pocket Cinema Camera 4k (BMPCC4k) with a Nikon 200mm Lens (400mm equivalent on full frame). Graded on BlackMagic Davinci Resolve.

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  • $\begingroup$ yet another excellent "exotic TLE" answer, thank you! :-) Cool link to video of the daytime reentry as well $\endgroup$ – uhoh Aug 30 '20 at 15:54
  • $\begingroup$ So while OGO-1 would have had regular and predictable periapsi at fairly low altitude amenable to radar tracking every few days, the frequency at which they happen at a convenient location for tracking would have been low I suppose. $\endgroup$ – uhoh Aug 30 '20 at 15:58
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    $\begingroup$ It may be they lack opportunities to get radar transmitter/receiver and target close together, particularly when you get a run of perigees at very far south latitudes. That doesn't entirely seem to cover it, though. It really seems as if Space-Track just isn't all that interested in objects in high orbits. $\endgroup$ – Bill Gray Aug 31 '20 at 18:10
  • $\begingroup$ I've been wanting to post a question something like "What is 'projectpluto' and how does it work?" since it's obviously not this in Wikipedia for a long time now. It's likely on-topic both here and in Astronomy SE. Do you have any thoughts on which site might be better served by such a post? $\endgroup$ – uhoh Sep 1 '20 at 9:02
  • $\begingroup$ I just found a video in YouTube and added the link to your answer, I hope you don't mind. If you do I can move it to the question instead. $\endgroup$ – uhoh Sep 1 '20 at 14:45

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