Bill Gray
  • Member for 3 years, 4 months
  • Last seen more than a month ago
What is the basic mechanics behind the way the orbits of the twelve Vela spacecraft evolved over decades?
Accepted answer
9 votes

The main culprit is the Kozai mechanism. Basically, with objects in orbits of this sort, you can swap from a highly inclined, nearly circular orbit to one that's more eccentric and less inclined. It'...

View answer
Why can't custom-made TLEs for the DSCOVR launch booster in orbit around Earth work with SDP4?
Accepted answer
7 votes

SDP4 has a lot of algorithmic nuances to handle objects in 12- and 24-hour orbits. You can get decent (km-level over a week or so) fits to the model. I see no evidence in the SDP4 algorithm that ...

View answer
Differences between SGP8 and the standard SGP4? Is it ever used in practice?
7 votes

About fifteen years ago, I very carefully implemented SGP8 and SDP8 (and the 'original' SGP) into my software (see https://github.com/Bill-Gray/sat_code ). It is hard to prove a negative -- i.e., ...

View answer
Is TESS really lost? What does Celestrak mean exactly?
Accepted answer
7 votes

You're right; these objects aren't lost. However, Space-Track doesn't actually track a lot of higher-orbiting objects. They just don't seem particularly interested in anything with a multi-day ...

View answer
In a 3D space defined by semimajor axis, eccentricity and inclination, what would be the shape of the space where SDP4 works better than SGP4?
4 votes

I have no formal, mathematical answer to this. When I have to fit TLEs to a new object, I try SDP4; if it fails, I drop back to SGP4. It's an entirely empirical "gee, SDP4 didn't work here&...

View answer
What is/was legacy TLE ephemeris type 2? (TLE, line 1, column 63)
4 votes

I find Ryan C's answer interesting... but I've never actually interpreted the documentation that way. It's difficult to be certain, since all official TLEs have had ephem type 0. But I (and others ...

View answer
What artificial satellite has the farthest orbit around the Earth?
4 votes

A late reply here, but perhaps it'll be of interest to those finding this post as I did. Space-Track is not especially helpful for tracking the really high-flying junk (it's not very important to ...

View answer
Where is Vela 1A? Is there any information what its orbit would be like now?
4 votes

This object was "lost" for some decades. In October 2014, the Palomar Transient Factory (which was looking for near-earth asteroids that might hit us) recovered it. That was apparently enough to ...

View answer
Has a TLE ever been issued for a spacecraft trajectory not bound to Earth orbit?
3 votes

Unfortunately, you can't generate a TLE for a parabolic or hyperbolic flyby. The eccentricity must be between 0 and 1, and the semimajor axis must be positive. In some cases, you can't even find ...

View answer
Why has no TLE been published for the DSCOVR satellite and the Falcon 9 R/B?
3 votes

I realize this is an old question, but in case someone stumbles across it (as I just did): It is true that TLEs for 2015-007B = DSCOVR booster are not provided by Space-Track. However, that object ...

View answer
Why does OGO-1's trajectory and imminent reentry come from sky surveys and NEO tracking rather than normal satellite tracking?
2 votes

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 ...

View answer