24

In a series of four tweets starting with this one (found in 2 satellites will narrowly avoid colliding at 32,800 mph over Pittsburgh on Wednesday)

1/ We are monitoring a close approach event involving IRAS (13777), the decommissioned space telescope launched in 1983, and GGSE-4 (2828), an experimental US payload launched in 1967.

LeoLabs mentions a predicted conjunction of two defunct satellites with a worrisome separation

2/ On Jan 29 at 23:39:35 UTC, these two objects will pass close by one another at a relative velocity of 14.7 km/s (900km directly above Pittsburgh, PA). Our latest metrics on the event show a predicted miss distance of between 15-30 meters.

Since neither spacecraft can be controlled we just have to sit back and watch what does or does not happen 900 km above Pittsburgh, PA.

Question: How did these two spacecraft end up in nearly counter-propagating orbits? Since their altitude is so high they must have both started not far above 900 km. Wouldn't it have been a good idea not to have them going opposite directions? (Or am I misunderstanding the situation?)

enter image description here

uhoh
  • 148,791
  • 53
  • 476
  • 1,473
  • 1
    companion question: What does “MSR” represent in the context of this predicted satellite conjunction? – uhoh Jan 29 '20 at 03:29
  • 4
    I didn't know there was a Pittsburgh in Panama (=PA) ;-) – gerrit Jan 29 '20 at 08:57
  • 4
    They were launched in 83 and 67. I'm yet to hear about predictions being accurate over a month, let alone after 37 years. There was just no way to predict that over time these two particular satellites would go into conjunction. And by the way, those kind of conjunctions happen often in LEO, they just don't usually make news. But notice that over the next revolution, these two satellites won't pass close to each other, hence why this usually isn't a problem. Every LEO operator without propulsion is afraid of being hit by another satellite, this just didn't happen (by accident) yet. – Mefitico Jan 29 '20 at 19:54
  • @Mefitico that's certainly correct; I think an additional answer to that effect would be instructive. – uhoh Jan 29 '20 at 23:17
  • 1
    Looks like they tunneled through each other. Simulation time step too long ... :-) –  Jan 30 '20 at 13:52
  • 1
    @ebv there are advantages to living in a simulation ;-) – uhoh Jan 30 '20 at 13:53

1 Answers1

26

How? Simple, because they launched into those orbits.

Why? Well, first, let me explain what their orbits actually are.

IRAS (13777) and GGSE-4 (2828) are both in high-inclination orbits, 70° and 99°, respectively. The latter is slightly retrograde, as is common for sun-synchronous orbits. However, to fully understand in what plane they are orbiting, we need two elements, not just the inclination. (Why two? Because you need two measures to indicate a unique direction in 3 dimensions.)

This second element is their right ascension of the ascending node, or, more simply, at what longitude (not relative to the rotating surface but rather to to the First Point of Ares, a fixed point in the sky) they cross the Earth's equator traveling north. These are 214.5008° and 027.7275°, respectively.

You can see those are nearly 180° apart. Combined with the inclinations, which are 10° and 20° off of a true polar orbit but in different directions, and we've got two satellites in nearly the same orbit, except moving in opposite directions.

So back to why...

IRAS was launched into a sun-synchronous orbit because it is an infrared survey satellite and it is useful to always have the sun in the (nearly) same position when shooting infrared photos. That way, they're all taken at the same time of day & you don't have to consider the changes in lighting (as much) for remote sensing applications. GGSE was launched into a high subpolar orbit because it was a '60 spy sat and it needed to take photos of high-latitude Soviet things.

However, it seems IRAS was launched just at the right (wrong?) time of day that GGSE's orbit was directly overhead, but going the opposite direction that IRAS launched. Because their inclinations are similarly off of 90°, but in different directions, that gave their two orbits a very low relative inclination of around 10° (actually, technically, it's around 180, because they're going nearly opposite directions, but that's not my point here, my point is they're in the same plane).

So if IRAS had launched 12 hours later or earlier from when it did, they would have been going the same direction but with a 30° relative inclination difference instead of a 11° one.

Anton Hengst
  • 10,687
  • 40
  • 78
  • 21
    tl;dr: NASA either didn't do their homework, or made a calculation error, re IRAS; or they didn't know about GCSE-4 (possibly its existence was still classified in 1983?) – Ian Kemp Jan 29 '20 at 11:52
  • 13
    @IanKemp I don't think that's necessarily a fair conclusion. At the time (and to a certain extent, today) most operators assumed that the number of objects in space was so low that it would be nearly impossible for any two of them to collide. Even now, nearly 40 years after their launches, they haven't collided (though how many close approaches there have been, I couldn't say) – costrom Jan 29 '20 at 15:41
  • 9
    Are you suggesting that a sun-sync and a non-sun-sync vehicle have maintained the same orbital relationship since 1983? They will be precessing at different rates. – BowlOfRed Jan 29 '20 at 20:03
  • 3
    Shouldn't "around 180" be "around 190"? – Dan Is Fiddling By Firelight Jan 29 '20 at 20:19
  • 1
    "because they launched into those orbits" Does a satellite really stay in the same orbit for 50 years? – JiK Jan 30 '20 at 13:03
  • 1
    @jik (and @ BowlOfRed too) Absolutely not. My answer is almost categorically false as far as a historical narration of what happened. Non-Keplerian perturbation on such low orbits of such smallish vehicles means that (I speculate) unless you had the original launch parameters, it'd be very hard to determine in which original orbit these satellites were placed. However, I believe my answer does hold merit in that it deepen the asker's understanding of (admittedly, Keplerian-only) orbital mechanics & helps them think about how such a situation could originate (on a much shorter timescale). – Anton Hengst Jan 30 '20 at 17:29
  • 1
    @AntonHengst I often ask questions to which I already have an idea of what the answer may be. My goal in these cases is to facilitate the generation of answers that benefit future readers. So sometimes I play the role of answer facilitator rather than genuine enquirer. – uhoh Feb 16 '22 at 00:11