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.

The image from one of the tweets is shown below. In addition to the two trajectories intersecting over the northeast US (Pittsburgh, PA) there is a highly elongated arc over Baja California, northwest Mexico and Texas of concentric narrow ellipses labeled "MSR".

Question: What does "MSR" represent in the context of this predicted conjunction? Is it related to the conjunction itself? If so, why is it shown so far away from the intersection of the two trajectories?

enter image description here

  • $\begingroup$ companion question: How did two satellites end up in almost the same orbit except moving in opposite directions? $\endgroup$
    – uhoh
    Commented Jan 29, 2020 at 3:30
  • 2
    $\begingroup$ It is amazing just how many overloads for that TLA exist and seem to be almost relevant. $\endgroup$ Commented Jan 29, 2020 at 10:38
  • $\begingroup$ @StarfishPrime what are TLAs and how does one overload them? $\endgroup$
    – uhoh
    Commented Jan 29, 2020 at 10:48
  • 5
    $\begingroup$ TLA: Three Letter Acronym. It is overloaded by having multiple meanings... medium spatial resolution imaging, microwave scanning radiometer, multi-sensor re-analysis, major satellite repeat (a biology term, confusingly), the list goes on. $\endgroup$ Commented Jan 29, 2020 at 10:52

1 Answer 1


MSR is one of LeoLab's radar stations (LeoLabs being the people who tweeted about the potential collision, a company whose busines is monitoring satellites). It lives in Midland, Texas, and as such is called the Midland Space Radar. I'm not sure what the contours show, but they're presumably related to the region that the radar can observe.

  • $\begingroup$ That's interesting, as it appears both the satellites are outside of that observation region? $\endgroup$
    – Skyler
    Commented Jan 29, 2020 at 14:47
  • $\begingroup$ @Skyler I'm not familiar with the orbits in question, but I'm assuming that at some point in the not too distance past they did pass through the radar's field of vision. LeoLabs also has some other radar stations elsewhere in the world which might have also spotted them, but are far enough away from the collision that they're not shown on the map. $\endgroup$ Commented Jan 29, 2020 at 14:57
  • $\begingroup$ Or maybe they intend to use the radar station to track part of the debris from a possible collision? $\endgroup$
    – Emil Bode
    Commented Jan 29, 2020 at 17:25
  • $\begingroup$ Oh that is so cool! i.sstatic.net/ILwjW.jpg The beam is narrow in the north-south direction because the phased array is long in that direction (note direction of sunset (sunrise?) to the right). The beam points nominally towards the zenith (the reflector looks like the left half of a parabola) but for some reason still spreads out along the east-west direction. $\endgroup$
    – uhoh
    Commented Jan 30, 2020 at 0:48
  • 1
    $\begingroup$ @uhoh if my google maps stalking skills serve me correctly (see here, taken from the position marked as MSR on the LeoLabs page) the array is in fact long east-west, which explains how it can look in both those directions despite having a reflector that's fixed to point in one direction. I could be wrong, of course. For some reason, google hasn't yet done street view out there to confirm what's on the ground. Laziness, I reckon. $\endgroup$ Commented Jan 30, 2020 at 10:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.