IRCB S73-7 Satellite Found After Going Untracked For 25 Years

When the United States launched the KH-9 Hexagon spy satellite into orbit atop a Titan IIID rocket in 1974, it brought a calibration target along for the ride: the Infra-Red Calibration Balloon (IRCB) S73-7. This 66 cm (26 inch) diameter inflatable satellite was ejected by the KH-9, but failed to inflate into its intended configuration and became yet another piece of space junk. Initially it was being tracked in the 1970s, but vanished until briefly reappearing in the 1990s. Now it’s popped up again, twenty-five years later.

As noted by [Jonathan McDowell] who tripped over S73-7 in recent debris tracking data, it’s quite possible that it had been tracked before, but hidden in the noise as it is not an easy target to track. Since it’s not a big metallic object with a large radar cross-section, it’s among the more difficult signals to reliably pick out of the noise. As can be seen in [Jonathan]’s debris tracking table, this is hardly a unique situation, with many lost (XO) entries. This always raises the exciting question of whether a piece of debris has had its orbit decayed to where it burned up, ended up colliding with other debris/working satellite or simply has gone dark.

For now we know where S73-7 is, and as long as its orbit remains stable we can predict where it’ll be, but it highlights the difficulty of keeping track of the around 20,000 objects in Earth orbit, with disastrous consequences if we get it wrong.

26 thoughts on “IRCB S73-7 Satellite Found After Going Untracked For 25 Years

    1. It had to fit on the top of a rocket stack. The satellite was effectively the “nose” of the rocket during launch, and then separated.
      At least in the 70s if you didn’t fit a proper sized nose cone onto your rocket, it wasn’t going to function as a rocket.

    2. Big cameras, lots of film. Across all 19 HEXAGONs, they achieved a ground coverage of 2.3 billion square kilometres. If evenly distributed, they would have photographed the entire surface of the Earth nearly 5 times over.

      KH-9 wasn’t nicknamed “Big Bird” for nothing!

    1. Not sure. These were analog film cameras, I suppose?
      If so, they’d rather “send” physical films back, with a parachute or something.
      I mean, technically it was possible to record on magnetic tape and send picture data back, but..
      The film cameras had better resolution in these days, I guess.

      1. Yes, these early keyhole satellites (the KH in KH-9) had film cameras. The rolls of film would feed past the camera in into one of the 4 recovery vehicles. Those would be jettisoned and drop until a parachute deployed. An aircraft would then snatch them form the air by snagging the lines. (At least the earlier versions of these satellites worked that way.) The 1974 flight was at the very tail end of the program, which was replaced by electronic systems that transmitted their data back down.

    1. The balloon is the satellite that was untracked all this time. The satellite with the cameras reentered Earth’s atmosphere in 1975. These satellites orbited only about 100 miles altitude, so they didn’t have a very long orbital life.

  1. The big satellite (“Big Bird”) was not the one that’s hard to track, though today it’s pretty hard to believe a 60 foot satellite was sent up for 4 months of service.

    Declassified in 2011

    > According to documents released by the NRO, each HEXAGON satellite mission lasted about 124 days, with the satellite launching four film return capsules that could send its photos back to Earth. An aircraft would catch the return capsules in mid-air by snagging their parachutes following the canisters’ re-entry.

  2. The headline & banner image do kind of bury the lead that this is in fact a small calibration balloon not the actual thing in the picture.

    Although the whole Hexagon thing is so bonkers it’s always worth a re-visit.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.