In order to better defend my position that trying to track Juno from Earth once it can no longer transmit would be Quixotic, I'd like to ask what the current record is for the farthest detection from earth (using radar, optical/IR, etc.) of an earth-launched spacecraft or other earth-launched object that was no longer able to actively transmit or rebroadcast (e.g. Doppler ranging).

The recent so-called 'WTF' object comes to mind, but there may be something farther, or otherwise more demonstrative of (at least non-military) capability.

The bold font phrase is meant to exclude detection of landers on the Moon or Mars from spacecraft orbiting those bodies. I'd also like to exclude lunar retro-reflector arrays from Apollo missions as well as LAGEOS because these can be monitored only because we already know exactly where they are supposed to be.


2 Answers 2


There are two potential candidates that I'm aware of. The first is J002E3, which is believed to be the Apollo 14 Saturn 5 upper stage. At the time of detection, it was orbiting Earth between 1-2 lunar distances away. Another candidate is 2007 VN84, also known as Rosetta, which was discovered via an optical survey while Rosetta was on a close approach path to Earth. Rosetta was transmitting at the time, but the detection was done without intentionally searching for it. According to Horizons, it was about 6.3 million km away at the time, about 16 times further than the Moon! And point of reference, this detection was first made on October 27th, according to the Minor Planet Center. On that date, it was twice as far away!

Bottom line, we can detect spacecraft sized objects at a distance perhaps equal to Earth's hill sphere, very large objects as far as maybe 16-20 lunar distances, but no further.

  • $\begingroup$ Super - Thanks! OK so it sounds like optical detection has some big advantages over radar. You get high resolution position measurements while measuring a (relatives) large area, instead of pointing a radar bam, and the sun is painting your target with a kilowatt of optical power per square meter, while a radar beam from earth might give you microwatts per square meter (at these distances). Both of these objects are actually in Horizons so we should be able to get those distances vs time. $\endgroup$
    – uhoh
    Commented Jul 7, 2016 at 14:09
  • 1
    $\begingroup$ By the way, I really really enjoy Emily Lakdawalla's Extensive Blog and Tweets. They make a nice historical record, including notable data, explanation, and of course dates, times, and names of things! $\endgroup$
    – uhoh
    Commented Jul 7, 2016 at 14:13
  • $\begingroup$ closer -> farther ​ ​ $\endgroup$
    – user2822
    Commented Jul 8, 2016 at 5:24
  • $\begingroup$ @RickyDemer radar bam -> radar beam :-) $\endgroup$
    – uhoh
    Commented Jul 19, 2016 at 3:02
  • $\begingroup$ Apollo 12 instead of 14, apparently. $\endgroup$ Commented Feb 8, 2018 at 22:17

I can name two other spacecraft, one of which was significantly further away than Rosetta:

  1. SOHO (The Solar and Heliospheric Observatory) near L1 (approx 1 million miles from earth).
    After a gyroscope failure, one of the instrument PIs took their Phase E funding and bought time at Arecibo to try to find the spacecraft. With additional help from Goldstone, they managed to find it, and successfully recovered the spacecraft.

  2. STEREO-Behind at approx. 1.95AU (16.214 light minutes).
    As the spacecraft was going to be passing behind the sun (relative to Earth) in 2014, alternate software was loaded onto the spacecraft to tell it to go into a slow roll. I believe that at least one of the gyros had already failed at that point (they were only using the laser gyros during maneuvers), and they suspect that the spacecraft went into a faster than planned roll. They managed to contact the spacecraft in August of 2016 and confirm its position, but were not able to recover it.

STEREO-Ahead went through similar maneuvers as STEREO-B, but they had learned their lesson, so they never formally "lost" it, although it was out of communications while it passed behind the sun. It was put into a safekeeping mode, and then recovered on the other side. Even though it was recovered at a greater angle of separation with the Earth, STEREO-A orbits the sun at slightly less than 1AU, so it wouldn't be by much.

(note: I used to work for the Solar Data Analysis Center & STEREO Science Center. I was not directly involved involved with either effort (SOHO was before I started there), and I'm working from (possibly faulty) memory of discussions around the office during the attempted recovery of STEREO-Behind.)

  • $\begingroup$ I'm not sure which dish managed to talk to STEREO-B, but I know that they were using Greenbank to try to find and recover it. $\endgroup$
    – Joe
    Commented Dec 10, 2020 at 1:06

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