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As far as I understand it, one can not hide spacecraft, because the heat from the engine would be very visible.

Now I tried searching for the farthest distance an actual spacecraft has been detected besides its radio signals that are intentionally sent to us. Unfortunately I could not find any data on this.

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    $\begingroup$ The answer will not impress you. If the International Space Station were at the distance of the moon, it would barely be detectable by Hubble. quora.com/Could-Hubble-take-a-picture-of-voyager-1 $\endgroup$ – called2voyage Sep 19 '17 at 13:42
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    $\begingroup$ @called2voyage Thats fine. I wish to only believe true things and not believe false things. So if my current "knowing" is wrong, I love to learn. $\endgroup$ – Angelo Fuchs Sep 19 '17 at 14:26
  • $\begingroup$ Unfortunately, I don't know the actual answer, because there are so many possible targets for farthest spacecraft within the range that is observable. I just wanted to give you a heads up that such was the case. $\endgroup$ – called2voyage Sep 19 '17 at 14:28
  • $\begingroup$ Furthest I can think of are Lunar and Mars probes that have been imaged on the ground by orbiting satellites (usually just enough to see that there's something there, or its shadow, rather than any detail). Since all spacecraft do have radio, there's no incentive to look for them using anything else - unless you;re looking for dead probes. $\endgroup$ – JerryTheC Sep 19 '17 at 15:16
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    $\begingroup$ You typically will not see engine heat except when the engine is firing. You are more likely to see reflections of sunlight. $\endgroup$ – Rory Alsop Sep 19 '17 at 16:08
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OSIRIS-Rex has been spotted on its approach to Earth, at a distance of approximately 7 million miles (12 million kilometers) away, with a brightness of approximately 25th magnitude. The Large Binocular Telescope was used for this observation, this has a pair of 8.4-meter mirrors.

enter image description here

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I saw the Cassini spacecraft visually through a 20 inch (500 mm) telescope when it flew by the Earth in August 1999. At the time of my sighting, it was just over 72000 miles (116000 km) away. The complete description is on the Seesat-L archives.

I do not have the information, but I think the Apollo spacecraft were seen visually at a greater distance.

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  • $\begingroup$ I believe that telescopes that are searching for near Earth asteroids routinely image objects that are farther than the Moon. The Gaia spacecraft has been detected at 1.5 million kilometers away! (about 4 times the distance of the Moon). Look up the discovery of "asteroid" 2015 HP116. $\endgroup$ – JohnHoltz Sep 19 '17 at 16:49
  • $\begingroup$ That comment is a good candidate for farthest observation, and it is also quite surprising! $\endgroup$ – called2voyage Sep 19 '17 at 17:48
  • $\begingroup$ @JohnHoltz Today I found out that not only has Gaia been accidentally observed, it is systematically observed every day (see my answer for details) $\endgroup$ – Angelo Fuchs Sep 20 '17 at 15:42
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When it was 5700km from Earth, the Rosetta spacecraft was misidentified briefly as an asteroid as it flew by Earth for a gravity assist. I cannot find any more information on other spacecraft observed without radio from further away (I'm happy to be corrected!).

If landers count; as mentioned in the comments, spacecraft orbiting Mars have observed their lander brethren on the surface.

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  • $\begingroup$ Is the distance of detection between Mars orbiter and lander equal to or greater than 5700 km? I think not. Also, probably worth noting that 5700 km is a lot closer to the ISS than it is the Moon (i.e. it's not really that far). $\endgroup$ – called2voyage Sep 19 '17 at 16:12
  • $\begingroup$ ISS is very close. At least to surface of the earth. Probably around 350 km. $\endgroup$ – jmh Sep 19 '17 at 20:03
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The GBOT (Ground Based Optical Tracking) is systematically looking at the Gaia Probe at L2 of the Earch-Sun System at about 1.5 Mio KM to locate it precisely (+/- 150 meters).

It has been tested on the WMAP and Planck Space probes around L2 before that.

Maybe the spotting of Rosetta was even farther, but I can't comprehend the protocol: http://www.minorplanetcenter.net/iau/mpec/K07/K07V69.html

Thanks uhoh for figuring out the distance:

At the first reported observation at 2007-11-07 07:37 UTC the distance between Rosetta and Earth was about 5.35 million km according to JPL Horizons. And the flyby was on the 13th, with a closest approach of about 11,700 km from the center of the Earth (1.84 Earth radii) so it all checks out! Graph

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  • $\begingroup$ This is really fascinating! I had no idea spacecraft could be seen so far away, but if you have a 1-2 meter telescope, time to do a sufficient exposure, and you know just where to look, and since it's placed almost exactly opposite the Sun all the time, then it's certainly possible assuming the spacecraft has some significant reflected light in the right direction. I was going to look up Rosetta's position; is "2007 11 07.31748" in your link the same as "JD 2454411.81748"? $\endgroup$ – uhoh Sep 23 '17 at 14:19
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    $\begingroup$ Assuming that's correct, at the first reported observation at 2007-11-07 07:37 UTC the distance between Rosetta and Earth was about 5.35 million km according to JPL Horizons. And the flyby was on the 13th, with a closest approach of about 11,700 km from the center of the Earth (1.84 Earth radii) so it all checks out! i.stack.imgur.com/4mIkr.png $\endgroup$ – uhoh Sep 23 '17 at 14:54
  • $\begingroup$ @uhoh I have no idea what the data in the protocol means. $\endgroup$ – Angelo Fuchs Sep 23 '17 at 16:50
  • $\begingroup$ JD = Julian Day, the fractions are fractional days (so 0.31748 of a day is 9h, 19m, 9s) $\endgroup$ – Hobbes Sep 23 '17 at 19:57
  • $\begingroup$ @Hobbes As far as "2007 11 07.31748" is concerned, I still get 07:37:10.3, and it's JD 2454411. 81748 because JD is offset by (exactly or almost exactly) 0.5 days from UTC. $\endgroup$ – uhoh Sep 23 '17 at 20:54
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This is unfortunately a vague question. If, for example, the spacecraft were reasonably large, you could detect it indirectly due to its gravitational pull on other (putative) nearby objects, or even gravitational lensing.

So far as heat-sensing or not, the spacecraft would have to be very cold indeed to not radiate in the microwave range at levels inconsistent with the cosmic background. How sensitive a radiotelescope would be needed is a separate issue.

And finally, if you had a really powerful lidar you'd easily "ping" the spacecraft -- perhaps vaporizing it in the process.

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    $\begingroup$ The question is intentionally formulated as "Whats the farthest it has ever worked" not "Whats the farthest it could theoretically work" to avoid any such vagueness. $\endgroup$ – Angelo Fuchs Sep 20 '17 at 15:22
  • $\begingroup$ There's absolute no chance anyone could detect the gravitational pull of spacecraft. They're so tiny it'd be all but negligible based on current measuring abilities. Unless by "reasonably large" you mean, the size of a small moon. The same holds true for gravitational lensing. Just not gonna happen. $\endgroup$ – zephyr Sep 21 '17 at 15:40
  • $\begingroup$ @zephyr have you already forgotten "....that's no moon..." ? $\endgroup$ – Carl Witthoft Sep 21 '17 at 17:20
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    $\begingroup$ But the question isn't about some theoretical galaxy far far away. It's asking about actual, current detections of currently existing spacecraft. Your answer my apply in theory, but it is not the correct answer for this question as it was stated. $\endgroup$ – zephyr Sep 21 '17 at 17:32

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