<p>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 <a href="http://www.satobs.org/seesat/Aug-1999/0217.html" rel="noreferrer">Seesat-L archives</a>.</p>

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

CC BY-SA 3.0
2017-09-19T16:49:07.810Z
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.
JohnHoltz
CC BY-SA 3.0
2017-09-19T17:48:08.433Z
That comment is a good candidate for farthest observation, and it is also quite surprising!
called2voyage
CC BY-SA 3.0
2017-09-20T15:42:43.847Z
@JohnHoltz Today I found out that not only has Gaia been accidentally observed, it is systematically observed every day (see my answer for details)
Angelo Fuchs
CC BY-SA 3.0
2017-09-19T16:26:36.540Z
2017-09-19T16:26:36.540Z
JohnHoltz
<p>The <a href="http://gbot.obspm.fr/" rel="nofollow noreferrer">GBOT</a> (Ground Based Optical Tracking) is systematically looking at the Gaia Probe at L2 of the Earch-Sun System at about <strong>1.5 Mio KM</strong> to locate it precisely (+/- 150 meters).</p>

<p>It has been tested on the <a href="http://gbot.obspm.fr/gbotpipeline/spie9149-25.pdf" rel="nofollow noreferrer">WMAP and Planck</a> Space probes around L2 before that.</p>

<p>Maybe the spotting of Rosetta was even farther, but I can't comprehend the protocol: <a href="http://www.minorplanetcenter.net/iau/mpec/K07/K07V69.html" rel="nofollow noreferrer">http://www.minorplanetcenter.net/iau/mpec/K07/K07V69.html</a></p>

<p>Thanks <a href="https://astronomy.stackexchange.com/users/7982/uhoh">uhoh</a> for figuring out the distance:</p>

<p>At the first reported observation at 2007-11-07 07:37 UTC the distance between Rosetta and Earth was about <strong>5.35 million km</strong> according to <a href="https://ssd.jpl.nasa.gov/horizons.cgi" rel="nofollow noreferrer">JPL Horizons</a>. 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! <a href="https://i.sstatic.net/4mIkr.png" rel="nofollow noreferrer">Graph</a> </p>

CC BY-SA 3.0
2017-09-23T14:19:06.693Z
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"?
uhoh
CC BY-SA 3.0
2017-09-23T14:54:26.740Z
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](https://ssd.jpl.nasa.gov/horizons.cgi). 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! https://i.sstatic.net/4mIkr.png
uhoh
CC BY-SA 3.0
2017-09-23T16:50:56.447Z
@uhoh I have no idea what the data in the protocol means.
Angelo Fuchs
CC BY-SA 3.0
2017-09-23T19:57:26.893Z
JD = Julian Day, the fractions are [fractional days](https://en.wikipedia.org/wiki/Decimal_time#Fractional_days) (so 0.31748 of a day is 9h, 19m, 9s)
Hobbes
CC BY-SA 3.0
2017-09-23T20:54:29.300Z
@Hobbes As far as "2007 11 07.31748" is concerned, I still get 07:37:10.3, and it's JD 2454411. **8**1748 because JD is offset by (exactly or almost exactly) 0.5 days from UTC.
uhoh
CC BY-SA 3.0
2017-09-20T15:41:38.107Z
2017-09-23T16:51:28.520Z
2017-09-23T16:51:28.520Z
Angelo Fuchs
<p>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.    </p>

<p>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.  </p>

<p>And finally, if you had a <a href="https://what-if.xkcd.com/141/" rel="nofollow noreferrer">really powerful lidar</a> you'd easily "ping" the spacecraft -- perhaps vaporizing it in the process. </p>

CC BY-SA 3.0
2017-09-20T15:22:39.710Z
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.
Angelo Fuchs
CC BY-SA 3.0
2017-09-21T15:40:23.210Z
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.
zephyr
CC BY-SA 3.0
2017-09-21T17:20:22.243Z
@zephyr  have you already forgotten  "....that's no moon..." ?
Carl Witthoft
CC BY-SA 3.0
2017-09-21T17:32:47.620Z
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.
zephyr
CC BY-SA 3.0
2017-09-20T13:20:59.270Z
2017-09-20T13:20:59.270Z
Carl Witthoft
<p><a href="https://www.nasa.gov/image-feature/goddard/2017/large-binocular-telescope-snags-first-glimpse-of-osiris-rex" rel="noreferrer">OSIRIS-Rex has been spotted</a> 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.</p>

<p><a href="https://i.sstatic.net/cv1Nc.jpg" rel="noreferrer"><img src="https://i.sstatic.net/cv1Nc.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2017-09-22T16:06:57.690Z
2017-09-23T19:51:33.023Z
2017-09-23T19:51:33.023Z
Hobbes
<p>As far as I understand it, one can not hide spacecraft, because the heat from the engine would be very visible.</p>

<p>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.</p>

CC BY-SA 3.0
2017-09-19T13:42:24.157Z
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. https://www.quora.com/Could-Hubble-take-a-picture-of-voyager-1
called2voyage
CC BY-SA 3.0
2017-09-19T14:26:18.790Z
@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.
Angelo Fuchs
CC BY-SA 3.0
2017-09-19T14:28:47.930Z
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.
called2voyage
CC BY-SA 3.0
2017-09-19T15:16:35.380Z
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.
JerryTheC
CC BY-SA 3.0
2017-09-19T16:08:00.347Z
You typically will not see engine heat except when the engine is firing. You are more likely to see reflections of sunlight.
Rory Alsop
CC BY-SA 3.0
2017-09-20T11:34:52.193Z
@RoryAlsop The question aims more in the direction: How far away could you see an engine firing or similar direct observation of a spacecraft?
Angelo Fuchs
CC BY-SA 3.0
2017-09-20T11:36:12.747Z
Angelo - if the engine is pointed away from Earth you aren't going to see it. Towards - and it will depend how bright.
Rory Alsop
CC BY-SA 3.0
2017-09-22T11:27:08.800Z
[related question on Space SE](https://space.stackexchange.com/questions/3585/is-there-any-way-to-find-and-track-pioneer-10-11-and-voyager-1-2/3586#3586). RTG-powered spacecraft could be detectable in the IR spectrum at long distances, but I don't know how well a telescope can detect objects smaller than its angular resolution.
Hobbes
CC BY-SA 3.0
2017-09-23T07:58:00.863Z
@Hobbes They could, but where they?
Angelo Fuchs
CC BY-SA 4.0
2021-04-25T22:18:45.330Z
How far was SpaceX's Starman (the tesla on the F9 second stage), when it was last sighted a full 44 days after launch?
PcMan
<distances><space-probe>
Whats the farthest distance a spacecraft has been detected (except by radio)
Angelo Fuchs
# Whats the farthest distance a spacecraft has been detected (except by radio)

(Asked by: username_0 on 2017-09-19)

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.

**Tags:** distances, space-probe
> **Comments:**
> **username_1**: 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. https://www.quora.com/Could-Hubble-take-a-picture-of-voyager-1  
> **username_0**: @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.  
> **username_1**: 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.  
> **username_2**: 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.  
> **username_3**: You typically will not see engine heat except when the engine is firing. You are more likely to see reflections of sunlight.  
> **username_0**: @RoryAlsop The question aims more in the direction: How far away could you see an engine firing or similar direct observation of a spacecraft?  
> **username_3**: Angelo - if the engine is pointed away from Earth you aren't going to see it. Towards - and it will depend how bright.  
> **username_4**: [related question on Space SE](https://space.stackexchange.com/questions/3585/is-there-any-way-to-find-and-track-pioneer-10-11-and-voyager-1-2/3586#3586). RTG-powered spacecraft could be detectable in the IR spectrum at long distances, but I don't know how well a telescope can detect objects smaller than its angular resolution.  
> **username_0**: @Hobbes They could, but where they?  
> **username_5**: How far was SpaceX's Starman (the tesla on the F9 second stage), when it was last sighted a full 44 days after launch?  

---

## 4 answers

### Answer by username_4 on 2017-09-22. Score: 12 | Accepted answer

[OSIRIS-Rex has been spotted](https://www.nasa.gov/image-feature/goddard/2017/large-binocular-telescope-snags-first-glimpse-of-osiris-rex) 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](https://i.sstatic.net/cv1Nc.jpg)](https://i.sstatic.net/cv1Nc.jpg)


---

### Answer by username_6 on 2017-09-19. Score: 9

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](http://www.satobs.org/seesat/Aug-1999/0217.html).

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

> **Comments:**
> **username_6**: 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.  
> **username_1**: That comment is a good candidate for farthest observation, and it is also quite surprising!  
> **username_0**: @JohnHoltz Today I found out that not only has Gaia been accidentally observed, it is systematically observed every day (see my answer for details)  

---

### Answer by username_0 on 2017-09-20. Score: 5

The [GBOT](http://gbot.obspm.fr/) (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](http://gbot.obspm.fr/gbotpipeline/spie9149-25.pdf) 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](https://astronomy.stackexchange.com/users/7982/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](https://ssd.jpl.nasa.gov/horizons.cgi). 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](https://i.sstatic.net/4mIkr.png)

> **Comments:**
> **username_7**: 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"?  
> **username_7**: 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](https://ssd.jpl.nasa.gov/horizons.cgi). 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! https://i.sstatic.net/4mIkr.png  
> **username_0**: @uhoh I have no idea what the data in the protocol means.  
> **username_4**: JD = Julian Day, the fractions are [fractional days](https://en.wikipedia.org/wiki/Decimal_time#Fractional_days) (so 0.31748 of a day is 9h, 19m, 9s)  
> **username_7**: @Hobbes As far as "2007 11 07.31748" is concerned, I still get 07:37:10.3, and it's JD 2454411. **8**1748 because JD is offset by (exactly or almost exactly) 0.5 days from UTC.  

---

### Answer by username_8 on 2017-09-20. Score: -3

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](https://what-if.xkcd.com/141/) you'd easily "ping" the spacecraft -- perhaps vaporizing it in the process.

> **Comments:**
> **username_0**: 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.  
> **username_9**: 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.  
> **username_8**: @zephyr  have you already forgotten  "....that's no moon..." ?  
> **username_9**: 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.  

---

