Which of the optical sensors in operation on the three spacecraft currently en route to Mars (China's Tianwen-1, UAE's Emirates Mars Mission (Hope), USA's Mars 2020 (Perseverance)) could image one of the other spacecraft, with resolution more than one pixel, assuming a correctly aimed sensor or spacecraft?

I couldn't find technical details of Tianwen-1's (orbiter's?) optical navigation sensor.

The Hope orbiter's EXI imager has an angular resolution of 22 arc seconds, but it might be turned off during cruise.

The Perseverance rover is inside an aeroshell. Photos from Lockheed Martin show that it has at least one small circular window. I couldn't find details of optical instruments outside Perseverance, nor whether an interior instrument is aimed at such a window.

So it might come down to Tianwen-1's sensor's angular resolution and how densely packed this convoy ever becomes.

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    $\begingroup$ It seems unlikely that you'll be able to get resolution of more than one pixel. Just for a comparison, the aeroshell of Perseverance is 4.5 m in diameter. For such an object to subtend more than 22 arcsec in Hope's field of view, it would have to be less than 42 km away. I don't know for certain whether they'll get that close, but I strongly suspect that they won't. $\endgroup$ Commented Aug 6, 2020 at 20:19
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    $\begingroup$ What's more, if you take into account the diffraction limit, a mirror of diameter $d_a$ can only see an object of size $d_b$ using light of wavelength $\lambda$ if it is closer than (approximately) $d_a d_b/\lambda$. For objects with $d \approx 5$ m and visible light, this works out to be something like 50,000 km, or about 8 Earth radii. If they never get any closer than that, there's no real hope. $\endgroup$ Commented Aug 6, 2020 at 20:26
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    $\begingroup$ Mars2020 is en route with about 39,600 kph, that is 11 km/s. In 24 hours that is 950,400 km. The spacecrafts launched some days sooner or later are millions of km apart. $\endgroup$
    – Uwe
    Commented Aug 6, 2020 at 20:43
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    $\begingroup$ It may be necessary to first have an answer to Will the Emirate Mars Mission, Tianwen-1 and Mars 2020 Mission get closer to each other or spread out on their way to Mars? before this can be answered. However, you might consider also asking in a new question if they can see each other as star-like objects without necessarily resolving each other. I think there is certainly a chance, but that calculation would not answer this. See also Have spacecraft photographed each other beyond Earth orbit? $\endgroup$
    – uhoh
    Commented Aug 6, 2020 at 22:56
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    $\begingroup$ The small circular window on Perseverance was used to install the MMRTG and is now closed. $\endgroup$
    – RAD6000
    Commented Aug 20, 2020 at 10:14

1 Answer 1


(If one of the commenters posts this as an answer, I'll delete my answer and accept theirs. They did the hard work.)

No, none of these spacecraft could image one of the others, for two reasons.

This answer based on trajectories in Horizons shows that the closest approach of any pair is (was) about 1 million km between Tianwen-1 and Hope, circa 2020 July 20.

That's 20 times farther than the diffraction limit for objects of that size, from Michael Seifert's comment's calculation.

Even if one of the spacecraft brandished a telescope as good as one on an observatory like Gaia or Chandra or Hubble, with roughly 0.1 arc second resolution for subjects this faint, from Michael Siefert's comment that 42 km is the greatest distance for subtending more than 1 pixel at 22 arc second resolution, the greatest distance would be 42 * 22 / 0.1 = 9240 km. The closest approach is 100 times farther.


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