On Oct 7, 2020, according to the latest predictions, the Tesla Roadster/ Starman will pass within 0.05 AU (7.4 million km). Could HiRISE image it with any success, and as anything but a dot?
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asked Feb 11, 2018 at 23:01
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$\begingroup$ If you ask could it be resolved - it will not. HiRISE has aperture 0.5 meter and the angular size of the target at distance 7.4 million kilometers will be far far beyond diffractional limit. en.wikipedia.org/wiki/… $\endgroup$– HeoppsJul 5, 2018 at 7:54
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Great question!
Using JPL's Horizons in Observer mode one can see their predictions for apparent magnitude as seen from Earth, including locations on Earth that contain the word "Mars" but unfortunately not from Mars the planet. Still the information is instructive.
I've started the math for calculating apparent magnitude in this answer but have not included the dependence on the angle of illumination. It wouldn't be the same for a slowly rotating long cylinder as it would for a sphere anyway.
At 7 million km from Earth, Roadster is predicted to have an apparent magnitude of about +22. This will have a few magnitudes uncertainty, as well as variability as the oblong rocket-body rotates (this variability is already observed).
There are also variations of a few magnitudes due to the phase angle of illumination.
From Mars, add +1 for the increased distance to the Sun also.
HiRise is designed to see the daytime side of Mars. Right now don't know about its performance in extremely low light levels or arc-second pointing stability for long exposures, but with a (huge!) 50 cm aperture there seems to be a possibility. Note that HiRise seems to be the winner for the question What's the largest aperture telescope sent beyond the earth-moon system?
above: From here. Click for full size.
If HST's limiting magnitude is +31, then $ 2.5 \log_{10}\left(\frac{2.4^2}{0.5^2}\right)$ suggests a similarly equipped space-based telescope not looking through an atmosphere with a 0.5 meter aperture might have a limiting magnitude in the ball park of +27, so I don't think this can be ruled out yet.
