What are some reasons that space telescopes would be placed farther from some planets than from others?

Question

I need to argue this on why but I don't understand the question.

I have to argue why telescopes need to stay farther away from some planets but no others, I have tried searching this up but have not found anything.

So I'd like to ask for some reasons why space telescopes might need to be placed far from planets, and why it might need to be farther from some planets than from other planets.

So far every telescope we have built either is on earth, in earth orbit or in heliocentric orbit. So they all stay away from other planets. — Polygnome, Dec 01 '19 at 23:51
Could be a little more specific please? Which planets? Which telescopes? Why? There are way too many possibilities to give a good answer now. — Bob Jacobsen, Dec 01 '19 at 23:53
well what I was told to is "student argues why the telescope needs to stay farther away from some planets but not others" — user34116, Dec 01 '19 at 23:59
What is the context here? Missions to solar system planets? Direct imaging of exoplanets? Without that it's impossible to answer this question. — AtmosphericPrisonEscape, Dec 02 '19 at 01:07
@uhoh: At the moment it is unclear that OP is asking what you think. You might have changed the question's context. — AtmosphericPrisonEscape, Dec 02 '19 at 01:21
I've edited and slightly modified your question to better fit the Stack Exchange style. It's a good question! I don't have time to write an answer, but I can think of at least three reasons. 1) You can be closer to a small planet than a larger planet if the planet must block no more than 10% of the sky. 2) If you are using cooled optics and image sensors to look for near-earth objects using thermal infrared light or just looking at infrared, you might want to be farther from a hot planet than a cold planet... — uhoh, Dec 02 '19 at 01:21
@AtmosphericPrisonEscape "unclear" is not an objective concept. It's quite clear to me. — uhoh, Dec 02 '19 at 01:22
@uhoh it is unclear whether the OP is referring to 'placed' or 'looking at'. Given that we generally don't place telescopes anywhere in the solar system, while we do look at places everywhere in the solar system, I'm inclined to think the latter. In any case, it sounds like the assignment given by the OP's teacher is poorly worded and the OP should try and ask some clarifying questions to their teacher. — Ingolifs, Dec 02 '19 at 01:32
that might be good advice in https://interpersonal.stackexchange.com/ but I don't think we should be driving student-teacher discourse here. If asking was easy/comfortable/possible mightn't the OP have done so already? I'm not comfortable second-guessing the situation. The edit is pretty mild. — uhoh, Dec 02 '19 at 02:36
@uhoh: That's good for you. It is very unclear to me, because your interpretation doesn't make sense, as other comments have pointed out. Not objective indeed, maybe you should refrain from policing this website as if it were your own. — AtmosphericPrisonEscape, Dec 02 '19 at 03:01
@uhoh It's clear from the OP's other post that they have been given an assignment of some form, possibly in another language, whose wording they don't understand and they are using stackexchange to try to clarify the meaning of the question rather than asking the teacher. Until the asker clarifies the question we can't answer it satisfactorily, and as it stands this question I don't think can be edited into an interesting general Q in the same manner as Muze's sometimes could be. — Ingolifs, Dec 02 '19 at 03:20
I've helped the best way I know how. I understand asking for clarifications can also be helpful, but I don't think that A should suggest B not to help until A's questions have been addressed to their satisfaction. — uhoh, Dec 02 '19 at 03:52
@AtmosphericPrisonEscape you are welcome to expand on that in Do I police this site as if it were my own? — uhoh, Dec 02 '19 at 05:04
Do "telescopes" in this question include cameras of space probes, like HiRIZE on martian MRO orbiter? For them it's quality vs quantity trade. — Heopps, Dec 02 '19 at 08:47
I've voted to close as unclear as well and have retracted it again, even though I really do no understand what this question is about. However, if someone did understand what OP wants and has a useful answer, it might as well stay open, IMHO. — DarkDust, Dec 02 '19 at 09:30

uhoh · Answer 1 · 2019-12-02T05:13:11.470

Space telescope: In orbit around a planet but looking at other objects in space

I can think of at least three reasons:

You can be closer to a small planet than a larger planet if the planet must block no more than X% of the sky. This answer gives the equation of the solid angle that a spherical body would block. $OP = R + h$ where $R$ is the radius of the planet and $h$ is the height of satellite above the planet's surface. If the planet is twice as large and you make $h$ twice as high, the solid angle blocked will be the same.

$$\Omega = 2\pi \left( 1-\frac{ \sqrt{2rh + h^2}}{R + h} \right)$$

If you are using cooled optics and image sensors to look for objects like asteroids or comets using thermal infrared light or just looking at infrared like the JWST, you might want to be farther from a hot planet than a cold planet to decrease the thermal loading from the planet's infrared radiation.
If your planet is very close to the Sun then reflected glare from the planet (e.g. Earthshine) will be more of a problem than for a planet far from the Sun. Baffles on the telescope help somewhat and you can coat the inside of your telescope with Vantablack but getting farther from the source of the scattered light will also help

Planet-observing telescope: In orbit around a planet and looking down at it

You can see below that a satellite in a higher orbit will move a bit slower, and so the blurring of the planet's surface due to the satellite's motion will be reduced (neglecting the planet's own rotation which you might not be able to do) but at a given image scale (meters per pixel) you'll need to increase your focal length and aperture diameter to keep the f/no. and image brightness the same. The speed of a circular orbit around a planet is given by

$$v \approx \sqrt{\frac{GM}{r}}$$

where $G$ is the gravitational constant about 6.674E−11 m^3 /kg s^2, $M$ is the mass of the planet and $r$ is the radius or semimajor axis of the circular orbit. You can also look up the product $GM$ for the Sun, Moon and planets as their standard gravitational parameter.

What are some reasons that space telescopes would be placed farther from some planets than from others?

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