11
$\begingroup$

I don't know of any space telescope that has been placed in geosynchronous orbit among the communication satellites. I wonder why not?

In GEO, a space telescope could use a single stationary radio dish on Earth for its data transfer. GEO is about 100 times further from Earth than LEO where for example Hubble is, so heating from Earth's radiational reflections should be much less of a problem if it has instruments that go a bit further into infrared. With lower angular orbital velocity It wouldn't need to turn as fast to stay focused, maybe increasing the lifetime of its reaction wheels. Also, the threat of collisions with space debris would be less. I struggle to find the reason why GEO hasn't been used for space science missions.

$\endgroup$
  • 22
    $\begingroup$ Hubble was built to be serviced by the shuttle, you can not do that in GEO. $\endgroup$ – Polygnome May 23 at 14:38
  • 3
    $\begingroup$ Geostationary orbits are primarily relevant for devices which needs to be easily and cheaply accessible to lots of end users, i.e. satelite dish owners. $\endgroup$ – Thorbjørn Ravn Andersen May 24 at 7:00
21
$\begingroup$

It wouldn't need to turn as fast to stay focused, maybe increasing the lifetime of its reaction wheels.

On Earth when you "turn" a telescope, you are really keeping it pointed in one direction! It's the Earth that's turning, and you have to turn the telescope mount to keep the legs pointed at the ground.

It's the same thing as having to move the antenna to keep it pointed at the ground!

As an aside, how do space telescopes handle pointing antennas at earth?

GAIA uses a phased array that can always beam towards earth as the telescope rotates (see this answer), and TESS stores, compresses and processes almost two weeks of data while measuring near apoapsis, then zooms past Earth and sends it in an 8 hour burst during periapsis. (See this answer and this answer).

In space you don't turn a telescope, you leave it mostly alone and it nearly keeps pointing in one direction. There are small tidal forces especially in LEO (see this answer), and other torques like solar pressure (see this answer) that will very slowly tilt a telescope, so the reaction wheels have to handle that.

But there is no major space-telescope-turning necessary to compensate for Earth's rotation. That's strictly an "Earth thing".

Why aren't space telescopes put in GEO?

It's a crowded place, and there's a lot of "space citizenship" necessary to stay there. You have to worry very much about station-keeping in order not to drift near any of your ultra-expensive hardware neighbors, and that can interfere with science scheduling.

A communications satellite can do stationkeeping at the same time it serves its primary function because its antennas don't need to be pointed with arc-second stability like an optical telescope would.

It took a lot of work to boost Hubble up to 540 kilometers above much of the atmosphere so that it wouldn't have to do station-keeping burns. Putting it in GEO could actually force it to do more station keeping, exactly opposite of what you want.


The chance that a future big-science optical or radio space telescope for Astronomy will be put in LEO or GEO as the best place for it is very small. These days spacecraft reliability is very high, on-board computing and image processing can sort and pre-process data somewhat, and X-band links even in deep space can do data-dumps during short periods of time. See this answer and this answer.

While MEO could avoid station-keeping that would be necessary in LEO or GEO, I think most telescopes will be much farther from Earth's reflected light and radiated heat, either like TESS which spends most of it's time almost as far from Earth as the Moon, or JWST out near Sun-Earth L2.

The moon is another option. Without any atmosphere so a UV telescope (like those on the Chang'e landers) or IR telescope could work there without needing to be attached to a spacecraft. A radio telescope on the far side would also be shielded from artificial electromagnetic radiation from the Earth, as well as light and heat from the Sun during the two week long lunar night.

There are so many places to put future telescopes, I don't think we'll see any major ones in LEO or GEO. The only reason for GEO would be a low-budget, like a cubesat or nanosatellite project where one uses a low-cost multi-satellite release (see this answer or this question or this question (not everyone can see it, you can also get an idea by looking at this question).

$\endgroup$
  • 1
    $\begingroup$ re: the first point: the OP is talking about the ground dish being stationary, not the one attached to satellite. This is of course pretty trivial when only a few ground dishes will be pointed at it, not millions of fixed dishes on private homes, as Thorbjørn commented. But +1 for the other points, was hoping someone had already addressed the OP's confusion about turning. And the rest was some interesting reasons I didn't know about. $\endgroup$ – Peter Cordes May 24 at 15:57
  • $\begingroup$ @PeterCordes Oh! Yes it seems I totally missed that! Okay I'll edit accordingly. Thanks! $\endgroup$ – uhoh May 24 at 16:02
  • $\begingroup$ Great answer! Things are sometimes up side down in space, compared with simple intuition here from the paradise garden. $\endgroup$ – LocalFluff May 24 at 20:03
  • $\begingroup$ @LocalFluff: if you want a earth-side model analogy of orbital angular momentum vs. local angular momentum: imagine a carousel ride. But instead of fixed seats, imagine the seats or horses are on frictionless pivots. Moving on a circular path is totally independent of which direction you're facing while doing it. Maybe your "simple intuition" is coming from an analogy of twirling something on a string? That would be like a tidally-locked object like the moon, where the force pulling it in a curved path does affect its facing. $\endgroup$ – Peter Cordes May 24 at 20:25
  • $\begingroup$ Thanks to the anonymous user for the edit. I kept the major fix (Earth → Sun) but I could not understand the addend parenthetical statement so I left it out. Please feel free to leave a comment and elaborate! If you don't have sufficient reputation to do so, then try again and if possible find a way to explain that more clearly, perhaps by expanding on it a bit. Thanks! $\endgroup$ – uhoh Sep 2 at 22:20
29
$\begingroup$

Spacecraft are placed into the orbits that they need to be in, given the objectives of the mission and the constraints during design. Nothing in space is arbitrary, since there is so much at stake if something goes wrong.

In fact, GEO is not a particularly special place for a space telescope and several telescopes are placed (or are planned on being placed) well away from there. For example, (if it's ever launched...) the James Webb Space Telescope will be moved to the Earth-Sun Lagrange point L2, for exactly the reason you stated - it needs to be away from Earth's IR radiation.

As Polygnome stated, the Hubble was built to be serviced by the shuttle, which limited where it could be placed. However, its orbit did not prevent it from doing exceptional astronomy.

There are many better places to put telescopes than LEO and GEO and there are arguments for each place. A telescope on the far side of the moon would be shielded from Earth's radio transmissions, so it makes a lot of sense to put one there for radio astronomy. However, doing so would be a huge technical challenge, so we haven't done so yet.

Some missions are launched on strange orbits that at first make little sense. For instance, Spektr-R was launched on a highly elliptical Earth orbit. Such an orbit allowed it to be used in conjunction with ground-based telescopes to increase resolution using a technique called Very Long baseline Interferometry, which gets better the further apart your telescopes are. Spektr-R would have been less effective if it were placed in GEO, since its orbit took it much further out than GEO and enabled it to produce better resolution images.

So to answer the question in TL;DR mode: GEO is not particularly special, and most space telescopes (if not all) are placed into the orbit that allows them to achieve their mission objectives.

$\endgroup$
  • 9
    $\begingroup$ Re: the James Webb: "At least the slope is less than one" $\endgroup$ – Michael Seifert May 23 at 14:54
  • $\begingroup$ Re. Earth-Moon L2 for observation, we haven't placed any 'classical' space telescopes there, but we have put satellites with observation capabilities there. $\endgroup$ – Jack May 24 at 10:52
  • $\begingroup$ I feel this answer would be better if it more clearly explained why certain features of different orbit are beneficial/detrimental for telescopes. The question specifically asks why GEO is not preferred over other possible orbits and list some potential benefits (but, as uhoh points out in their answer, the antenna would still have to move etc). $\endgroup$ – glaux May 24 at 12:42
  • $\begingroup$ @glaux That isn't entirely possible because it depends on the mission. The crux of my answer was that the mission is placed in the orbit that it needs to be in so that mission goals can be fulfilled. The example of Spektr-R implies that orbit design is highly dependent on the science goals and instrument specifications, so we couldn't really have a meaningful answer about different types of orbits. $\endgroup$ – Michael Stachowsky May 25 at 13:29
10
$\begingroup$

At least one space telescope has been put into a geosynchronous orbit (but not geostationary) due to its unique mission:

The Solar Dynamics Observatory (SDO) (Wikipedia)(Official mission site).

One of its three sensor packages, the Atmospheric Imaging Assembly (AIA), creates 4096x4096 images of the sun across 12 wavelengths, at a cadence of every 12 seconds (or 1 image per second, taking 12 seconds to revisit a specific wavelength). This produces a very high volume of data, so rather than storing images until it can connect to a DSN ground station or use communications satellites in geostationary orbit to relay the data, it has its own dedicated ground station in New Mexico.

Because its mission is to provide constant coverage of the sun, it can not be in a pure geostationary orbit, where the Earth would occlude it twice each day. Rather, by having an inclined orbit, it has two seasons each year where the Earth occludes the sun for up to 90 minutes at a time twice per day, and the moon can occlude the sun as well. (Rarely, they can both occlude the sun at the same time.)

Also, because of its high data rate, it can't be placed further from the Earth like other heliophysics space observatories such as SOHO, the STEREO A and B probes, and the Parker Solar Probe... both because it would completely saturate at least one DSN ground station for the entire time it's in view of that station, and because greater distances require using lower frequencies, and lower frequencies have less bandwidth, resulting in a data rate that's too low for the image cadence that SDO creates. While SOHO's mission is to provide real time coronagraphs, its image cadence and resolution are much lower, so only occasionally transmits to DSN stations, and both the STEREO probes and Parker Solar Probe are designed to store images for extended periods of time for offline processing, so can wait for ideal times to connect to a DSN station. Data from the solar observatories that are outside of Earth's orbit are further relayed through terrestrial networks after being received by a DSN station, which adds yet another delay in processing that SDO's unique orbit among space-based observatories is designed to avoid.

$\endgroup$
1
$\begingroup$

It's a late answer, but to complement @Ghedipunk's :

The last GOES geostationary satellites have solar telescopes (EXIS and SUVI). (See wikipedia GOES-16)

Geosynchronous orbit was also considered for future WFIRST space telescope because of its planned high data rate. But finally Sun-Earth L2 was chosen instead because of lower radiation, stable temperature and no light pollution from Earth and Moon.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.