China's telescope on the moon
newscientist.com
newscientist.com
-- Trivial pointing; no need for active pointing stabilization (aside from thermal drifts).
-- If you can do it on the earth-facing side of the moon, you can do it on the dark side of the moon, escaping stray light from the earth.
-- You can watch one target for a very long time. Earth-orbiting telescopes have to be very careful about keeping the earth out of the frame. Even geosynchronous systems will point near the earth (for some azimuths) once a day. This will come near the earth and sun once a month (each).
-- No need to maintain orbit. Can't hit anything, either.
-- It's really cool.
Edit: Here's the wiki page: https://en.wikipedia.org/wiki/Chang%27e_3
-- You can look at Earth trivially.
-- The UV imaging system is one of many instruments on the lander. Why not get a free ride to a great vantage point?
2. If you are on the dark side of the moon you can't transmit imaging back to earth for significant periods of time. Spacecraft computers need to be primitive as crap to deal with radiation and probably aren't packing gigabytes of storage outside the Van Allen belt. Also, I don't think stray light from Earth would conceivably be a problem except when the terminus is near the axis of the telescope.
3. Also true of satellites in geosynchronous lunar orbit.
4. I think this is more than outweighed by the environmental conditions.
5. Agreed. And on further consideration cost is probably a concern. eg China is sending a moon probe, would you rather have a telescope on the surface of the moon or none at all. A telescope in hand beats one two in the bush, and all.
http://freemars.org/l5/aboutl5.html
Just intuitively - think about the rockets you need to get a satellite level payload to geosynchronous orbit vs what it took to get space probes to lunar orbit (let alone exolunar or returning a craft to earth). Bear in mind that staging is an exponential problem, the more mass and the more total delta-V you need, the problem gets exponentially worse. Small satellites can be launched what amounts to a SRBM and a small kicker, to get even a small probe to lunar orbit you need an ICBM plus a pretty decent kicker to handle translunar injection and lunar orbiting. To get a spacecraft there you need, well, a Saturn-V plus a decent kicker plus a small rocket for everything else.
Geosynchronous satellites started in 1963 (Syncom 1) but the Lunar Orbiter program didn't start until 1965. That doesn't make sense if geosynchronous orbit was beyond lunar orbit. And if geosynchronous orbit was an equal distance, it would be trivial to put satellites into trailing orbits and cover the lunar blind spot. Lagrange points are a different (but very useful) orbital equilibrium. Particularly the Sun-Earth LaGrange points - these are indeed very far from Earth and have a very useful vantage point for all sorts of things. So far it's mostly been used for things like solar observatories.
> New Horizons recorded scientific instrument data to its solid-state memory buffer at each encounter, then transmitted the data to Earth. Data storage is done on two low-power solid-state recorders (one primary, one backup) holding up to 8 gigabytes each. Because of the extreme distance from Pluto and the Kuiper belt, only one buffer load at those encounters can be saved. This is because New Horizons will require approximately 16 months after it has left the vicinity of Pluto (or future target object) to transmit the buffer load back to Earth.
If we can pivot this a bit: "The lunar gravity well collects any debris onto the surface of the stellar body. In the case of collision with another object, the debris does not simply spray into space."
(I'm not actually sure how true this is)
Every time someone says dark side of the moon a lunar geologist kicks a kitten. Far side, please. It's only dark half the time, when the near side is lit. :)
In principle having a telescope on the Moon gives you a longer continuous period of observation (and a longer period of inability to observe) for a single object, when compared to a telescope in geosynchronous orbit. Roughly speaking, the continuously obserable stretches for an object on the celestial equator would be ~28x longer for a telescope on the far side of the moon (of course, this particular telescope+probe is on the near side). This is just that the Moon takes ~28x longer to go around the Earth's center of mass, compared to a satellite in geosynchronous orbit.
Thinking about future applicatoins, if humans ever put a base on the Moon, a telescope on the surface would be far easier to maintain/upgrade than one in orbit. So, it is potentially worth testing small telescopes there now to determine their performance in that environment/location.
If the target is in the plane of the ecliptic then the period of observation is probably not a full 28 days - for roughly half of that period a target would be eclipsed by the moon, plus some change for the Earth. I have no idea the precise time this would be better or worse than the Earth given the Moon's lack of magnetosphere or atmosphere pushing the drag down and decreasing the orbital radius vs the smaller angular view of the moon and so on. It's still probably longer than the Earth's when comparing the radii equivalently, I'd think. It's 28x and change for sure, but with significant confounding factors.
Counterpoint: the Moon has near-trivial gravity, so getting to lunar orbit isn't much of a problem. Assuming we had a moonbase, we could service lunar-orbital telescopes very easily. Remember, lunar orbit was attained by the Apollo LEM, compared to a S-V to get off the terrestrial surface. The rocket equation is a bitch, and strong gravity wells suck ass.
The Hill radius[1] for the Earth-Moon system is only 24,000 km. The orbital period for a satellite at this distance from the moon should be about 90 minutes. Increasing the distance to increase the period would put the object outside the Hill Radius, causing the satellite to eventually go into orbit around the Earth, rather than the Moon. So, it seems that lunar synchronous orbits do not exist, except at some lagrange stability points of the Earth-Moon system.
[0] https://en.wikipedia.org/wiki/Geosynchronous_orbit#Other_syn...
> 2) Unlike space-based observations, the Moon provides a large stable platform for maintaining astronomical instruments in permanently stable configurations. The rotation angular velocity of the Moon is about 0.55″/s, which is about 27 times slower than the Earth. The slow rotation allows long term monitoring without interruption for as long as about 10 days.
Plenty of goodies there.