
China's telescope on the moon - curtis
https://www.newscientist.com/article/dn28323-china-has-had-a-telescope-on-the-moon-for-the-past-two-years/
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paulmd
Not that it's not a cool project, but what possible reason is there to place
the telescope _on the surface of the moon_ as opposed to say, in
geosynchronous lunar orbit? As the article notes, the environment is much more
extreme and the imaging base much less stable, and I don't see any particular
benefits.

~~~
ISL
Off the top of my head:

\-- 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](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?

~~~
paulmd
1\. This doesn't change versus a satellite in geosynchronous lunar orbit.

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.

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hueving
geosynchronous lunar orbit is basically just a satellite sitting at the same
distance from the earth. It's so far away from the moon that it's basically
pointless to put something there unless your are observing the surface of the
moon.

[https://en.wikipedia.org/wiki/File:L4_diagram.svg](https://en.wikipedia.org/wiki/File:L4_diagram.svg)

~~~
paulmd
It's actually not, the L4 Lagrange orbit is something totally different from a
typical satellite geosynchronous orbit. Consider the following link.

[http://freemars.org/l5/aboutl5.html](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.

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hueving
Thanks for the info, but I wasn't talking about geosynchronous orbits. I was
talking about the term the GC used: 'geosynchronous lunar orbits'. i.e. an
orbit synchronous to the rotation of the moon.

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sevengraff
This article is not particularly useful. I like space and space tech, but this
article just declares something to exist and doesn't go into any interesting
details. I might as well read a press release from the chinese space agency.

~~~
ISL
The link at the bottom links to their arXiv preprint:
[http://arxiv.org/abs/1510.01435](http://arxiv.org/abs/1510.01435)

Plenty of goodies there.

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ommunist
in 5 years from obscure and forgotten article at arxiv.org we shall all know
that China has had geology probe successfully operating on Mars since 1999.

