5 ms·

China's telescope on the moon

61 points by curtis 11y ago

paulmd 11y ago
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 11y ago
  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 11y ago
    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.
    - hueving 11y ago
      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 11y ago
        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.
        hueving 11y ago
        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.
    - gus_massa 11y ago
  - stephengillie 11y ago
    > No need to maintain orbit. Can't hit anything, either. 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)
  - dwc 11y ago
    > you can do it on the dark side of the moon 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. :)
- privong 11y ago
  I assume this was a case of "we're sending a probe to the moon, what else can we put on it?" rather than "we want to put a telescope on the moon, let's send it on a probe". 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.
  - paulmd 11y ago
    I agree with this (see: my post below). It's probably a case of "we're sending a probe, would you like to put a telescope on it or have nothing at all". I edited that I mean geosynchronous lunar orbit in particular. 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.
    - nitrogen 11y ago
      What is a "geosynchronous lunar orbit"? I can't find that term on the interwebs and I'd like to know more. What body is being orbited, and at what distance and period?
- jessriedel 11y ago
  Note sure this is a good justification, but it's the only claimed advantage I could find in their main paper: > 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. http://link.springer.com/article/10.1007/s11433-011-4255-7 http://link.springer.com/article/10.1007/s11433-011-4255-7
deleted 11y ago
[deleted]
sevengraff 11y ago
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.
- c3534l 11y ago
  Yeah, until we find out they've had a secret colony on the moon for two years, saying their probe's telescope still works and isn't that interesting is kinda a silly article to be the #1 article.
- ISL 11y ago
  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.
ommunist 11y ago
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.

61 points by curtis 11y ago

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 11y ago
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 11y ago
  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.
  - hueving 11y ago
    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 11y ago
      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.
      - hueving 11y ago
        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.
  - gus_massa 11y ago
- stephengillie 11y ago
  > No need to maintain orbit. Can't hit anything, either. 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)
- dwc 11y ago
  > you can do it on the dark side of the moon 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. :)
privong 11y ago
I assume this was a case of "we're sending a probe to the moon, what else can we put on it?" rather than "we want to put a telescope on the moon, let's send it on a probe". 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.
- paulmd 11y ago
  I agree with this (see: my post below). It's probably a case of "we're sending a probe, would you like to put a telescope on it or have nothing at all". I edited that I mean geosynchronous lunar orbit in particular. 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.
  - nitrogen 11y ago
    What is a "geosynchronous lunar orbit"? I can't find that term on the interwebs and I'd like to know more. What body is being orbited, and at what distance and period?
jessriedel 11y ago
Note sure this is a good justification, but it's the only claimed advantage I could find in their main paper: > 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. http://link.springer.com/article/10.1007/s11433-011-4255-7 http://link.springer.com/article/10.1007/s11433-011-4255-7

deleted 11y ago

[deleted]

sevengraff 11y ago

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.

c3534l 11y ago
Yeah, until we find out they've had a secret colony on the moon for two years, saying their probe's telescope still works and isn't that interesting is kinda a silly article to be the #1 article.
ISL 11y ago
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.

ommunist 11y ago

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.