China to land probe on the far side.

[Dalhousie]

The Chang'e 5-T1 mission carrier spacecraft ended up being parked in L2 for a while to test relay operations.  It returned some very nice images of the Earth and lunar far side.  There are some screen shots from Chinese TV here, which also show the halo orbits quite nicely

http://www.spaceflight101.net/change-5-test-mission-updates.html

[Zakalwe]

https://theconversation.com/chinas-plan-to-be-first-to-far-side-of-the-moon-could-unveil-inner-lunar-secrets-53253

Thanks for sharing the links.
I can hear the hoaxies already screaming "fake" as they struggle to get their heads around illustrations such as this:

People like Hunchbaked and Tarkus will have their tiny minds warped by small objects that are near appearing larger than further objects that are actually larger. Time to roll this classic out in a pre-emptive strike 

[smartcooky]

The Lagrange points are where the gravitational potential gradient is zero in an orbital system with two massive bodies. In English, those are points in space where the gravity of the two objects (e.g., the sun and the earth, or the earth and the moon) cancel and you can stay there more or less indefinitely.

There are 5 such points, numbered L1 through L5. L1 is along the line between the two bodies where their gravitational attractions match. The earth-sun L1 point, about 1.5 million km toward the sun from earth, is now well used; the most recent addition to the fleet there is DSCOVR, the spacecraft that returned that great sequence of the moon moving past the earth last summer. It joins ACE, SOHO and a bunch of others.

But none of the spacecraft are actually at the L1 point, because that would put them right in front of the sun as seen from earth. The sun generates radio noise, and this would interfere with reception of the satellite's signal. So they are slowly moved around the actual L1 point so that, from earth, they appear to slowly circle the sun, far enough from it that the dish antennas on earth can exclude the sun. (I think it's once per year, but I'm not sure). It does take fuel to do this, but with careful planning you can keep station for years. That's a halo orbit.

The L2 point is on the same line but not between them; it's closer to the smaller body. Similarly, L3 is on that line but closer to the larger body. (There was once a whole genre of science fiction about a "parallel earth" situated at the Sun-Earth L3 point, placing it permanently on the other side of the sun where we cannot see it. Unfortunately, our fantasies were dashed when our spacecraft saw nothing there, just as they show a lifeless, desolate desert on Mars, and an uninhabitable hell on Venus...)

L4 and L5 are in the orbit of the smaller body, 60 degrees behind and ahead of it (or maybe it's ahead and behind, I can never remember). These are the only two Lagrange points that are dynamically stable, i.e., you could put a rock there and it would stay indefinitely. The other three are metastable, kind of like balancing on a fence; it takes active control to stay there, but as long as you keep fairly close it doesn't take much energy.

A picture helps

[bknight]

The Lagrange points are where the gravitational potential gradient is zero in an orbital system with two massive bodies. In English, those are points in space where the gravity of the two objects (e.g., the sun and the earth, or the earth and the moon) cancel and you can stay there more or less indefinitely.

There are 5 such points, numbered L1 through L5. L1 is along the line between the two bodies where their gravitational attractions match. The earth-sun L1 point, about 1.5 million km toward the sun from earth, is now well used; the most recent addition to the fleet there is DSCOVR, the spacecraft that returned that great sequence of the moon moving past the earth last summer. It joins ACE, SOHO and a bunch of others.

But none of the spacecraft are actually at the L1 point, because that would put them right in front of the sun as seen from earth. The sun generates radio noise, and this would interfere with reception of the satellite's signal. So they are slowly moved around the actual L1 point so that, from earth, they appear to slowly circle the sun, far enough from it that the dish antennas on earth can exclude the sun. (I think it's once per year, but I'm not sure). It does take fuel to do this, but with careful planning you can keep station for years. That's a halo orbit.

The L2 point is on the same line but not between them; it's closer to the smaller body. Similarly, L3 is on that line but closer to the larger body. (There was once a whole genre of science fiction about a "parallel earth" situated at the Sun-Earth L3 point, placing it permanently on the other side of the sun where we cannot see it. Unfortunately, our fantasies were dashed when our spacecraft saw nothing there, just as they show a lifeless, desolate desert on Mars, and an uninhabitable hell on Venus...)

L4 and L5 are in the orbit of the smaller body, 60 degrees behind and ahead of it (or maybe it's ahead and behind, I can never remember). These are the only two Lagrange points that are dynamically stable, i.e., you could put a rock there and it would stay indefinitely. The other three are metastable, kind of like balancing on a fence; it takes active control to stay there, but as long as you keep fairly close it doesn't take much energy.

A picture helps

Nice image!
ka9q, the issue that I was asking, how do you orbit a point in space in this case a point where the gravitational forces cancel.

[Echnaton]

So if you put a relay satellite at earth moon L4 or L5 point, it would cover part of the far side.  Something like 2/3 of the side it were on? That would give enough coverage to make keep communications up. 

[Ishkabibble]

Just occurred to me thatlLibration hasn't been factored in. As I understand it, we can see something like 58% of the near side, which means that anything on the other side of the terminator at full phase we'd be able to see 58% of it due to libration, right? Doesn't that open up all but the most "central" parts of the far side?

[Dalhousie]

Liberation only allows an addition 10 degrees or so to be seen, so most of the far side (84%) still cannot be seen.  That's a lot more than just the central part. Even those areas that can be seen are seen only at very low angles.  This makes communications with anything on the surface difficult, as it will often be obscured by mountains, crater rims etc.

[Dalhousie]

So if you put a relay satellite at earth moon L4 or L5 point, it would cover part of the far side.  Something like 2/3 of the side it were on? That would give enough coverage to make keep communications up.

Less than half I think, one satellite at each point  (L4 and L5) would give more complete coverage.  But one satellite at L2 will give complete coverage and better coverage of the poles.  Which is who China (and others planning far side missions) plan on putting one there.

[Ishkabibble]

Liberation only allows an addition 10 degrees or so to be seen, so most of the far side (84%) still cannot be seen.  That's a lot more than just the central part. Even those areas that can be seen are seen only at very low angles.  This makes communications with anything on the surface difficult, as it will often be obscured by mountains, crater rims etc.

I don't think I explained myself nearly well enough to be confident anyone understood what I was thinking. I had this image in my head, but not the vocabulary to describe it. So I drew something. Most definitely not to scale, by the way.

When I say "central parts" I mean the area shaded reddish on the diagram attached. If the green plane is the orbital plane in which the relay commsat is placed, then it can only stay in communication with the line-of-sight areas in the non-shaded area of the far side (and likewise the near side, but that would be unnecessary).

So since they're aiming at polar regions, wouldn't this setup work, with less chance of cockups?

[Dalhousie]

Liberation only allows an addition 10 degrees or so to be seen, so most of the far side (84%) still cannot be seen.  That's a lot more than just the central part. Even those areas that can be seen are seen only at very low angles.  This makes communications with anything on the surface difficult, as it will often be obscured by mountains, crater rims etc.

I don't think I explained myself nearly well enough to be confident anyone understood what I was thinking. I had this image in my head, but not the vocabulary to describe it. So I drew something. Most definitely not to scale, by the way.

When I say "central parts" I mean the area shaded reddish on the diagram attached. If the green plane is the orbital plane in which the relay commsat is placed, then it can only stay in communication with the line-of-sight areas in the non-shaded area of the far side (and likewise the near side, but that would be unnecessary).

So since they're aiming at polar regions, wouldn't this setup work, with less chance of cockups?

Thanks, that helps.  In reality the red area (yes, I know it's diagramatic) is proportionally much larger.

I don't think anyone, least of all China, is talking about a polar orbiting relay satellite, which would be visible to the lander for only a few minutes each orbit (depending on altitude of course), even for a polar mission.

Because the SPAB is offset from the SP (it reaches as far north as -17 degrees) a lander could well be out of sight of a polar satellite orbiting satellite for three weeks out of four (again, the details would depend on altitude).

A L2 halo orbit is by far and away the best location for a far side relay.  Note that two Chinese spacecraft have been tested in the lunar L2 position, Chang'e 2 and Chang'e 5 T1.

[smartcooky]

Just occurred to me thatlLibration hasn't been factored in. As I understand it, we can see something like 58% of the near side, which means that anything on the other side of the terminator at full phase we'd be able to see 58% of it due to libration, right? Doesn't that open up all but the most "central" parts of the far side?

Libration adds very little to the amount of the moon we can see from the earth...

[smartcooky]

Liberation only allows an addition 10 degrees or so to be seen, so most of the far side (84%) still cannot be seen.  That's a lot more than just the central part. Even those areas that can be seen are seen only at very low angles.  This makes communications with anything on the surface difficult, as it will often be obscured by mountains, crater rims etc.

I don't think I explained myself nearly well enough to be confident anyone understood what I was thinking. I had this image in my head, but not the vocabulary to describe it. So I drew something. Most definitely not to scale, by the way.

When I say "central parts" I mean the area shaded reddish on the diagram attached. If the green plane is the orbital plane in which the relay commsat is placed, then it can only stay in communication with the line-of-sight areas in the non-shaded area of the far side (and likewise the near side, but that would be unnecessary).

So since they're aiming at polar regions, wouldn't this setup work, with less chance of cockups?

Thanks, that helps.  In reality the red area (yes, I know it's diagramatic) is proportionally much larger.

I don't think anyone, least of all China, is talking about a polar orbiting relay satellite, which would be visible to the lander for only a few minutes each orbit (depending on altitude of course), even for a polar mission.

Because the SPAB is offset from the SP (it reaches as far north as -17 degrees) a lander could well be out of sight of a polar satellite orbiting satellite for three weeks out of four (again, the details would depend on altitude).

A L2 halo orbit is by far and away the best location for a far side relay.  Note that two Chinese spacecraft have been tested in the lunar L2 position, Chang'e 2 and Chang'e 5 T1.

100% agree with this. Its the best option by far.....

[Ishkabibble]

Liberation only allows an addition 10 degrees or so to be seen, so most of the far side (84%) still cannot be seen.  That's a lot more than just the central part. Even those areas that can be seen are seen only at very low angles.  This makes communications with anything on the surface difficult, as it will often be obscured by mountains, crater rims etc.

I don't think I explained myself nearly well enough to be confident anyone understood what I was thinking. I had this image in my head, but not the vocabulary to describe it. So I drew something. Most definitely not to scale, by the way.

When I say "central parts" I mean the area shaded reddish on the diagram attached. If the green plane is the orbital plane in which the relay commsat is placed, then it can only stay in communication with the line-of-sight areas in the non-shaded area of the far side (and likewise the near side, but that would be unnecessary).

So since they're aiming at polar regions, wouldn't this setup work, with less chance of cockups?

Thanks, that helps.  In reality the red area (yes, I know it's diagramatic) is proportionally much larger.

I don't think anyone, least of all China, is talking about a polar orbiting relay satellite, which would be visible to the lander for only a few minutes each orbit (depending on altitude of course), even for a polar mission.

Because the SPAB is offset from the SP (it reaches as far north as -17 degrees) a lander could well be out of sight of a polar satellite orbiting satellite for three weeks out of four (again, the details would depend on altitude).

A L2 halo orbit is by far and away the best location for a far side relay.  Note that two Chinese spacecraft have been tested in the lunar L2 position, Chang'e 2 and Chang'e 5 T1.

100% agree with this. Its the best option by far.....

That image explains what I couldn't visualise.

Especially with laser communications, yes, that absolutely would be the best option, and is the real representation of what I actually was trying to describe, but about the planar orbit I was thinking about. My only question is, how far on the other side is the L2? With laser communications it wouldn't matter how far it was though, because all comms would be nearly instantaneous.

I think that with ESA and China taking steps towards manned landings resuming, that we as a nation might possibly move to go back ourselves. At least I hope so.

Can you imagine the views that will come out of manned landings with today's digital imaging technology? That bulky Hasselblad attached to the chest will be supplanted by aerospace versions of go-pros built into the helmets. Panoramic cameras showing all around views all at once. The mind boggles at the possibilities.

Too bad out-of-shape, overweight, middle-aged historians aren't being sought by NASA for the next Astronaut corps... I'd be a shoo-in!

[bknight]

China will either have to built a network similar to NASA's or rent bandwidth from them.

[smartcooky]

Interestingly, the Sun - Earth L2 point is where NASA/ESA et al  plan to place the James Webb Space Telescope when it is launched in 2018.

From the JWST Wikipedia page

"The JWST will be located near the second Lagrange point (L2) of the Earth-Sun system, which is 1,500,000 kilometers (930,000 mi) from Earth, directly opposite to the Sun. Normally an object circling the Sun farther out than Earth would take longer than one year to complete its orbit, but near the L2 point the combined gravitational pull of the Earth and the Sun allow a spacecraft to orbit the Sun in the same time it takes the Earth. The telescope will circle about the L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius of approximately 800,000 kilometers (500,000 mi), and take about half a year to complete. Since L2 is just an equilibrium point with no gravitational pull, a halo orbit is not an orbit in the usual sense: the spacecraft is actually in orbit around the Sun, and the halo orbit can be thought of as controlled drifting to remain in the vicinity of the L2 point. This requires some station-keeping: around 2–4 m/s per year from the total budget of 150 m/s. Two sets of thrusters comprise the observatory's propulsion system."