[X] My Assistant

[rlorenz]

New images from the science cameras:

http://www.jaxa.jp/press/2007/11/20071116_kaguya_e.html
Very promising!

Looks very good - sort of HRSC at the moon. Is there going to be anything useful left for LRO
to do ?

It'd be really interesting/useful if someone put together a comparison table of the various cameras that have
been/are going to the moon (resolution, coverage, colors, stereo etc.) Maybe it exists (surely anyone
in the USA flying a camera has to justify it - maybe the Indian/Japanese/Chinese are all 'tech demos' like
AMIE on Smart-1

[Phil Stooke]

LRO brackets the Kaguya resolution: global stereo and multispectral at 100 m/pixel, high resolution at 1 m/pixel (approx. values).

Somewhere I have seen a table comparing the current set of cameras. I'll try to find it and post it.

Phil

[edstrick]

It's probably not really difficult to maintain a 10 or 15 km lunar orbit. You need 2 things.. maybe 3.

1.) A GOOD gravity model, reasonably high resolution, including the farside. You need to predict the exact spacecraft trajectory several orbits in advance to 1 km or better accuracy.

2.) Good tracking to verify what's the real orbit as opposed to the predict. A real-time engineering-ops lidar or RF altimiter wouldn't hurt.

3.) A propulsion system, maybe ion, maybe hypergolics running low thrust engines

You simply need to design a vehicle that will do an orbit trim maneuver or two every orbit. Just a 1/2 km tweek... just keep it circular within a small amount.

[dvandorn]

I can see a sustained 25km lunar orbit, maybe, but I think a 10km orbit would be pushing it. A lot. After all, the Moon is lumpy, and not just gravitationally -- there are some mountains (well, basin and crater rims, mostly) that rise nearly 10km up from their surrounding terrains.

And, as the gallows humor that surrounded the Apollo 8 flight had it, "60 miles? Just wait 'til you get a load of that 61-mile-high mountain on the Far Side...!"



-the other Doug

[mps]

Looks very good - sort of HRSC at the moon. Is there going to be anything useful left for LRO
to do ?

It'd be really interesting/useful if someone put together a comparison table of the various cameras that have
been/are going to the moon (resolution, coverage, colors, stereo etc.) Maybe it exists (surely anyone
in the USA flying a camera has to justify it - maybe the Indian/Japanese/Chinese are all 'tech demos' like
AMIE on Smart-1

Best spatial resolution: Chang'e 120 m/pix, Kaguya 10 m/pix, Chandrayaan-1 5 m/pix, LRO 0.5 m/pix
So, yes, LROC is very useful

P.S. It's my first post here. My english isn't very good, I hope this will not be a big problem.

Mps

[Stu]

No problem at all Mps, welcome to UMSF!

[Del Palmer]

You simply need to design a vehicle that will do an orbit trim maneuver or two every orbit. Just a 1/2 km tweek... just keep it circular within a small amount.

I remember reading an AIAA paper that suggested "frozen orbits" exist at low altitude, with a caveat that the orbit's inclination matters significantly. It concluded that at certain inclinations (27º, 50º, 76º, and 86º) it is possible for a spacecraft to remain in a low-lunar orbit indefinitely, without OTMs.

[nprev]

Wow...a stable 86 deg low orbit sounds like a real opportunity for an ultra-high resolution global mapping mission, or perhaps escaping volatile sensing/monitoring...

[ugordan]

You simply need to design a vehicle that will do an orbit trim maneuver or two every orbit. Just a 1/2 km tweek... just keep it circular within a small amount.

Of course, the original idea was lowering Kaguya's orbit at the end of mission. Neglecting the fuel required to actually lower the orbit, you'd still need propellant to maintain circularity. Spacecraft are usually running on fumes at end-of-mission so that's not very plausible.

What happens if you have unforseen downtime in your tracking support?

[Phil Stooke]

I'm not so sure about the 'frozen orbits'. I could be wrong - I seem to remember being wrong once... - but here, I think this is probably talking about the orbit plane being fixed with respect to the sun, so you have the same surface illumination all the time. Certain inclinations and orbit shapes give just the right combination of gravitational effects that the orbit is always, let's say, 15 degrees from the terminator.

Low lunar orbits are unstable because of the 'lumpy' gravitational field caused by mascons, which in typical low orbits cause the perilune to migrate up and down over time in complex ways - if I understand it correctly, right now, Kaguya's perilune is ascending, but later it will descend until it has to be corrected. The effects of the Earth, the sun and mascons together make life complicated. I don't think this can be 'frozen'.

Phil

[ugordan]

I'd think mascons* are by far the greatest destabilizing factor for such a low orbit (10 km), with Earth and Sun getting progressively more important (and mascons less important) at higher orbits.

* mascons and high mountain peaks, that is.

[djellison]

We've got two more cratering experiments due before Kaguya, Chang'e 1, LRO and Chandaraan 1 will be anywhere near end of life. I'm hoping that they go for barn-storming low-altitude science instead of crashing them into polar crater

[Del Palmer]

Regarding frozen orbits, I couldn't find the original paper, but here's an article that is derived from it.

[remcook]

I'm not so sure about the 'frozen orbits'. I could be wrong - I seem to remember being wrong once... - but here, I think this is probably talking about the orbit plane being fixed with respect to the sun, so you have the same surface illumination all the time. Certain inclinations and orbit shapes give just the right combination of gravitational effects that the orbit is always, let's say, 15 degrees from the terminator.

Judging from the link above it seems that these frozen orbits are different from sunsynchronous orbits of which Phil is speaking. J2 seems quite small for the moon, so I'm not sure sunsynchronous orbits are even possible for the moon. It looks like frozen orbit means that the lumpy gravity field will give nudges to the orbit that are not unstable in these cases.

[spdf]

Here is some info about the problem which delayed Kaguyas launch.

http://www.yomiuri.co.jp/dy/features/scien...120TDY03104.htm