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General Discussion => Advanced Concepts => Topic started by: Robotbeat on 03/09/2012 04:49 AM

Title: Phobos surface access from Mars orbit (tether-based)
Post by: Robotbeat on 03/09/2012 04:49 AM
http://www.mrc.uidaho.edu/~atkinson/IPPW-3/posters/11_Hamelin_fobos.pdf

Very interesting... it talks about a Phobos tether, about 4km in length (comparable to other tethers that have been deployed in LEO, with likely even less strength required), being used for Phobos sample return in order to avoid the necessity to land on Phobos itself. Basically, a Mars orbiter would stay near Mars-Phobos Lagrange 2 and unreel a tether to the surface to collect regolith.

I do wonder about an architecture like this being applied to a human mission to the surface. This sort of thing would allow propulsive-less EVAs to the surface, or perhaps allow a pod to be lowered to the surface via a cable (this would allow the orbiting spacecraft to remain stably at the Lagrange point for long periods of time, getting rid of careful station-keeping requirements). Astronauts could ascend and descend on this tether.

If this was essentially the same thing as the Exploration Gateway, this could provide a convenient foothold in Mars orbit, with ready access to the surface of a celestial body without requiring high thrust (thus all propulsion from Earth orbit to Phobos surface could be very, very high Isp). Thus would allow processing of Phobos regolith and easy transfer to Mars orbit for either further processing or use as shielding material, perhaps eventually used for ISRU life support and/or propellant. But the neat thing is that it'd require no new exotic technology and would be a pretty good spot for servicing a reusable Martian lander as well.

And if the cable broke, the delta-v required to get to the exploration craft at Lagrange 2 (or slightly beyond) from the Phobos surface would be well within the range of a Shuttle-era MMU (Phobos escape velocity is 11m/s, MMU can do 24.4m/s, so you could even do a round-trip), so the astronauts would not have to worry about being stranded on the surface.

The nice thing is that the cable (which could be VERY lightweight and still have an enormous factor of safety) and an unreeling mechanism and some space suits with MMUs could substitute for designing a landing craft.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: kkattula on 03/09/2012 05:30 AM
Phobos gravity is less than 0.001 g, so high thrust wasn't ever required for landing. Tiny cold gas thrusters would suffice.

The only good reason I can see for this concept is to avoid surface sample contamination with the probe's exhaust.  Which may be problematical anyway, if it's reeling a sample in through the L1/2 region it's been station keeping in. But it's likely only Nitrogen or maybe Helium.


Landing an unmanned probe on a non-rotating, < 0.001g, vacuum surface does not sound more difficult.  Presumably one would bring the lander to a few hundred feet above the target area and hover for a few hours while close-up pictures & radar/lidar images are sent back to Earth, examined, and a final touchdown point decided on and up-linked. Maybe 10 to 15 m/s of delta v for the hover.

Once landed, the probe could extend sample collectors beyond the tiny contamination zone, if desired.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: Nomadd on 03/09/2012 04:02 PM
 Why would you need to avoid landing on Phobos in the first place? You could throw a baseball into orbit from there. And you'd be constanly expending fuel in orbit. It's hard to stay place when you keep reeling rocks up.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: Robotbeat on 03/09/2012 04:47 PM
Why would you need to avoid landing on Phobos in the first place? You could throw a baseball into orbit from there. And you'd be constanly expending fuel in orbit. It's hard to stay place when you keep reeling rocks up.
You didn't read the article, did you?

This is about staying at Mars-Phobos Lagrange point which would mean you don't need to constantly expend fuel (i.e. it's like the Earth space elevator concept, except without the need for anything exotic like nanotubes and while having a tether four orders of magnitude shorter) except for typical station-keeping purposes. You are held in gravitational balance between Mars and Phobos.

If you are anchored to the surface (either with a harpoon-type method or with having 1/4 or your total spacecraft mass resting on the surface of Phobos), you are totally stable and don't even need standard station-keeping (i.e. set it and forget it).

Regolith could also be used to anchor the bottom of the tether.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: A_M_Swallow on 03/09/2012 06:32 PM
A 500 kg vehicle means that the article was thinking of an unmanned probe.

For manned visits to Mars a base could be built at the Mars Phobos Lagrange point.  The transfer vehicle being parked at the base/spacestation.  The reusable Mars lander's hanger could either be part of the base or on the surface of Phobos.

If heavy items are going to be moved up and down the Phobos space elevator a counter weight on a long tether above the base will be needed to balance the items' mass.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: alexterrell on 03/09/2012 07:12 PM
I assume this is on the basis that Phobos regolith might be abrasive and disruptive in the way lunar regolith is.

On the other hand, it might be preferable to bury the habitat module in the regolith for radiation protection. Radiation versus abrasive dust?
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: Robotbeat on 03/09/2012 10:58 PM
I assume this is on the basis that Phobos regolith might be abrasive and disruptive in the way lunar regolith is.

On the other hand, it might be preferable to bury the habitat module in the regolith for radiation protection. Radiation versus abrasive dust?

You could bring the regolith (in sealed bags?) up to the Mars-Phobos Lagrange point Exploration Gateway*, thus allowing you to avoid both the abrasive regolith and getting lots of radiation shielding (approaching the levels you could get on the Martian surface).

*(just a name, I'm not suggesting this is near-term)
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: A_M_Swallow on 03/10/2012 12:08 AM
The empty bags would have to be brought from Earth making them very expensive.

It may be possible to turn the regolith into a solid, possibly by heat sealing.  The large bricks could then be lifted up to the platform to create the radiation barrier.
Title: Re: Phobos surface access from Mars orbit (tether-based)
Post by: Nomadd on 03/10/2012 02:00 AM
Why would you need to avoid landing on Phobos in the first place? You could throw a baseball into orbit from there. And you'd be constanly expending fuel in orbit. It's hard to stay place when you keep reeling rocks up.
You didn't read the article, did you?

This is about staying at Mars-Phobos Lagrange point which would mean you don't need to constantly expend fuel (i.e. it's like the Earth space elevator concept, except without the need for anything exotic like nanotubes and while having a tether four orders of magnitude shorter) except for typical station-keeping purposes. You are held in gravitational balance between Mars and Phobos.

If you are anchored to the surface (either with a harpoon-type method or with having 1/4 or your total spacecraft mass resting on the surface of Phobos), you are totally stable and don't even need standard station-keeping (i.e. set it and forget it).

Regolith could also be used to anchor the bottom of the tether.
Maybe you better figure in the horizontal acceleration of the mass you're raising up before you declare ship to be "perfectly" stable. And the oscillations of the tether that long. Station keeping means more than staying a constant distance from the moon.
 I read Wizard of Oz when I was a kid too. That doesn't mean I took it as practical.