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Journal brandido's Journal: Using a Space Elevator for a Mars Sample Return 30

Journal by brandido
Lately, the Space Elevator concept has been getting a fair amount of airplay recently, and has been on my mind for a while. That, and thinking about the renewed push towards a human trip to Mars, and the steps that it would take to get there, has me pondering. I knew that we did not have the capabilities to build a Space Elevator for Earth as the Carbon Nano Tube technology is not yet strong enough. At the same time, I kept thinking about the Mars sample return trip and how this particular mission also seemed to be beyond our current capabilities. It is the first mission to Mars that would require either a huge step forward in our financial and technical capabilities or a paradigm shift. Based on the current methodology, the mission might look like:
  • Launching a mission to Mars,
  • Safely landing the probe, probably using parachutes and airbags, with which we do not have a great record,
  • Providing fuel to get the sample back into orbit by either:
    • Bringing enough fuel for the return trip, a fairly hazardous endeavor to crash land on a planet, or
    • generating the fuel there which requires a fair amount of potentially delicate hardware
  • Launching back into orbit without any of our standard launch equipment, and
  • Using fuel on the orbiter to send the sample back to earth.

While we have successfully sent orbiters and landers to mars, sending either a large supply of fuel or sending the equipment to process fuel once we are there is a significant step beyond our current abilities. Sending the fuel with the lander requires sending significantly more mass with the mission, as it must be enough to orbit the sample and return it to earth. Potentially more importantly, bringing the fuel poses significant risks to the mission from the increased chance of a fatal explosion.

On the other hand, trying to send the equipment required to manufacture fuel while Mars requires sending significantly more mass to Mars, is prone to failure either due to the harshness of the landing or simple mechanical failure, and is severely dependent on chemical distribution at the landing site. In addition, neither of these options provides for any infrastructure on the planet for future missions - sending the fuel leaves nothing there while the fuel processing equipment will be hard to return to (landing is currently a very imprecise science).

As an alternative, a Mars Space Elevator shows great potential as a means of performing a sample return mission. While the current Carbon Nana Tube (CNT) cables that can be produced are not yet strong enough for a Space Elevator on Earth, they are strong enough for one on Mars due to its lower gravity (0.38g). In addition, the Aerosynchronous (Mars Geosynchronous) orbit is lower than the Geosynchronous orbit, so the cable could be shorter. So, technologically, we should have the capability to do a sample return mission using a space elevator. So, what are the steps?

  • The mission would be comprised of the orbiter/cable and the lander.
    • The orbiter/cable would be used for lowering the lander to the surface of Mars while gradually sending a counterweight farther out into space. It would also server as a communication relay.
    • The lander would be lowered to the surface, anchor itself to the surface, and take a sample.
  • Once a sample has been taken on Mars, the sample will be sent up the cable using a cable crawler.
  • The crawler would use solar energey to crawl up the cable until it reaches Aero-synchronous orbit, at which point, it will start accelerating away from the planet as gravity looses its hold.
  • This acceleration will be used to fling the sample back to earth, with fuel needed only to adjust its flight back.

Using the Mars based Space Elevator provides a few significant advantages:

  • No need to use parachutes and air bags to try and do a controlled crash landing on Mars, usually the most deadly portion of a Mars landing. Instead, the lander is lowered to the surface using the Space Elevator.
  • Fuel does not need to be sent to Mars or processed there, as the crawler that climbs back up the cable can operate on electricity generated by solar energy, either on the crawler or beamed down from the orbiter. While climbing the cable of a Space Elevator is significantly slower than riding a rocket, it is also quite a bit safer!
  • Future return trips would already have the way paved for using the Space Elevator, reducing future landing costs and dangers.
  • Would provide a proving ground for the Space Elevator prior to technology being advance enough for an Earth version. Would also help minimize any unforeseen dangers by testing it out a long way from anyone.

There are also some significant issues that would need to be resolved:

  • Probably the biggest, both literally and figuratively, is Phobos - at only 9000 kilometers distance, it is well within the minimum required length of the cable of 17,200 km (aerosynchronous orbit). Deimos may also be a problem, but this could counter acted by using a heavy counter weight on the Elevator. This may be able to counter act by inducing oscillations in the cable to miss the moon(s)? Thoughts?
  • Transporting the cable could be an issue, as I have no idea what the weight of a CNT cable that is capable of providing for a sample return would be, so may be prohibitively expensive to launch to mars.
  • Currently, the longest CNT cable length I have read of is about 5 km - a Mars Space elevator would need one at least 5000 times longer. However, I do not know if the 5k limit is a hard limit, or simply because they stopped.
  • Current costs of CNTs is still very high, so may make cable production costs prohibitively high.
  • Dust storms on Mars can have very high windspeeds that could break the cable. While the relatively thin atmosphere mitigates this, it would need to be investigated.
  • Many others that I have missed - any thoughts?

Thanks if you have taken the time to finish reading my thoughts on this subject. I am sure that there are some glaring mistakes that need to be addressed, and I welcome feedback. I also realize that there have been many previous discussions about a Martian Space Elevator, in science fiction such as "{Red|Blue|Grenn} Mars" and in research such as Dr Edwards report on the Space Elevator. However, I have not previously read a discussion of using a Space Elevator as a means to explore Mars prior to having one on Earth. If anybody has any references to some reading covering this, it would be greatly appreciated.

Thanks,

brandido

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Using a Space Elevator for a Mars Sample Return

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  • Why not use Phobos or Demios as a counterweight?

    My astronomy is on the weak side, so I must ask an extremely ignorant question: Are they geostationary? If not, is it anywhere near feasible to make them so? They _are_ miserably tiny by planetary standards. Rocks the size of a big city...

    Doing that would save you, what, half to two thirds the cable length? (in addition to saving you the problem of accidentally having your cable smack into one of them)

    Theoretically you could even do it on an earth SE - you
    • Theoretically you could even do it on an earth SE - you could balance a SE at 36000km instead of 91000km if you happened to have something big enough floating around...

      Finally a use for the ISS! Oh wait, that's in LEO... :-(
    • First off, I love this idea. However, I have a somewhat better approach to getting an elevator erected on Mars.

      We got to the moon by taking a series of steps, each only so much more complicated than the last. With that in mind, why not take a step back from such a grand plan and aim a touch lower.

      If a space elevator is the crux of such a Mars mission, clearly we need something approaching a working prototype first before sending it months away to another planet. Why not make such an elevator for Earth'
  • It's Areosynchronous, not Aerosynchronous (because Ares is the Greek equivalent of Mars, and synchronous is bastard Greek...)
  • I didn't know that anyone had built a CNT cable. Got a link?
  • You don't want to take along the extra weight of a fuel manufacturing unit or extra fuel, yet you're perfectly fine with hauling around tens of thousands of miles of (admittedly very light, but it all adds up) cable and a counterweight?

    • You don't want to take along the extra weight of a fuel manufacturing unit or extra fuel, yet you're perfectly fine with hauling around tens of thousands of miles of (admittedly very light, but it all adds up) cable and a counterweight?

      Actually, my issue with the extra fuel or the fuel manufacturing is not primarily the extra weight, it is more the issues of trying to land such things on the surface of Mars and the lack of reusable infrastructure. With the space elevator, the bulk of the mission is sen

      • Good point, but tens of thousands of miles of cable is still a lot, especially when you're talking about using it to haul loads off the surface of a planet with solar-powered satellite. Frankly, a space elevator on Earth is a stupid idea. There is WAY too much stuff in our planet's orbit, especially stuff that we put up there. The key issue is that the elevator will be stationary to the Earth's surface, while stuff floating around in LEO is moving several kilometres a second relative to the Earth's surface
        • I forgot one thing I meant to add... Take a look at how fast the landers on Mars are moving on the surface. Your space elevator would probably achieve comparable speeds. We could probably terraform and colonize Mars in the time it would take to lower the probe to the surface.

          • There is no reason that the cable would be moving at rates comparable to the Mars Landers - such a statement is sheer hyperbole. The landers are trying to cover uneven and potentially hazardous terrain, using algorithms that favor safety over speed, while the space elevator is simply being lowered through free space with free acceleration (it is basically falling) - no terrain to speak of until it reaches its final destination.

            Lowering the probe to the surface could be done quite quickly (meaning in a f

        • I think that a significant amount of the difference in opinion that you and I are having could be resolved by reading the report the Dr. Edwards produced for NIAC [spaceelevator.com].

          To address your point of mass, in this report, one of the issues that is covered is the mass of the various components for a system to lower a cable to Earth (see table 3.1). Looking at these numbers, the weight of a cable for Earth is ~20000kg, which is a lot. But take into account that the Mars Space elevator will be less than half the lengt

  • Actually, why not do it on the moon first? The moon's gravity is about 1/6 of earth's, resulting in a shorter cable. Of course, the moon turns so slowly (1 day is about 1 earth month) that there is little centripital force.

    It's closer than Mars, too.
    • Actually the slow rotation of the moon means that geosync (lunasync?) orbit is much farther away than earths, making a space elevator more difficult on the moon. Geosync orbit on the moon is the same as the earth-moon distance, so we couldn't put a satellite there. The only place it would be feasible would be the l1 lagrange point. Building a cable from the moon, through l1, and part way back to earth, the mass of the cable on the earth side of l1 would be the counterweight that held the cable taught thr
      • What about a "no centripital force" compromise?

        How about no "space elevator" on the moon, but a tall platform, perhaps 50-150km high as a platform, to allow low gravity escapes from orbit? Basically, you could ride up a sturdy, tall platform, and launch using less chemical propellant....

        You know, that's probably just too lame and too expensive....
        • You've still got to build up enough lateral velocity to achieve orbit, which is most of the fuel spent in taking off from a planet.
        • A 50-100km tower might be a nice launching point from earth, as it would get you past most of the atmosphere so there would be less drag during the launch, but on the moon it wouldn't make much difference. The gravity diff at only 50-150k isn't enough to make a tower that huge worth building. On the moon, an electromagnetic launcher, like a big rail gun, would be a better choice. You have the advantage of plentiful solar energy (and fuel for fusion if we figure that out), no atmosphere, and low gravity,
    • The moon is tidal-locked to the Earth. Ever notice how you can only see one side of it? A lunar-synchronous orbit would only be achievable by parking yourself at one of the Earth-Moon Lagrange points.

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