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I was reading a FAQ on Mars missions when I came across the following snippet:

NASA does have the rocket technology to get humans to Mars. However, current technology is not advanced enough to get us there efficiently. We are continuing to develop technologies to improve that efficiency. Another challenge we are actively working to overcome is the heat shield that will be needed to protect the returning capsule as it passes through Earth's atmosphere.

The last sentence is what gets me the most. I know the atmosphere is the most effective way we have to shed excess velocity, and if something made it back to Earth from Mars it would have to be going pretty darned fast. But are there other alternatives? Could a Mars mission instead shed its velocity with an anti-gravity-assist on the Moon or some other means and proceed to dock up with the ISS? Or is that simply too much excess velocity to shed given mission constraints...

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    $\begingroup$ An entry heat shield is not that much of a challenge. It would be far greater challenge to do something different. $\endgroup$ – Mark Adler Feb 8 '16 at 19:47
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No. Not with a lunar gravity assist for LEO at least. Moon's average orbital speed is about 1 km/s. Mars return for a rendezvous mission to Mars has an Earth reentry speed of about 11.4 km/s and more, depending on exact chosen trajectory. Since the Moon could never offer large declination angle for a retrograde gravity assist at such large inbound hyperbolic excess velocity (it's surface escape velocity is only ~ 2.4 km/s, decreasing in inverse square relationship to its radius as you select a higher periselene for your gravity assist flyby), it could only offer a reduction of a tiny fraction of its orbital speed for Earth reentry.

Earth's atmosphere could be used for aerocapture into elliptical orbit and later propulsive circularization at apogee, but this hasn't been done yet at such high entry speeds and at Earth. See Could lunar mining be used to service Low Earth Orbit? and How can aerobraking be used to enter high orbit without landing? where this is discussed a bit more.

There's also been some work done on Mars sample return capture trajectory for later retrieval from lunar trailing orbit, following Earth propulsive flyby and lunar gravity assist (see Mars Sample Return Using Commercial Capabilities: Concept for the Earth Return Vehicle Trajectory and Related Capture Requirements, N. Faber et al., NASA Ames, PDF), but that's not LEO either.

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  • $\begingroup$ Are there any other (perhaps longer) alternatives, like Venus or the Sun? $\endgroup$ – corsiKa Feb 8 '16 at 19:57
  • $\begingroup$ Doesn't help. Basically, you can't fall towards the Earth from outside its Hill sphere with your speed being less than ~ 10.8 km/s at ISS orbital altitude, even if you've removed all hyperbolic excess velocity first. There's a couple of ways to get to that same number, one is simply using escape velocity $v_e=\sqrt{2GM/r}$ at target orbital altitude. You'll still have to remove at a bare minimum of about 3,173 m/s and then circularize from eccentricity of ~ 0.02 to basically 0 (about 0.0005 for ISS). First part can be done with aerobraking, second with a not too delta-v heavy impulse burn. $\endgroup$ – TildalWave Feb 8 '16 at 21:25

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