[X] My Assistant

[Guest_Zvezdichko_*]

I read a very interesting article about the problems when it comes to big landers on the Red Planet ( including Mars Sample Return and manned Mars landers ).

http://www.universetoday.com/2007/07/17/th...the-red-planet/

Some quotes:

The major conclusion that came from the session was that no one has yet figured out how to safely get large masses from speeds of entry and orbit down to the surface of Mars. "We call it the Supersonic Transition Problem," said Manning. "Unique to Mars, there is a velocity-altitude gap below Mach 5. The gap is between the delivery capability of large entry systems at Mars and the capability of super-and sub-sonic decelerator technologies to get below the speed of sound."
Plainly put, with our current capabilities, a large, heavy vehicle, streaking through Mars' thin, volatile atmosphere only has about ninety seconds to slow from Mach 5 to under Mach 1, change and re-orient itself from a being a spacecraft to a lander, deploy parachutes to slow down further, then use thrusters to translate to the landing site and finally, gently touch down.

Apollo and Soyuz capsules and the proposed Crew Exploration Vehicle (CEV) will all decelerate to less than Mach 1 at about twenty kilometers above the ground just by skimming through Earth's luxuriously thick atmosphere and using a heat shield.

then:

Parachutes can only be opened at speeds less than Mach 2, and a heavy spacecraft on Mars would never go that slow by using just a heat shield. "And there are no parachutes that you could use to slow this vehicle down,��? said Manning. "That's it. You can't land a CEV on Mars unless you don't mind it being a crater on the surface."

If what's said here is true, we don't have any chance to land in a way we know - using a standart heat shield, parachute or landing thrusters.
That "Hypercone" concept is interesting and surely could be integrated for a Mars Sample Return mission, but what about a human landing mission? Wouldn't this be a challenge even to be launched with the proposed rockets like Ares 5?

[dvandorn]

My suggestion for trying to maintain a high l/d for as long as possible was simply to give yourself a lot of time to continue to bleed off energy. The longer you can stay up in the air and use it to brake, the less time you'll have to spend using rockets to slow you down.

As for very slow approaches to Mars, Doug is right, terminal velocity on Mars is still so high that you continue to have to address the Mach 5 problem. And if you're taling about landing humans there, you must remember that the greatest hazard in going to Mars is going to be from the interstellar radiation environment. Any long-lasting human settlement on Mars (or the Moon, for that matter) is going to need to cover its hab modules with enough dirt to block energetic cosmic rays, and you'll need to spend as little time as you can possibly manage en route. Otherwise, you'll be condemning your crew to an early death.

So, for human transport, you don't want a slow, leisurely trajectory -- you want the fastest possible trajectory you can manage. And, not incidentally, one that clears the van Allen belts as quickly as possible, spending the least amount of time traversing them as possible.

-the other Doug

[mchan]

And if you're taling about landing humans there, you must remember that the greatest hazard in going to Mars is going to be from the interstellar radiation environment. Any long-lasting human settlement on Mars (or the Moon, for that matter) is going to need to cover its hab modules with enough dirt to block energetic cosmic rays, and you'll need to spend as little time as you can possibly manage en route. Otherwise, you'll be condemning your crew to an early death.

Solar outbursts like the one that crippled Nozomi would also not be very pleasant for a human crew. There have always been descriptions of a heavier shielded section of the ship for sanctuary when the high energy particles arrive.

Of course, galactic cosmic rays energies are too high to be effectively shielded against in an interplanetary spacecraft, but isn't that also the case for earth orbiting spacecraft? The Earth's magnetosphere does not do a lot to mitigate high energy cosmic rays, does it? How much worse off is a crew on a 9 month transit to Mars than a crew in a 9 month endurance mission in Earth orbit?

[dvandorn]

Of course, galactic cosmic rays energies are too high to be effectively shielded against in an interplanetary spacecraft, but isn't that also the case for earth orbiting spacecraft? The Earth's magnetosphere does not do a lot to mitigate high energy cosmic rays, does it? How much worse off is a crew on a 9 month transit to Mars than a crew in a 9 month endurance mission in Earth orbit?

Well, Earth's magnetic field does block some galactic cosmic rays -- or more accurately, it deflects some of them. And the mass of the Earth blocks some -- you're close enough to Earth in LEO that it provides a pretty fair shielding effect. The overall exposure is less in LEO than in interstellar space, though not by a huge amount.

The atmosphere is our best protection from them -- it presents enough mass to keep most (though not all, of course) from getting through. And, obviously, the mass of the Earth blocks a lot from any given point on Earth's surface. However, airline pilots have restrictions on how many total hours of flight time they can log in their careers, with flight times near and poleward of the antarctic and arctic circles counting greater in a weighted average. Beyond a certain time frame, airline pilots face unhealthy long-term exposure to cosmic rays. This is why many airline pilots are forced to retire in their 50's -- they've logged as many high-altitude hours as they're allowed for their lifetimes.

I think the maximum NASA is prepared to allow a single human to spend in LEO is on the order of 18 to 24 months -- any longer than that and you accumulate too many rads from cosmic rays. That's why most ISS expedition members are allowed to fly on two and only two long-term expedition crews. Russia has flown some of its cosmonauts for longer periods, and AIUI some of those guys have shown deleterious effects from the radiation exposure.

-the other Doug