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Comment by revicon | Hacker News Reader

Regarding slowing down upon arrival, could they enter a low orbit and skim the atmosphere of Mars to reduce velocity?

Definitely not. Back of the napkin shows that to get to Mars in 3 days would need a speed in excess of 200 km/s. That is an order of magnitude larger than Mars escape velocity. Your only chance of being captured by Mars is by pancaking.

Make the first vehicle nothing but a giant mirror, crash it into Mars and then reflect the laser off the debris to slow down the second vehicle?

I think you're having trouble grasping exactly how fast 200km/s actually is.

The "debris" would a fine dust scattered over millions of square kilometers at that speed.

Mars is actually a pretty bad planet to try landing on. It has just enough atmosphere to burn up a spacecraft that enters too quickly, but not enough to use for air-braking (skimming or parachutes).

Given that aerocapture and parachutes have been successfully used for Mars probes in the past, that seems unlikely to be true.

What jobu probably means, is that Mars' atmosphere is too thin for parachutes to slow a reasonably sized craft down to low speeds (that's why MSL's landing required additional rocket propulsion [1]). Compare with Earth: Here, the terminal velocity of a reentering capsule is slow enough for parachutes to take over, which in turn slow it down enough for touchdown.

[1] https://en.wikipedia.org/wiki/Mars_Science_Laboratory

It works great for lighter unmanned missions, but becomes less useful as you increase in mass. NASA has had to come up with some interesting solutions in order to land their rovers. Mars Pathfinder used a giant inflatable airbag to scrub the remaining speed it had when hit the martian surface(Lithobraking.) MSL Curiosity, after it jettisoned it's heat shield and parachutes, used a rocket-powered 'sky-crane' in order to slow down the last 200 MPH for a soft touchdown.

This is one of the reason SpaceX is focusing on rocket-powered landings of boosters and capsules vs. parachutes. This is a vital technology required to be able to put large amounts of mass on the surface of Mars

Landing under a parachute is extremely problematic, but they can still be useful for braking before landing, and there's plenty of atmosphere to brake into orbit from an interplanetary trajectory.

Doing that last bit from a 200km/s arrival speed is going to be challenging to put it mildly, but because of the massive deceleration required and the massive heating.

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