It's because of the thin Mars atmosphere. It is thick enough so that the density varies a fair bit. And also thick enough so that as soon as you hit it, you are committed. You can't orbit just above the surface as they did for Apollo. But it is not quite thick enough to descend all the way by parachute.
Also - we can only control them with a time lag of 8 minutes, (more if Mars is further away). So the whole thing needs to be automated - by the time we hear about it, it's far too late to do anything to fix any issues that turned up.
So - you can't decide exactly where to land on the surface. So - even though we know the surface of Mars pretty well, from orbit, to meters resolution - we can only land on wide flat plains and such like with hardly any craters or cliffs for many kilometers in all directions - and even then is a risk of our rovers landing on a boulder or such like, even if you know they are there, you can't avoid them unless you can arrange to have no boulders at all in the landing ellipse..
This boulder was close to Viking 1, and would have destroyed it if it had landed on it.
Also makes the descent more complicated. You have to have three stages - the aeroshell, the parachute descent - and then airbags or rocket propulsion for the last stage.
And each has to be timed exactly to a fraction of a second.
It's far harder to land on Mars than on the Moon or Earth.
Also historically about 50% of all landers on Mars have failed, including quite a few in recent times. Though NASA has had a remarkable run of success with only 1 out of 8 missions failing (the Mars polar lander). ESA, UK, and Russia all had close to 100% fail rate for Mars (some of the Mars landers sent some data back from the surface before they failed).
And though we do our best, chances are that we will get many more failures when we try to land on Mars because there is so much more to go wrong there.
Also - we can only control them with a time lag of 8 minutes, (more if Mars is further away). So the whole thing needs to be automated - by the time we hear about it, it's far too late to do anything to fix any issues that turned up.
So - you can't decide exactly where to land on the surface. So - even though we know the surface of Mars pretty well, from orbit, to meters resolution - we can only land on wide flat plains and such like with hardly any craters or cliffs for many kilometers in all directions - and even then is a risk of our rovers landing on a boulder or such like, even if you know they are there, you can't avoid them unless you can arrange to have no boulders at all in the landing ellipse..
This boulder was close to Viking 1, and would have destroyed it if it had landed on it.
Also makes the descent more complicated. You have to have three stages - the aeroshell, the parachute descent - and then airbags or rocket propulsion for the last stage.
And each has to be timed exactly to a fraction of a second.
It's far harder to land on Mars than on the Moon or Earth.
Also historically about 50% of all landers on Mars have failed, including quite a few in recent times. Though NASA has had a remarkable run of success with only 1 out of 8 missions failing (the Mars polar lander). ESA, UK, and Russia all had close to 100% fail rate for Mars (some of the Mars landers sent some data back from the surface before they failed).
And though we do our best, chances are that we will get many more failures when we try to land on Mars because there is so much more to go wrong there.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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