First manned mission to another planet

[Alltimegreat1]

Everyone talks about Mars as if it's the only logical choice. What about Mercury? It's about the same distance from Earth as Mars and the timing of the return trip back to Earth would more flexible. I read the high northern latitudes of Mercury have moderate temperatures and are near water ice. Why is a mission to Mercury never discussed?

 

[fresh_42]

Everyone talks about Mars as if it's the only logical choice. What about Mercury? It's about the same distance from Earth as Mars and the timing of the return trip back to Earth would more flexible. I read the high northern latitudes of Mercury have moderate temperatures and are near water ice. Why is a mission to Mercury never discussed?

How do you plan to shield the enormous radiation?

 

[rootone]

Mercury has almost no atmosphere at all, which limits the methods available for soft landing on it, and also makes setting up a habitable shelter more difficult.
There is less chance of useable water resources being found close to landing site.
Some ice in small amounts may be present at the poles but Mars has a lot of water ice at the poles and quite likely there could be subsurface water in other parts.
Dangerous solar radiation is way stronger than on Mars, (again with no atmosphere to moderate it.)
Finally it's gravitationally locked to the Sun which has the result that some areas are almost permanantly exposed to Sun and have a ground temperature of around 500C, while other area are almost permanent night with a temperature around -200C, so much of the surface is unsuitable to land on due to that.

 

[Alltimegreat1]

Fair enough. I suppose they'd want to land near the Martian equator then because it's less cold there.

 

[rootone]

I'd say a site near the equator would be more generally favoured, and especially so if the existence of subsurface water was proven.

 

[Vanadium 50]

It's about the same distance from Earth as Mars

It's not the distance, it's the energetics. Mercury is moving very fast - your spaceship needs to gain a lot of energy. That's why a flight to Mercury (orbit) takes 7 years.

 

[fresh_42]

It's not the distance, it's the energetics. Mercury is moving very fast - your spaceship needs to gain a lot of energy. That's why a flight to Mercury (orbit) takes 7 years.

That's what I've always wondered about: why can't the sun's gravity be used?

 

[Vanadium 50]

What is the velocity of the sun with respect to the sun?

 

[fresh_42]

What is the velocity of the sun with respect to the sun?

Thank you. (Blind side hit.)

 

[Alltimegreat1]

It's not the distance, it's the energetics. Mercury is moving very fast - your spaceship needs to gain a lot of energy. That's why a flight to Mercury (orbit) takes 7 years.

It is necessary to get into Mercury's orbit before attempting to land there?

 

[Janus]

That's what I've always wondered about: why can't the sun's gravity be used?

It's because of the Sun's gravity that it harder to get to Mercury than it is to get to Mars. In order to enter an orbit that will intersect with Mars, you need to gain 2.96 km/sec. To intersect with Mercury you have to shed 7.51 km/sec*. With rockets it takes fuel to shed velocity just as much as it does to gain it.

It is necessary to get into Mercury's orbit before attempting to land there?

If you want to land without crashing you have to match velocity with Mercury, which is the equivalent of getting into the same orbit as it is in. And to do that you have shed over 9 km/sec of the speed you would be moving when you reach Mercury by the method above.

* A rocket that just leaves Earth is still traveling around the Sun at the same speed as the Earth, which is ~30 km/sec and is enough to keep the Earth at its present distance. To get closer to the Sun, you have to lose enough of that velocity to allow you to fall in as far as Mercury. If you don't lose enough velocity, the following happens: you start to fall in towards the Sun, as you do so, you gain speed, this extra speed tends to make you whip around the Sun faster and eventually you swing around the Sun and begin to climb back out before you even reach Mercury's orbit. It is one of the little peculiarities of orbital mechanics that it takes more energy to move in towards the Sun from Earth's orbit that it does to move out away from it. It is easier to launch a rocket completely free of the Solar system than it is to hit the Sun with it.