For the foreseeable future, anything we could do with a manned mission can be done faster, cheaper and much safer with robots.
There are just too many unknowns in the technology which would be required to send humans. Such a mission must necessarily take at least two years, for orbital reasons. We do not know how to build such a craft which won't break down in that interval.
Worse, we do not have the capability of reasonably building a craft which could carry with it to Mars enough fuel to get back. All the current proposals involve manufacturing the fuel for the return flight on Mars before the return trip begins. This is extremely scary; if it doesn't work, there's no return. And it means that the craft carrying the people must land almost exactly on top of the system which has manufactured the fuel; if they miss by twenty miles, they're dead.
No matter how it's done, it would be grossly expensive. Humans are large and fragile and consume enormous amounts of resources.
Because of this, it's only possible for us to mount a single mission, with a landing at one place. We'd learn a lot about that one place (a radius of at most twenty kilometers around the landing site), but Mars is large and varied. It has the surface area of North and South America combined. Turn it around and assume we were Martians attempting to explore the earth. If our one and only landing happened in the Yukon, how much would we learn about the Mojave Desert, or the Amazon rain forest, or the Mississippi delta?
Robots are better. Robots are smaller, and while it's uncomfortable if we lose one, it's not a tragedy. Robots are current technology; they already work and we've already succeeded with them. They take a one-way trip, and they require no life-sustaining resources while they travel between the planets, except for a small amount of renewable electricity from photocells. They can sustain dozens of gravities during reentry, making reentry far easier to engineer. (And even at that, we've had trouble with that kind of thing.)
Because we can build and send many robots for the cost of a single manned mission, we can land them all over Mars, thus getting a better idea of the entire sphere, instead of a very concentrated look at one microscopic piece of it.
It's been argued that the advantage of humans is versatility, and that if human explorers find something unexpected they have the ability to adapt and investigate it. This argument doesn't wash, because robots can do the same thing. It just takes longer.
If a robot spots something unusual, we can send another robot four or six years later designed specifically to check it out. The result is the same, and I'm not in a hurry.
For the same monetary investment, we can get far more and far better science done with robots than with a manned mission, because we won't be spending vast sums to transport oxygen, food and water to keep fragile humans alive during a long voyage.
And it will be better science, because it will be spread all over the globe.
It will be less glamorous, less spectacular. But those are dreadful reasons for risking the lives of precious humans. And if we send humans, everything has to work perfectly the first time.
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