We all know that ‘Robots NASA’ is the term which makes us understand the technology which NASA is following this entire time to explore the universe outside. This means rovers, since NASA, ESA, and the Soviet Union have sent automated spacecraft to nearly all the Solar System’s larger celestial bodies.
Rovers have been sent only to the Moon and Mars, and I will consider the prospects of sending them elsewhere.
Robots NASA: Some Of the Developments
Most of the Solar System’s celestial bodies are airless. That makes it challenging to land on them, because one needs rockets to slow down to land properly, as opposed to crashing. Currently, we have only two main kinds of rocket engines, high-thrust, low-exhaust-velocity (chemical engines) and low-thrust, high-exhaust-velocity (electric motors). Pushing up the exhaust velocity makes the rocket use less fuel, but lower thrust means longer to change speed — typically too long for landing on any sizable celestial body. So while one may use an electric engine to travel to such a celestial body, the final bit of the way must be by the chemical engine.
Mercury would be the most difficult to land on, with its low-orbit velocity of 3 km/s. By comparison, the Moon’s is 1.7 km/s, and Jupiter’s four big moons are 1.8, 1.4, 1.9, and 1.7 km/s. Most other airless bodies are much easier.
Of those with atmospheres, the outer planets do not have solid surfaces or even well-defined liquid surfaces. Their atmospheres fade off into a liquid state with increasing pressure. Of the remaining ones, Triton and Pluto have very thin atmospheres, roughly comparable to the Earth’s atmosphere at about 70 km altitude. Venus and Titan, however, have atmospheres thicker than the Earth’s, and one can slow down by hitting the atmosphere, as one does for the Earth.
Now for surface conditions. Mercury and Venus are both inside the Earth’s orbit, and their surfaces are hotter than the Earth’s. Of the two, Mercury would be easier to land on and travel on, since one can land near a pole and not have very hot daytime. A sun shield should work well since the Sun would be near the horizon, and since reflected sunlight and re-radiated heat will not be very great. Venus, however, is hot all over, and at about 450 C. I’ve seen some proposals for some rovers that can survive such temperatures and even some work on transistors that can continue to work at such temperatures, but that’s about it.
Outside the Earth’s orbit, it is much better, since overheating will be much less of a problem. Titan, for instance, has an average surface temperature of about 98 K or -171 C.
Turning to travel to these celestial bodies, there are some big problems. Travel time, velocity change (delta-V), and for the outer solar system, distance are some problems. These problems make it difficult to travel to Mercury and the outer Solar System, and that is why missions to those places have been relatively infrequent compared to missions to Mars.
We can conclude that the technology is developing and is making significant progress, which can even unveil the mystery of warm holes in the future.