Which of our solar system’s planets could we actually set foot on without dying with today’s technology?
Well, Mercury and Mars are the only two planets we could conceivably land on without dying a horrible death (one much more quickly than the other), though both propose their own set of challenges we must overcome in order to set foot on another world beyond our own.
First and foremost, it would require an exponential amount of energy to land on Mercury because the spacecraft would have to accelerate to incredible speeds before reaching the innermost planet in our solar system. This is due in part to its close proximity to the sun (the sun’s immense gravity would pull at any orbiting spacecrafts) and its slightly unstable orbit. Since it is almost tidally locked (always showing the same face to one another) to the sun, one side of the planet is very, very hot (reaching temperatures around 427 degrees C [801 degrees F]), while the opposite side is generally very cold. (about -173 degrees C [or -279 degrees F]) Therefore, you would have to find a sliver of the surface where the temperatures are tolerable for humans, which would be very difficult (if not impossible entirely).
Radiation would be a serious hurdle for a manned flight to Mercury, or to any other object in our solar system, for that matter. Even astronauts must travel through the van Allen radiation belts before reaching the moon. Thankfully, our atmosphere and magnetic field ward off most of the dangerous particles to us on Earth.
Venus is a no-go, though it would be much more practical in the long run to colonize than Mercury if the need ever arises to do so. For short-term exploration, the pressures on the planet are too high, paired with a temperature hot enough to melt lead (it’s actually much hotter than Mercury, due to its tenuous atmosphere and the level of CO2 in it) and the fact that it sometimes rains sulfuric acid, with clouds of the hostile gas lurking in the atmosphere. The environment is so volatile that we have only attempted landing a probe there a mere dozen or so times (the soviets did during the space race through the Venera project, which we have written about before). Each of which only lasted a maximum of a few hours after landing and sending information back to Earth.
Beyond Mars, the rest of the planets are composed primarily of gas, which means there is no solid surface to land on. Even if there were – deep, deep within the planet’s atmospheres – you would be crushed by the immense pressures of the atmosphere before you ever reached said solid surface. The moons, on the other hand, are another story. Though, again, each would have their own issues we would have to overcome. Quite obviously the temperatures on each of the moons would be a huge problem, since many of them are very, very cold.