Could we put humans on Mercury?

Mars, whatever. What about Mercury? It’s only super hot on the part that faces the Sun. Is this doable?

Super cold on the other side.

Mercury’s Temperature

Mercury’s surface temperatures are both extremely hot and cold. Because the planet is so close to the Sun, day temperatures can reach highs of 800°F (430°C). Without an atmosphere to retain that heat at night, temperatures can dip as low as -290°F (-180°C).

IIRC the cold side is no colder than the moon, so that wouldn’t be a significant technical barrier. I’d guess the toughest part would be getting there - for big chunks of time Mercury is on the other side of the sun from Earth, so the orbital path to get there might be very complicated. But I imagine it could be done, and probably easier than going to Mars - I believe Mercury is usually considerably closer to Earth than Mars is.

But Mercury isn’t tidally locked, is it? So there isn’t a permanent cold side?

Mercury is in a 3:2 resonance with the Sun (which immediately made Niven’s “The Coldest Place” wrong as soon as it was published.)

Though I am curious how cold it would be if it was tidally locked.

Something I’ve learnt here on the SDMB is that it takes a lot of energy to get close to the Sun, more so than going to outer planets. I would think that applies to going to Mercury as well?

The astronauts are going to need some pretty thick Ray Bans.

“The goggles - zey do nozzink!”

It does. Gravitational assists can be used, but they take a lot of time. The MESSENGER probe took 7 years to reach Mercury orbit, with 6 flybys of Earth, Venus, and Mercury. You could cut this with more delta-V in your vehicle, but it’ll probably still take years.

A direct transfer to Mars (~9 months) needs something like 3.6 km/s delta-V, and you can land with pure aerodynamics or (for larger vehicles) somewhere under 1 km/s for landing. For Mercury, you need about 13 km/s just to enter orbit, and another 3 km/s to land. That’s a really crazy amount, which is why gravity assists are used. Unfortunately it means manned missions become much more difficult.

Can you explain, in words of only a few syllables, why that is so? I always thought that the Sun was at the bottom of a super-gravity well that would make it easier to get there than to the outer planets. I know it’s been explained here, but I’ve only been able to understand the bottom line in the discussions.

Nobody’s ever been on the moon at night.

The problem with getting to Mercury isn’t the thermal heating from the Sun (although you’d definitely want a lot of shielding for both that and the solar proton radiation) but actually, as you note, the impulse or change in velocity (Δv) required. The minimum Δv from Low Earth Orbit for a Hohmann trajectory is about 13000 m/s to get to orbit around Mercury versus 5700 m/s to Mars. You might be able to do some fine tuning of this by selecting a particularly optimum Lunar swingby maneuver although I suspect the necessary alignment is not much more frequent than it would be to go to Mars, and your best bet is actually to head first for Venus, which only requires a Δv of 3850 m/s for intercept and borrow some of the lady’s orbital momentum to catch a free ride to Mercury. That is, if you can spare the time, as it will take significantly longer than a direct approach.

The problem is that the Earth is going at a much different speed than Mercury, and you have to lose almost all of that speed to “slow down” enough to catch up to Mercury. Technically, in terms of linear speed Mercury is actually going faster but has much lower orbital momentum. It is a peculiarity of orbital mechanics that to speed up you have to slow down. There is no good way to explain this without mathematics, and it is so counterintuitive that Gemini astronauts had problems maneuvering in orbit, so you just have to accept it as something a primate-derived brain is never really going to intuit. To actually go to the Sun you have to lose all orbital speed, almost 30 km/s at Earth’s orbit about the Sun, so it is literally easier to go to interstellar space than it is to fly into the Sun.

Stranger

Here’s a good layman’s discussion of the challenge with getting into orbit around Mercury. Getting there isn’t the problem; slowing down to enter orbit is the difficult part.

When we send a spacecraft to Mercury, it has to get much closer to the Sun than the Earth is.

But the laws of physics dictate that the spacecraft’s total orbital energy will stay the same unless we find a way to shed some. Reducing the part of orbital energy related to the distance from the Sun automatically increases the part related to speed by the same amount. Consequently, as the spacecraft gets closer to the Sun, it begins to speed up like a car driving downhill.

Speeding up is not necessarily a bad thing. According to Kepler’s third law of planetary motion, Mercury whizzes around the Sun on its tight orbit much quicker than Earth. So, our spacecraft heading to Mercury will have to gain some speed to catch up with the tiny planet.

Unfortunately, without shedding some orbital energy, the spacecraft will pick up too much speed and reach Mercury moving far too quickly to be captured by its gravity and will fly right past.

So it is the case that the Sun is at the bottom of a massive gravity well - but it’s the landing (ie controlled orbit) that is the problem? I’m starting to get it. I think.

There’s an alternative: play a lot of Kerbal Space Program. Games are great at building intuition. It clicks, eventually.

The first piece of understanding is that in space, you have to change your velocity here in order to change your position there. On the surface of Earth, we think of changing position as a kind of immediate effect of movement, but the distances in space are so large that this approximation is no longer true.

Mercury is closer to the Sun than Earth, which means we have to lower our orbit. If we fire a rocket in Earth’s orbit, slowing us down, we don’t change our position here–but we have lowered the spot that we’ll be half an orbit from now. The tricky part is that since we are now falling closer to the Sun, we speed up, and the lowest point will be the fastest.

Assuming we’ve done things right, that lowest point will come close to Mercury (I almost said “intersect Mercury”… hopefully not quite). But orbits repeat, so if we do nothing we’ll rise back up to Earth’s orbit again. To stay at Mercury, we have to fire our rockets again, slowing down. Again, that doesn’t change our current position, but does change where we’d be half an orbit from now. Fire the rocket long enough, and that opposite point will lower from Earth’s orbit down to Mercury.

This is easier at Mars. Vehicles can use the atmosphere to slow down (despite being so thin). But Mercury has no atmosphere and you have to use rockets or a gravity assist.

Both. Lowering your orbit down to Mercury takes delta-V, then entering Mercury orbit (instead of flying past it) takes more delta-V, and then landing from Mercury orbit takes yet more delta-V.

Just for understanding the principal I would recommend Simple Rockets over Kerbal SP (and also over Simple Rockets 2). The learning curve is just faster and the game pretty good too.

I’m not sure it’s impossible to think about why it’s difficult without math. Here’s a silly but hopefully illustrative analogy.

You are wizzing along on a surface on your frictionless bike on a frictionless surface. There is a bottomless well with a lip on it. A dozen metres above your head there is another surface with a hole that corresponds to the placement of the well.

There’s two spots you’d like to attempt to reach. One (we’ll call it Mercury) is a spot 1000m down the well. The other is the surface above you which we will call Mars. In a sense it sounds easier to get to Mercury than Mars because all you have to do to get to Mercury is go down the well, but to get to Mars you have to somehow climb upward.

There’s a few restrictions:

  • You have no pedals, no brakes, and no ropes to lower you down the well gently. You have only rockets.

  • You are going so fast that if you steer to go over the well, you don’t fall down it you just hit the lip and jump over it.

To get to Mars, all you have to do is use a relatively small amount of rocket fuel to speed up enough that when you go over the lip of the well, you fly up a dozen metres.

But to get to Mercury, you need to use substantial rocket fuel just to slow you down enough to fall down the well. And then once you do fall through, you will start heading down to your goal, but by the time you get there, you are going to be falling very fast, so you need to use even more rocket fuel to prevent yourself just wizzing past Mercury down at the 1000m mark.

This is obviously an inaccurate and tenuous analogy, but it does hopefully give the picture. In a frictionless, orbital world, where you are already moving very fast, what makes getting somewhere hard or easy isn’t whether you need to go up or down, it’s how similar your situation is compared to where you want to go. Your goal may be “down” from you, but that doesn’t help you if you are going so fast you can’t fall towards it; and if - even if you can start falling towards your goal - you will end up falling so fast toward it, you can’t stop.

Actually, the problem is that we are falling around the “gravity well” of the Sun so quickly that we cannot fall into it without great effort just as a ball swung at the end of a rope will not spontaneously hit the wielder. People tend to think of massive objects like the Sun as a whirlpool sucking things in, but in fact almost everything in the solar system outside of the Sun are objects that are perpetually stuck in orbit of it, burdened with too much kinetic energy ever fall into it. The Sun itself is expelling mass at the phenomenal rate of almost six million metric tons per second, a tremendous amount of material that it nonetheless will not be affected until long after it fuses most of its hydrogen, while the planets, asteroids, and comets are flung about it by the inextricable laws of orbital mechanics, forced to worship what they can never intimately know (at least, not in the biblical sense).

Landing on Mercury is no big deal; although it has the second highest density of any major known body in the solar system, smaller than Titan or Ganymede, and just not much larger than Earth’s Moon (although with more than twice the surface gravity, almost the same as Mars) it would be almost trivial to land upon due to the lack of atmosphere. But slowing down enough to even cross its orbit, much less avoid plowing into it at a few hundred meters per second, is a delicate dance requiring the expenditure of considerable impulse.

Stranger