Could we put humans on Mercury?

Factual Questions
22

The other problem is where to land.

Even though Mercury rotates (as pointed out, a 3:2 resonance with it’s “year” since it’s rotation been sort of captured by Venus). The logic would suggest "the poles will always be in twilight and moderately warm.

However, Mercury’s orbit is about 47 to 70 million km radius. the sun is 1.4 million kilometers diameter. The sun will be up to 1.7 degrees across in the sky, about 3 times the apparent diameter of it (or the moon) from earth. Likely the poles will be in perpetual sunset, brighter than twilight and still quite hot. The sun will simply turn around the horizon as the planet rotates. The only real solution would be to find a polar crater deep enough that no matter what direction the “sunset” gets to, the landing zone is sufficiently shielded from the sun that it does not get too hot.

And of course, someone has found a nice piece of real estate.

Of course, we haven’t mentioned solar radiation emissions yet.

As for the energy issue - think of being in orbit at a certain distance from the gravity source as being the amount of potential energy you have. (If you stopped dead and fell, that would be converted to kinetic energy.) The higher (further away) your orbit, the more potential energy you have, but to get there you need to add energy. To maintain that orbit, you have a certain orbital speed. Faster, means higher orbit. slower, lower orbit.

To get to Mars you need to speed up - add energy so that you are going faster. (add too much speed, you get escape velocity where you leave the gravity source - sun - and fly off on a five year mission to seek out intelligent life, to boldly go where no man has gone before ♫ )

To get to Mercury or Venus, you need to shed velocity. Go slower, fall inward, get a smaller orbit.

23

This, at least, isn’t an issue (or rather, it is an issue, but not a significant one): Any planet will spend approximately half of its time on the opposite side of the Sun from us. All that ends up meaning is that you have to choose your launch times such that the planet will be there when you get there. Which is not at all a difficult calculation: It could be done by hand by a first-year student of astronomy, using only techniques known to Kepler.

24

I was under the impression one of the biggest issues of going to Mercury is the amount of energy needed to make a return trip back to Earth and getting out of the Sun’s gravity well.

Also would be that close to the Sun interfere with radio communications in any way?

25

A Mercury mission would be uniquely suited to using a solar sail. The acceleration is low but you have an unlimited maneuver capability so the velocity change requirements aren’t an obstacle. Plus of course that solar sails work better the closer to the sun you go.

26

A solar sail might be good for a probe to Mercury. But solar sails are both slow and low-payload, neither one of which is consistent with a manned mission.

27

The MESSENGER probe communicated with the Earth by radio for several years as it orbited Mercury, so it doesn’t seem like a big issue.

Note that we have never landed anything on Mercury. A manned mission is a long way away, if ever.

28

In theory you can have any payload if the sail is just big enough. As for slow, well compared to what? If you can do a trip in the time considered acceptable for e.g. Mars missions ~280 days each way not counting surface stay. You don’t need a very large acceleration if it’s constant. The challenge is making and unfurling a sail big enough, like miles across, yet thin enough to have little mass. We can’t do both yet but it should be possible.

29

The problem is that a solar sail has a fixed amount of mass per unit; for a 2 mil metallized Mylar (PET) sheet, the area mass density about 70 g/m2. That may not sound like a lot but given that solar radiation pressure at Earth orbit is ~4.5⋅10-6 N/m2, the maximum possible acceleration of the solar sail by itself is ~6.4⋅10-5 m/s2, or 0.0000065 g, which is not enough to even escape Earth’s sphere of influence without additional propulsion. At the orbit of Mercury the radiation pressure jumps up by around a factor of 10 (depends where the planet is in its orbit because of the eccentricity of e=0.205) so you can knock that down by one zero, which still is about the same amount of acceleration as staring at someone really hard across the room.

Of course, once you start adding any significant payload (i.e. anything larger than a CubeSat) the amount of surface area necessary to get anywhere becomes literally enormous because the mass fraction of the payload has to be tiny to even get and acceleration within the same order of magnitude as the bare sail, notwithstanding all of the structure necessary to keep the sail relatively rigid and transfer load to the rest of the spacecraft which it is pulling along. If you had a spacecraft with a payload mass fraction of 0.1 and a total mass of 1000 kg, the solar sail would have to be almost 13000 m2 to get an acceleration of 0.00000585 g at Earth orbit; big and awkward, but within the range of feasibility. For some minimum spacecraft that could potentially carry a small crew of humans to Mercury—say, 1000 metric tons—it would have to have a solar sail of almost 150 km2 and all of the associated structure to support it, and it would take a few decades to get from Earth to Mercury, so the spacecraft would somehow have to have essentially 100% recycling of food and water because the mass of consumables that would be required would dwarf the rest of the ship.

For what it is worth, years ago I worked on a proposal effort to develop solar orbiting satellites to provide communications and positioning systems for interstellar missions, and one of the things we looked at were membrane structures for both solar electric power and solar propulsion for stationkeeping. At the time the state of the art of ‘inflatable’ solar arrays was still in its infancy and we ended up eschewing that for onboard nuclear reactors (which killed the proposal since that was never going to happen). Today, deployable solar arrays of a few hundred square meters would be feasible. Station-holding using solar radiation pressure, however, was a complete non-starter, a conclusion I initially disagreed with until a more experienced engineer walked me through just what it would take to build, test, and deploy the required several square kilometers of sail area, and how much mass that would add to a payload that was already pushing the capability of the Delta IV Heavy. Solar sails are great for small payloads that can spend many years languishing between planets, but they don’t scale up to larger payloads or crewed missions well at all.

Stranger

30

That’s all predicated on the sail thickness. You give calculations for 2 mil metallized Mylar which as you point out is hopelessly too thick; something akin to gold leaf or even graphene would be required. As I said we can’t currently make anything that thin in anywhere near the required amounts. But it’s not inherently undoable.

31

Please go back and read the post because it is not “all predicated on the sail thickness”. Regardless of how thin you can make the material (gold leaf has a negligible tensile strength, and even graphene is going to be limited by the strength of the σ bond of the lattice even if you assume you can somehow connect structure to it that wouldn’t concentrate loads) a supporting structure is still needed to be able to orient the sail, control its shape, and attached to the towed payload, all of which would add significant mass. It is easy enough for science fiction authors to hand wave infinitesimally thin sail material, tiny habitat systems with lossless recycling, and orders-of-magnitude greater radiation pressure than the Sun outputs, but in reality these do not physically exist nor are plausible with any extension of existing material or environmental management technology, and it would make no sense to send a crewed mission to Mercury with a propulsion system that would take many years or even decades to get there.

Stranger

32

Could we put humans on Mercury?

If it would afford NASA another least little opportunity to squander the taxpayers’ money, they would certainly give it a try.

33

Squander? You do realize that money’s actually spent on Earth, right? And scientific research is not squandering either.

34

Another point to keep in mind with solar sails is that, while the force on them increases as you get closer to the Sun, the gravitational force from the Sun increases in the same proportion. This isn’t necessarily a deal-killer, since in a realistic solar sail design, you don’t pit the light pressure directly against gravity, but the relative strengths of the two forces is still relevant.

35

Moderator Note

NASA funding and whether or not it’s a waste of money is a political topic, not a factual topic. Feel free to discuss the issue, just not in GQ.

36

Fair enough. Sorry.

37

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

I don’t think that is apt, its not really a case of being able to speed up to go slower when want to get to Mercury. When you are ascending, then yes you can go to fast and get to very far out, and when you are very far out from the sun, you can then be very slow, because you went faster and got yourself a very elliptical orbit…

So better to think about orbit heights. You slow down HERE to reduce your orbit THERE (the other side of the orbit. ) You speed up HERE to increase your orbit height THERE. Best to do these at aphelion and perihelion…

What gemini crew might have done is forgot that a small thruster burn here can sum to a large distance at the other side of the orbit. They’ve got it perfect… but half way around the earth… dam, the error is worse…

You don’t have to do it in the minimum fuel burn way… You can aim at Mercury, do a long burn, and descend (relative to sun gravity well ) at great speed to Mercury… Its a fast way to get there… but it costs a heck of a lot of energy to stop at Mercury when you get close.

Turns out the lowest energy way to do an orbit transfer , eg Earth’s orbit to Mercury, is to get free of earth’s gravity well (can’t avoid that energy requirement) , do a retrograde burn at point that you want to be your new aphelion. ( retrograde means stick your butt into the direction of your orbit, and thrust to slow down. )… then you change the orbit height over there at the new perihelion. You still have to do a retrograde burn when you get near Mercury , but total fuel used is the minimum… Then when you are near Mercury, which you have arranged to be your perihelion too… you do a retrograde burn to again reduce velocity, this will reduce the opposite end of the elliptical orbit… dropping the aphelion to be the same height as Mercury … which means almost circular since thats the same as your perihelion … then you have matched orbit speed with Mercury … you have wiped out your (Mercury) escape velocity …

Now consider what you have done if you have pointed your nose at Mercury and did a burn … its similar to prograde (in direction of orbit.) burn . That makes the other side of the ellipse move further out… it will make your orbits aphelion grow further from the sun, really makes the orbit much more elliptical… like a comet doing a sol flyby… hence the point about the fuel required to stop when you get close to Mercury… if you decide not to stop at Mercury you would have to burn fuel to enter circular orbit around sol… (thus matching speed with earth. ) when you get back at earth’s orbit height… no cheap abort to return home ?

BTW, if you were in orbit around Mercury and want to get back to earth with minimum fuel spend, well jump out to free space near Mercury, and you would have made sure you opposite ( relative to the sun ) of where you want to meet Earth (or earths orbit, if thats a different spot. ) … do a prograde burn (point nose in direction of travel…exactly the direction of travel at the time. ) … this shifts your aphelion out from being at Mercury’s orbit height, to be higher, and you pick up speed and grow aphelion to be up at earth’s orbit height. Because you are at aphelion on a more elliptical orbit, you can know you are slower than earth… but earths gravity is enough to catch you if you are close enough. if you got the ellipse to go past earth and go around the back U bolting around earth… You can drop into earths gravity well… Quadruple check your relative to earth velocity when doing this for real lol. Because if you did miss earth by too great a distance, you will soon drop back toward Sol and then be doing these elliptical orbits of the sun …

38

We used to put humans on Mercury all the time…to cure syphilis.

39

What if instead of using a very light and wide sail to capture the sun’s photons, we used a more compact and sturdy device to capture photons, then blasted it with powerful lasers pre-placed around the solar system?

40

I know NASA funding has been declared off topic, but isn’t it fair to say that the resources necessary for such a feat are far more daunting than any technical obstacles? Not to mention the will necessary to do something so costly with so little foreseeable return on the investment.

41

AIUI, solar sails are useful for propelling things away from the sun. Mercury is closer to the sun than Earth, so how would a solar sail help a vehicle get from here to there? Am I misunderstanding how solar sails work, or how they would be used for this mission?

Also, Mercury is super-hot on the sunny side and super-cold on the far side. So maybe we can find a decent landing spot at the boundary between the two hemispheres, at some spot where Mercury’s horizon shields our landing zone from most of the solar disc. But what about when the vehicle is still in space, at the same proximity to the sun that Mercury is at? Mercury is 35M miles from the sun, Earth is 93M miles, so insolation there is roughly 93^2/35^2 = 7 times what it is near Earth, so somewhere around 10 kW/m^2. What kind of engineering challenges does that pose?