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I read that it would be more difficult to land (not crash land) on Mercury or even send a spacecraft in orbit around it because of the gravitational pull of the Sun and the lack of planetary atmosphere. Does current technology allow landing a spacecraft on Mercury to study the surface in detail?

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    $\begingroup$ It's true that Mercury is closer in to the sun so requires a higher delta v to fly there directly. (Higher than Mars I believe.) So the question does have points worth answering if anyone would like to have a go. (upvoted.) $\endgroup$ – Andy Jul 20 '16 at 15:22
  • $\begingroup$ I get about 16 km/s from LEO to soft landing on Mercury. And that doesn't take into account Mercury's ~7º inclination or gravity loss during descent. The 100K to 700K temperature swings would be an engineering challenge, I suspect. $\endgroup$ – HopDavid Jul 20 '16 at 15:56
  • $\begingroup$ @HopDavid get down quickly into a crater near the terminator? $\endgroup$ – kim holder Jul 20 '16 at 15:58
  • $\begingroup$ Mercury rotates slowly - only a few metres per second - so you could just keep moving camp. :) $\endgroup$ – Andy Jul 20 '16 at 16:08
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    $\begingroup$ Messenger actually cut back on it's fuel usage considerably by using it's solar panels as solar sails. It still takes quite a bit of fuel for the landing part if nothing else. $\endgroup$ – PearsonArtPhoto Jul 20 '16 at 17:29
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The closest we have done thus far is to orbit a spacecraft around Mercury. This was accomplished by a number of flybys first, of Earth, Venus, and Mercury. The mass of Messenger was about 500 kg, minus fuel. The orbit was still highly elliptical. Per this chart, to land on Mercury from that orbit requires about 4200 m/s, roughly. That is an enormous amount, more than any other spacecraft to date has had!

There has been some work towards a lander, however. The closest to succeeding is the BepiColombo mission, which proposed to send a 7 kg lander to Mercury, known as the Mercury Surface Element. The main mission was approved, but the lander was rejected. Approximately 80% of its mass would be fuel. It would land near a pole, to avoid the temperature extremes that come due to the long days.

Bottom line, it might be possible, but it would be very difficult. Messenger was a low cost mission, and was able to orbit it. A lander was proposed, but nixed, due to it's expense and expected relatively little scientific gain.

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    $\begingroup$ To get from earth orbit to a circular orbit around Mercury, couldn't you use solar-powered ion propulsion? $\endgroup$ – Ben Crowell Jul 20 '16 at 17:14
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    $\begingroup$ That would work, but that wouldn't work for the landing itself. That still would require 3 km/s of fuel... $\endgroup$ – PearsonArtPhoto Jul 20 '16 at 17:16
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    $\begingroup$ To clarify that, powered descent to Mercury's surface has to be done over a period of about 10-15 minutes, whereas an ion drive can deliver 3km/s over a period of months. What could work is an ion drive bus stage to get to Mercury, with a lander that uses a chemical rocket to do orbital insertion and landing. Doable but costly. $\endgroup$ – Russell Borogove Jul 21 '16 at 0:23
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    $\begingroup$ Keeping propellant for a few years not really a problem (witness Juno). But you need a fuel mass fraction of about 80%, as mentioned above, so a half-ton science/power/guidance/engine/landing package means a 2.5 ton craft, which would have to be carried by a still-larger multiple-ion-engine-powered bus stage, which would require a lot of power. On the bright (ha!) side, since it's spending much of its travel time in the inner system, there's lots of solar power to be had... $\endgroup$ – Russell Borogove Jul 22 '16 at 2:46
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    $\begingroup$ Keeping the fuel for that long isn't as much of an issue, except for the fact that it requires a lower ISP fuel, which in tern, as you mentioned, requires a larger fuel mass fraction. The mini lander idea was actually interesting, and the best I think that could be hoped for. $\endgroup$ – PearsonArtPhoto Jul 22 '16 at 14:48

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