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If you want to fly a spaceship with human passengers as close to the Sun as possible, then what effects would the spaceship have to be designed to counteract in order to keep the passengers alive and how close to the Sun could you get before there would be no way to counteract the effects ?

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Assuming you're in orbit around the Sun (presumably a highly elliptical orbit) you won't feel any force due to gravity. In principle you might feel tidal forces, but for an object the size of a spaceship these are negligable even if you graze the surface of the Sun.

The most obvious problems are the heat from the Sun and the radiation it emits. The radiation is a mixture of electromagnetic radiation and charged particles, both of which are not good for anything relying on it's DNA remaining intact.

It's difficult to do much about the heat because in space the only way you can cool is by radiation. What you'd probably do is surround your spaceship with a mirrored shell and keep a layer of vacuum between the shell and your ship. Even with very good mirroring the shell will heat up, but for a while at least it will keep the heat off your spaceship. The MESSENGER probe in orbit round Mercury uses a reflective shield, and contains internal refridgeration - I don't knw exactly how this works but presumably it uses a radiator on the side of the probe pointing away from the Sun.

There isn't a lot you can do about the radiation except surround your spaceship with a thick layer of lead, and that much lead would be difficult to put into space.

The Solar Probe Plus is planned to get within 4 million miles of the Sun's surface, and this will be the closest we've managed to get any spacecraft. However the SPP doesn't have any human passengers to worry about. I suspect radiation is the real problem for human passengers. Even for a hypothetical manned Mars mission the radiation dose the astronauts would receive is a worry, and the intensity of the radiation goes up as the inverse square of the distance.

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Yes, you would effectively have mirrors on the sun-side of things, but as I understand radiative physics, being close to the sun doesn't mean that you have to thermalize to its temperature. The most solid angle the sun can take up is $2 \pi$, and the other half of your field of view is, indeed, cold. Of course, the best you could do on the dark side is to be a full blackbody, but there's no theoretical limit to how well you could insulate the sun side. With an accurate picture of radiative physics, I think you could theoretically hang out right above the surface at Earth temperature. –  AlanSE Aug 30 '12 at 17:35
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I guess it would depend on how perfect your mirror is. You can cool the mirror and use a radiator on the dark side of your spaceship, and if you have enough power available the efficiency of cooling could be arbitrarily high. All of which is only relevant after you've jettisoned the humans :-) –  John Rennie Aug 30 '12 at 17:40
    
Yes yes, the question comes down to the mirror efficiency, so one may defer to other questions for that part. Naturally, that only covers E&M radiation, whereas charged ion radiation can't be reflected, but could be deflected with electromagnets. Looking at the Solar Space Probe design, I am reminded the 2 pi angle assumes an infinitely thin spaceship. The closer you get to the surface, the more cramped the umbra cone gets. –  AlanSE Aug 30 '12 at 17:50
    
""There isn't a lot you can do about the radiation except surround your spaceship with a thick layer of lead, and that much lead would be difficult to put into space."" Is lead the "ideal mirror" You are looking for? –  Georg Aug 30 '12 at 21:09
    
Negligible tidal forces. That's good to know. Thanks for link to Solar Probe Plus. –  vtt Aug 31 '12 at 3:49
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If you dig deep into the Earth and then wait billions of years for the Sun to turn into Red Giant then the Sun would have come to you. Deep in the Earth avoids radiation.

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If you wait trillions of years for Sun to cool down into a black dwarf then you could fly close without the heat.

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If you fly past the Sun at high velocity, like large percentage of c, then you spend less time near the Sun so less time to heat up and less exposure to radiation so could probably get nearer than a craft flying slower.

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If you get really close just above the surface and a coronal mass ejection hits the ship then the kinetic kick of being hit would probably shatter the ship or at the least give such high impulse g-forces as to splatter the human contents of the ship against the ship walls. Need to have inertial dampeners in the form of a really enormous ship with massive mass.

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If you get really close but have thick enough shielding to avoid radiation, and are travelling fast enough that you don't spend enough time close to heat up too much, then there still could be problem of Sun's magnetic field. If field is strong enough it could disrupt biological function.

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The magnetic fields used in MRI scanners are 5,000 to 30,000 gauss. The very strongest magnetic fields on the Sun (in sunspots) are around 3,000 gauss. So the Sun's magnetic field is not a concern. –  John Rennie Aug 31 '12 at 5:51
    
+1 for comment on field strength. Can't vote cause I clicked twice and the software won't let me vote again. –  vtt Aug 31 '12 at 14:27
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Hi vtt, and a late welcome to the physics exchange! You can edit your answers to update them with further details instead of posting new answers. If you put a * in front of the lines, you can also have an iteration for a better overview. Nice idea waiting for the Red Giant inside earth btw :-7 –  Tobias Kienzler Sep 5 '12 at 19:43
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