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pavel_lishin ◴[] No.42472143[source]
> Now, you might naively think that it's the easiest thing in the world to send a spacecraft to the Sun. After all, it's this big and massive object in the sky, and it's got a huge gravitational field. Things should want to go there because of this attraction, and you ought to be able to toss any old thing into the sky, and it will go toward the Sun.

Yes, yes, speak orbital dynamics to me!

> The problem is that you don't actually want your spacecraft to fly into the Sun or be going so fast that it passes the Sun and keeps moving. So you've got to have a pretty powerful rocket to get your spacecraft in just the right orbit.

What?! No! I mean, yes, you don't want your spacecraft going right into the sun itself, but that's not the major reason why it's difficult! It's that at launch, the spacecraft is already in orbit around the sun - since it came from the Earth. And left to its own devices, it won't want to "fall" into the sun any more than it already is, any more than the Earth is falling into it. Changing orbital parameters that much is expensive in terms of delta-V!

As I recall, the "cheap" way of getting into a low-enough orbit to get that close to the sun is to counterintuitively first expand your orbit massively, and then do a retrograde burn at the highest point. (But I'm guessing the Parker Solar Probe used gravity assists.)

I wonder if some editor cut a large part of this paragraph.

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Ancalagon ◴[] No.42474111[source]
would a solar sail be a feasible - albeit long time scale - method of getting the delta-v to decrease the orbit? Just point it retrograde and wait a long time?
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1. tifik ◴[] No.42474403[source]
I might be missing something, but here is my thinking... the radiation coming out of the sun would always be perpendicular to your direction of travel around the sun at any given moment, so it would only ever be able to add delta-V and increase your orbit, not reduce it.

Unfortunately you can't do upwind sailing in a vacuum.

That being said, you can still use it for the method described in parent post, but you'd still need a different propulsion method to slow you down at the apogee.

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2. floxy ◴[] No.42474590[source]
You should be able to tilt your mirror/sail at 45°, so that the reflected light heads off in the direction of your travel, so that the momentum it imparts works against your current velocity, slowing you down, and degrading your orbit. Right?
3. emilamlom ◴[] No.42474611[source]
They can be used to decrease orbit as well. Since you just need to bleed off the speed from Earth's orbit, you could angle the sail diagonally so the the reflected light is pushing against your direction of orbit (sort of like how the fins on a pinwheel are angled).

While I was googling, a couple places likened it to tacking into the wind, but that's a different kind of phenomenon that works because of friction and pressure differences.

4. pavel_lishin ◴[] No.42474620[source]
I think that if you're constantly being thrusted radially out, you don't actually gain delta-v or increase your orbit - you just shift it. Your apoapsis increases, but your periapsis decreases.

(It's been awhile since I've played KSP, I could be wrong.)

5. josho ◴[] No.42474644[source]
Sailors have figured this out centuries ago to travel against the wind (called tacking). Some of the same principles apply, like orienting the sail so that photons push against the sail reducing the angular momentum.
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6. andrewaylett ◴[] No.42475155[source]
Tacking works because you have resistance against two media (air and water) which are travelling at different velocities -- you need a keel in the water. Solar sails don't have an analogous second medium.
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7. UniverseHacker ◴[] No.42478680{3}[source]
But they do! The (sort of) analogous second medium is gravity. You can “sail upwind” with a solar sail by angling it to reduce your orbital velocity.