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164 points pseudolus | 2 comments | | HN request time: 0s | source
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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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1. imglorp ◴[] No.42474628[source]
I'm not wild about the title either. In English, "fly into the sun" implies permanence and they exploited that for title bait.

Better, "closest approach" or even "dip into" would say that Parker will keep doing its job afterwards, maybe even lower the next time!

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2. LorenPechtel ◴[] No.42476018[source]
I don't think it has the delta-v to go lower. Anything it loses to the solar wind on the flyby comes off the apoapsis, not the periapsis. Really messed up mission to Eeloo, returning the capture burn for Kerbin would have been around 2,000m/s and I only had half of that. After many reloads I managed to make it work: I put the encounter distance in the upper atmosphere and waited until the burn would complete a bit past periapsis to light the engine. The booster was destroyed by the heat just after the engine shut down, a good portion of my heat shield burned off but I came out of the encounter slowed just barely enough for capture. I turned around and waited. Every time around my apoapsis would drop, the periapsis stayed almost constant through many orbits. When my apoapsis was low enough I didn't expect to get another orbit I turned back around and went in.