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Comment by zacharycohn | Hacker News Reader

I read this article this morning. I wonder why they didn't launch one satellite into Orbit A, wait 6 months[1], then launch the second satellite into Orbit A, but from the other side of the sun. Then, once they maneuvered into position, we'd have satellites stationed on both sides permanently.

It seems like that would be a lot faster, and would also ensure we can maintain our full view of the sun for a long period of time.

[1]Or however long it would take to get to the opposite side of the sun, accounting for the first satellite's movements.

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The stereos aren't staying in place, they start in the same orbit as Earth (and thus, with the same orbital speed). So we need to gently speed them up and slow them down. It would be inefficient to have the first probe burn a lot of fuel to stop, burn some fuel so it won't get pulled into the sun, and then 6 months later burn some fuel to resume orbit.

What he's saying is gently slow one down, then six months later gently slow down another. Then there will always be two opposing sats.

Only reason I can think of is that there is either other experiments on-board that they need to recover (solar wind collectors, etc), or that they were both taken up in the same launch vehicle, which would be the lion's share of the expense of this experiment.

Nope, you're not understanding the issue.

If you launch the first sat into Orbit A, it still has the orbital velocity of the Earth. Unless you can slow the first sat down a lot, it will continue to track along in about the same place as the Earth.

Waiting 6 months will accomplish very little in that case. The reason is, in 6 months, the Earth and the first sat will be in nearly the same place.

The key concept is known as "delta-v" (http://en.wikipedia.org/wiki/Delta-v). It's the change-in-velocity allowed by a certain amount of fuel. Here's some more on the STEREO mission trajectory: http://stereo.gsfc.nasa.gov/orbit.shtml It's worth it to watch the movie.

Gravity assist can help a lot, but it also takes a lot of time.

Agreed. I missed it. You'd probably have to wait something like 8 years for a long-term solution. With American ADD, wouldn't count on it actually happening.

Also, the people who design the missions eventually get old and retire. At some point, they have to take the human time scale into account.

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