[AR] Re: Falcon 9 flight today
- From: Henry Spencer <henry@xxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 9 Oct 2013 15:58:52 -0400 (EDT)
On Wed, 9 Oct 2013, Keith Henson wrote:
> However, I don't think you can orbit in the earth's shadow. You can in
> the sun, i.e., sun synch, but I can't think of a way to stay in shadow.
You can't really stay entirely in the sun either, at least not at
reasonable altitudes. You might think that a "dawn-dusk" sun-synchronous
orbit, with the orbit plane roughly at right angles to the sun axis, would
be in permanent sunlight. However, the precession of the orbital plane
that makes it sun-synchronous rotates the orbit plane around Earth's axis,
not around an axis perpendicular to the sun axis. So in sun-sync LEO,
once a year there is an eclipse season -- typically lasting a couple of
months -- in which the spacecraft passes through Earth's shadow briefly
once an orbit.
(MOST is in an 820km dawn-dusk sun-sync orbit, and eclipse season is a
nuisance -- it would have been easier to make the power budget close if
we'd been able to put the spacecraft in hibernation for eclipse season,
but some important science targets were best viewed then, so that was
ruled unacceptable.)
The only orbit that is entirely in Earth's shadow, sort of, is Earth's L2
point. But it's far enough out that Earth doesn't entirely eclipse the
Sun there. (And spacecraft "at L2" normally are in "halo orbits" around
the point itself, and I believe those are usually wide enough that they're
entirely outside the shadow.)
Henry Spencer
henry@xxxxxxxxxxxxxxx
(hspencer@xxxxxxxxxxxxx)
(regexpguy@xxxxxxxxx)
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