🚀 go-pugleaf

In article <1992Oct12.105531.11992@aio.jsc.nasa.gov>, tes@nomad.uucp (Thomas E. Smith [LORAL]) writes:
|> >Forwarded from:
|> >PUBLIC INFORMATION OFFICE
|> >JET PROPULSION LABORATORY
|> 
|> >     Today Mars Observer is about 4.5 million kilometers (2.6
|> >million miles) from Earth, traveling at a speed of about 10,800
|> >kilometers (7,600 miles per hour) with respect to Earth.  The
|> >spacecraft is traveling at a heliocentric velocity of about
|> >119,000 kilometers per hour (74,000 miles per hour).
|> 
|> Only 7,600 mph relative to Earth? Is that right? That's not even escape
|> velocity (~25,000 mph). I also can't think of a way for the craft to be
|> going 7,600 mph relative to Earth, and 74,000 mph relative to the Sun.
|> Can someone clear this up for me?

Objects that want to escape Earth's gravity well ballistically need a high 
initial velocity. This is lost as the Earth pulls them back, until in the 
end they just float gently away, their final velocity being just enough 
for them to carry on out where Earth's gravity is irrelevant. 

We don't launch spacecraft that way though. They might break :-)
They start slow, and burn fuel on the way up, to accelerate them up (and a 
bit along) as gravity accelerates them down. Mars Observer has ended up with
enough to get away, *plus* 7,600 mph.

For wrt think "frame of reference", not "radial". 1 AU = 9.3E7 miles. 
If that was a circle it would be 9.3E7 * 2 * pi = 5.8E8 miles around. 
Go around that in a year, 5.8E8 / 365 * 24 = 66700 mph. If Mars Observer
is cruising ahead of us, pretty much in our orbit for now, we add 7600 mph
wrt Earth to 66700 (Earth wrt Sun) and get 74300 mph (Observer wrt Sun).

Seems reasonable, Alan

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   Maidenhead itself is too snobby to be pleasant. It is the haunt of the
   river swell and his overdressed female companion. It is the town of showy
   hotels, patronized chiefly by dudes and ballet girls. 

   Three Men In A Boat, Jerome K. Jerome, 1889
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