re: Question on visibility of orbit changes

Philip Chien (
Sat, 14 Feb 1998 05:28:11 -0400 (Mark Borg) asks:

>I believe that when an interplanetary probe (e.g. the Mars Pathfinder)
>is launched, it is first placed in a parking earth orbit.  Assuming such
>a craft is low enough and large enough to be visible with the naked eye,
>when the time comes for it to leave earth orbit, will an observer on earth
>standing in the right place and at the right time, see the change in
>In fact, right now the Apollo missions came to my mind.  Did someone see
>such events when they left earth orbit?.

As a general rule most planetary probes make at least half an orbit around
the Earth before their upper stage sends them on to their destinations.

They're rather tricky to find in low earth orbit, primarily because each
mission has its own specific trajectory, usually optimized for the highest
C3 performance.  And since we don't usually get keplerian elements ahead of
time, we have to make a 'best guess'.

In addition upper stages are usually fairly small and difficult to spot
even while they're burning.  So you have to be really lucky to get the sun
angles optimized to reflect as much as possible to your location.  In the
case of Mars Pathfinder you're talking about a Star 48 solid motor, not a
very large rocket stage.

An additional problem for myself is that I'm usually much too close to the
launch pad (typically 1 to 5 miles) to see them while they're in their
short orbital stays around the Earth.  Well, I suppose seeing them launch
in person makes up for that ...

As far as a "change in trajectory" only Superman, the Starship Enterprise,
and UFOs make sharp turns in space ... (well, there are RUMINT about secret
DoD projects, but I rarely believe rumors ...)  any orbital changes are
very mild in comparison with getting up there in the first place and it
takes a while before any change becomes apparent to all but the most
precise observers.

As far as Apollo was concerned, the results of the Apollo 13 explosion were
observed by engineers at the Johnson Space Center who viewed the expanding
oxygen cloud from the breeched tank in a telescope.

Neverthess occasionally amateur satellite observers have noticed upper
stage burns for planetary spacecraft, some observers in Australia saw the
Cassini probe's Centaur stage send it out of Earth orbit.  And I believe
Spacewatch even took a time-exposure image which shows two tracks - Cassini
and the Centaur.

Philip Chien, KC4YER
Earth News
world (in)famous writer, science fiction fan, ham radio operator,
all-around nice guy, etc.