Satgen 549   Planetary Parking  by GM4IHJ   (BID SGEN549)  1999-10-02

The illfated Mars Climate Observer (MCO), was launched on Dec 11th
1998, and was scheduled to arrive in Mars space on 23rd Sept 1999.

The braking rocket was fired that morning, but after passing behind
the planet and into radio silence , nothing was heard when MCO should
have come back into view as it rounded the planet 20 minutes later.
NASA/JPL will no doubt debate the reasons for this failure. But given
that the number of failed Mars missions outnumbers the successes , it
may be useful to review what it is that makes this planetary parking
problem such a difficult one.

Starting from earth MCO took an orbit slightly longer than the
standard 259 day Hohmann transfer via a simple half ellipse around the
Sun. Because of planetary alignments at the start MCO went for a
longer 285 day transit. In which time earth moved about 284 degs round
its orbit and Mars moved 150 degs to finish up with the two planets
separated by about 250 million kms. So radio signals took about 28
minutes for the round trip Earth Mars Earth.

At which point the challenge was to have MCO aligned and going at just
the right speed to enter just inside the edge of the base of a cone
whose tip finished up about 100 kms high in the thin Martian
atmosphere. Only a thin ring just inside the edge of the entry cone
guaranteed safety. Go inside this thin rim and you hit the atmosphere
at too sharp an angle and burn up , go in deeper towards the centre of
the cone and you hit the planet dead centre. Go just outside the cone
and you miss the planet entirely. Equally important, you must get into
this narrow corridor at just the right speed.

How do controllers on earth know enough about MCOs position course and
speed to get this right ? It is far from easy. The vehicle is 250
million kms from earth. The doppler shift from its transmission can be
extrapolated over time to give distance along its track but this is
not very accurate. What you need is distance and direction from Mars,
plus if possible navigation fixes versus the Sun and the fixed stars,
which must come from instruments on MCO. You do not have a pilot there
to give you instant checks. Everything you get back on earth via
telemetry is 14+ minutes late. Any correction you upload takes 14
minutes to action and 28 minutes + before earth observers can see the
effect. 

NASA reports suggests that MCO may have gone too deep or the wrong
speed,inside the entry cone , got too near the planet , and burnt up
as the friction of its passage heated it and broke it apart. 

Perhaps it is still a little early to talk about amateur radio
satellites to Mars.