Question from ASTRO

Philip Chien (kc4yer@amsat.org)
Thu, 12 Dec 1996 00:20:26 -0500

Willie Koorts <wpk@saao.ac.za> asked for Andres Valencia <avalencia@true.net>:

>According to Eric J. Chaisson in "The Hubble Wars";  HST was down to a
>356 miles orbit when serviced by Space Shuttle Endeavor, STS-61, on December
>4, 1994 ("it had fallen some 25 miles in 43 months [since it was originally
>deployed at 380 miles on April 24, 1990 by the Space Shuttle Discovery,
>STS-31]").

I would take anything which Eric Chaisson says in his book with a grain of
salt.  Eric was the head of education at the Space Telescope institute, and
not somebody who was within the NASA system, or understood how the NASA
system works.  In the words of one person he was "the ultimate outsider who
thought he should have been an insider."

In any case when Hubble was deployed on the STS-31 mission in April 1990 it
was at an altitude of about 331 nautical miles.

By the STS-61 Hubble servicing mission in December 1993 it had slipped down
to 311 nautical miles due to atmospheric drag.  The orbit was raised to
about 321 nautical miles during the mission.  It's important to note the
the orbital raise was done while the new solar arrays were still stowed
against the side of the spacecraft (e.g. undeployed).  Commander Dick Covey
stopped the burn because of coupled loads between Hubble and the shuttle
which were starting to increase.

The planned rendezvous altitude with Hubble on the STS-82 mission next
February is 317 nautical miles.  It's hoped that a reboost can be performed
if propellant is available.  This time, however the solar arrays will be
extended since there are engineering concerns about retracting and
reextending the arrays.  STS-79 and STS-80 tested techniques which uses the
shuttle's lowest force thrusters, the verniers, to 'rock' the shuttle back
and forth for minimal accelerations during the orbital raise.

On the next servicing mission, currently scheduled for STS-103 in December
1999, a reboost will be more critical since the sun will be at its maximum
in its eleven year cycle, which will result in a higher drag factor.  There
are concerns since the rest of the shuttle fleet will be assembling space
station Columbia has been assigned to this flight, and Columbia has less
capabilities due to its higher dry mass.

And OBJ on-topic comment for the Seesat listserver -

During the first servicing mission I saw Hubble pass over the Kennedy Space
Center about an hour after Endeavour's launch.  About 30 minutes later I
saw Endeavour - starting its chase around the globe.  For the entire
servicing mission I would watch each of the EVAs on television, and then go
outside just before dawn to see Endeavour/Hubble come overhead.  After the
second EVA I saw a flashing object just behind Endeavour/Hubble - the
discarded solar array.

And for this upcoming servicing mission no crummy lousy observers in New
Jersey better spot any extra pieces tossed overboard before I do - you have
been warned!


Philip Chien, KC4YER
Earth News - space writer and consultant
note new E-mail address - pchien@digital.net