Hubble pointing and Iridium flares

Philip Chien (
Thu, 12 Feb 1998 20:51:46 -0400

Hubble's operating constraints include specific constraints to avoid
pointing close to the Sun, Earth, or Moon.  There's a 50 degree sun
exclusion constraint and 10 degrees for the moon.

If for some reason the software fails and Hubble approaches closer than a
specific value then backup protection goes in to effect.  Sun avoidance
sensors command the door to automatically close if the Sun comes within +/-
35 degrees of the primary axis.  It takes no more than 60 seconds to close
the door.  This probably would only be required if there was a drastic

 (I'm not absolutely certain but I believe the door will automatically
close if it comes too close - e.g. if it's lost gyro control and is in a
safe hold mode where it may not be able to control itself completely).

The limits can be overrode through software control, in fact it's required
to look at Venus or other targets close to the Sun.  Typically the Earth or
moon's edge will be used as an occulting disk.

Hubble's cameras are aimed towards the Earth purposely for 'flat field'
measurements, but these are just blank neutral images -- hardly the
mythical spy satelilte which some have claimed Hubble could be if it was
turned around!

I doubt that the Hubble engineers take Iridium flares in to account when
programming Hubble's software because of the statistical unlikelyhood of
any damage.

Hubble's field of view is *EXTREMELY* narrow.  It's similar to looking
through a soda straw.  Unless an object is almost precisely on Hubble's
long axis - it won't fall within the field of view.  So the odds of having
a flare occur at the wrong moment when Hubble happens to be viewing is
incredibly low.

Neverthless, I will bring this up with a couple of friends in the Hubble
project - more out of curiosity than anything else.

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