QUESTION:
Can the Hubble Space Telescope learn anything about the planet Earth?
Do they ever turn it around and take a look at home?

ANSWER from Bill Hathaway on April 18, 1996:
This has two answers - yes and no.

Yes, the HST often, indeed routinely, looks at the Earth.  We don't even
need to "turn it around" to do so.  Suppose it is looking at a star or
galaxy or other celestial target - since the telescope orbits around the
Earth approximately every 96 minutes, for about half that time, the Earth
blocks our view of that part of the sky as it moves between the telescope
and the target.  If we are making a short observation and complete it in
the 45 or so minutes that is available, we will normally turn the telescope
to point to a new target at a different part of the sky that may not be
blocked at that particular time.  However, if the observation requires
additional orbits, we will be spending as much as half of each orbit pointing
right at the Earth while waiting for it to move out of the way.

Most of the time none of the cameras or spectrographs are actually turned on
during this time of Earth occultation as we call it.  The bright light would
damage the more sensitive instruments which are designed to only look at
very faint sources.  But, one of the more important cameras, the Wide Field
and Planetary camera (WFII) uses CCD detectors (Charged Coupled Displays),
which can handle bright light sources without damage.  In fact to properly
use a CCD requires that they be exposed fairly often to a uniform light
source.  This allows us to measure their sensitivity and calibrate their
response to light.  This is essential for knowing how they behave during
normal observation.

So the people who monitor the sensitivity of the WFII actually NEED
the camera to be turned on while the HST is pointed at the Earth,
which is used as a 'flat-field' source of light.  We schedule such
observations several times each week, using several of the different
filters to provide a full wavelength coverage.  We don't have to even
tell the telescope to turn - we just look for times when it is already
looking at the Earth (waiting for the sky to return to clear view) and
schedule an "Earth Flat" observation at those times.

So now the NO part of the answer.  Since the spacecraft is moving so fast
around the Earth, any observation of the land or oceans or clouds zipping
past underneath it is completely blurred.  Even with the shortest exposures
(less than one second in time), no details on the Earth's surface can be seen.
So we can not learn about the Earth by looking at it this way.  We merely
use it to determine how the cameras are operating.

If someone could think of a way to make the pointing of the telescope
also move as fast as the Earth is moving below it, it could be possible
to follow a particular feature on the surface of the Earth and obtain
a picture of it.  Earth observation satellites, such as weather and
spy satellites are designed to follow the motion of the Earth underneath
and can do a far better job of making clear Earth pictures than HST
could.  To do this with HST, the person proposing to use it in this
way would also have to convince the Space Telescope Science Institute
that using the HST is the only way such a picture could be taken and
that it had important scientific merit.  Every observation approved
for the HST must satisfy these criteria.  Since better observations
can be done with those other satellites designed for that purpose,
none are ever done with HST.