QUESTION:
How many interplanetary weather satelites have we sent up that is are 
used in conjunction with the hubble to give us weather data?

ANSWER from Bill Hathaway on 11 June, 1996:
The short answer to this is none.  It would be nice to think that we 
would have sent a fleet of satellites to each of the planets to orbit 
them as weather satellites just like we have for monitoring the Earth's 
weather.  We could imagine TV cable channels (or web pages) that anyone 
could turn to at any time to see the weather on every planet.  This 
has not been done.  

Of course however, we have sent both probes and orbiters to most of 
the planets that have told us valuable information about the weather 
on them.  Mercury had a fly-by probe which was gone long before HST 
went up.  Both Venus and Mars had fly-bys and orbiters which sent back 
much data on its clouds and Jupiter had the Voyager fly-bys.  I believe 
only Jupiter currently has the (Galileo) orbiter which returns information 
on clouds, winds and the like.   HST is actively scheduling observations 
coordinated with Galileo observations to learn much about Jupiter's 
weather.  Saturn, Uranus and Neptune have so far only had the Voyager 
fly-bys.  A Pluto fly-by is planned.  

Again, although not exactly interplanetary weather satellites, we also 
have quite a fleet of earth-orbiting scientific satellites that can 
give us additional information on the planets, such as IUE (ultraviolet 
spectroscopy), XTE (x-rays), ROSAT, Compton Gamma Ray Observatory, 
and so on.  While most of these primarily look at objects outside 
the solar system, be assured that planetary scientists glean any 
possible information available from their solar system observations.  

An interesting exercise would be to search the NASA sites on the 
World Wide Web to create a list of all Earth-orbiting scientific 
satellites that can be used for planetary observations.  Add to 
these the probes that are already out there (Galileo) and ones 
that are planned (Cassini for the Saturn system, etc.) and use 
this to keep an eye on what the planetary meterologists are up to.  
Don't forget the many ground-based telescopes that point to the 
planets.  Amateur astronomers have been the 'weather watchers' long 
before the Space Age began and continue to contribute valuable 
observations.  Even modestly equipped telescopes with modern 
CCDs allow images of astonishing clarity.  Sky & Telescope 
magazine often prints these kind of pictures.  A little known 
fact is that the human eye can pick out detail on planets 
that conventional photography cannot record.  For example 
the first alerts of a storm on Saturn in 1990 (which the 
HST imaged in detail) came from visual observers on Earth.  
So even you can add to the accumulated knowledge of weather on 
other planets with a good quality telescope (need not be large) 
and a trained eye.  Having instruments in space only adds to 
the value of us watching from home, rather than making human 
observers obsolete.