QUESTION:
How big do the craters have to be on Pluto so you can see them in the 
photos?  How many craters did you count so far?

ANSWER from Meenakshi Sahu on April 15, 1996:
Pluto, the solar-system's farthest known planet, is about two-thirds the 
size of the Moon but 15,600 times farther way. Its size on the sky is so 
small that 23,400 Plutos would need to be lined up to match the diameter 
of the Moon. So it is impossible to directly see any significant detail 
on Pluto's surface even if we use the largest ground based telescope. 
Studying Pluto with ground based telescopes has been so hard that although 
Pluto itself was discovered in 1930, its satellite Charon was discovered 
48 years later, in 1978.

In the 1980s, because of the favorable alignment of Pluto and Charon, 
astronomers were able to infer indirectly that Pluto had an icy polar cap 
and significant surface structure. This was done by carefully studying how 
the light from Pluto changed in brightness when Charon passed in front of it. 
It is interesting to note that Pluto is very similar to Neptune's large 
moon, Triton, which about the same size as Pluto and has a very large and 
prominent ice cap.

Earlier this year, using a space telescope, the Hubble Space Telescope (HST)
with the improved optics, astronomers were able to have a good look at 
Pluto's surface. They took snapshots of almost the entire surface of Pluto, 
while it rotated through a 6.4-day period. What these pictures show is that 
Pluto has several bright and dark patches and confirmed the presence of 
the icy polar cap inferred indirectly from the earlier ground based 
observations.

The patches could possibly be craters like those found on the Earth's Moon.
But astronomers think that it is more likely that the bright and dark patches 
are frosts that move across Pluto's surface. Pluto is so far away from the 
Sun that even nitrogen, carbon dioxide and methane partially freeze on 
its surface and cause these bright and dark patches. These patches 
constantly shift depending on how far Pluto is from the Sun. Since these 
bright and dark patches keep moving around and keep changing in size, 
nobody knows exactly how many of them are there or their exact sizes.