QUESTION:
How do you keep track of "junk" in space so that the Pathfinder or 
Surveyor won't collide with it on its way to Mars?

ANSWER from Pieter Kallemeyn on April 25, 1997:
Medium to large-sized asteriods, when discovered, are observed for a few weeks
or months to determine their actual orbit about the Sun.  This data is
circulated around NASA and the astronomincal community, so we know exactly where
all 'known' asteroids are.  Whenever we design a mission, we check to see if
that mission would fly close to one of these asteroids, and make corrective
measures if we feel the risk of impact is too high.

But that only covers the asteroids we know about.  There are also many smaller
pieces of sub-planetary 'junk', each one in it's own orbit, that we're unaware
of.  Fortunately, the odds of striking a piece of this stuff is extremely
remote.  

Furthermore, spacecraft are usually covered with a blanketing material that
serves to protect the delicate electronics and fuel tanks against an impact from
interplanetary dust and very, very small asteriods (the size of a small pebble). 
This blanketing also helps keep the spacecraft's insides at just the the right
temperature.