Dr. Robert Zubrin, 47, an astronautical engineer and founder of
the Mars Society, a group advocating travel there, was having
coffee in Manhattan on a recent morning. Dr. Zubrin, who heads
the Pioneer Astronautics engineering company, which develops
ideas for NASA, had come to New York from his home in suburban
Denver to talk about his latest book, ''Entering Space: Creating
a Space-Faring Civilization'' (Tarcher-Putnam, $24.95). His 1996
volume, ''The Case for Mars,'' had made a sensation in
astronautical circles with an interesting notion for a cut-rate
program for Mars exploration known as the Mars Direct Plan. The
proposed program would use existing technology and would keep
fuel weight at a minimum by having the astronauts manufacture
propellant for the return trip from material on Mars. ''Entering
Space'' elaborates on the concept. Q. In both your books, you
suggest that the United States will enjoy a kind of national
rebirth once it sets up a Mars colonization program. Why do we
need a national rebirth