In 1996, the X PRIZE Foundation was troubled by what it saw as 40 years of stagnation in spacecraft innovation, so they "sought to bring about a radical breakthrough in the advancement of human spaceflight." Eight years later, the foundation sparked headlines around the world when they awarded their $10 million prize to aerospace designer Burt Rutan and Microsoft's Paul Rubin for successfully launching a personal spacecraft into the earth's orbit twice in two weeks.

Now they've set their sites on spurring innovation closer to earth. We learned today from Chris Baskind at Lighter FootStep that the X PRIZE Foundation has formally announced its rules for the new Automotive X PRIZE. The multi-million dollar prize will be awarded to the teams in two categories who build cars that get at least 100 miles per gallon of gasoline (or its equivalent with other fuel types).

The first category is for a "mainstream class" automobile that can carry four or more passengers, has four or more wheels, and has the size and general capabilities that consumers are used to. There is also an "alternate class" category for a two or more person car with no size or wheel limits.

X PRIZE isn't just looking for nifty science fair projects, though. They want the winners' cars to be desirable, cost-efficient, and mass producible. Automotive X PRIZE rules (PDF) state that the winning vehicles cost "must be reasonable enough to justify sales of 10,000 units per year in the intended market(s)", and that it is doubtful that any car priced at more that $75,000-$80,000 will meet this requirement.

Other Automotive X PRIZE requirements include:

• C02 emissions per mile to be no more than 200 g/mi

• Accelerate from zero to 60 in under 12 seconds

• Have a minimum top speed of 100 mph (80 mph in the alternate class)

• Have safety equipment, air conditioning, heat, feedback gauges, lights, horns, and most other common features consumers have come to expect in a vehicle.

X PRIZE will begin accepting registration applications later this year and will announce the prize winners sometime in 2009.