Rocket Ride Spaceships

Looking at the development of a variety of new safe and fast technologies to propel explorers across the solar system.

more Astronauts Edward White (left) and James McDivitt wait for liftoff inside their Gemini IV spacecraft.

"Mum, are we there yet?"

Every parent has heard that cry from the back seat of the car. It usually begins about 15 minutes after the start of any family trip. Good thing we rarely travel more than a few hundred or a few thousand miles from home.

But what if you were travelling to, say, Mars? Even at its closest approach to Earth every couple of years, the red planet is always at least 35 million miles away. Six months there and six months back - at best.

"Chemical rockets are just too slow," laments Les Johnson, manager for in-space transportation technologies at NASA's Marshall Space Flight Center. "They burn all their propellant at the beginning of a flight and then the spacecraft just coasts the rest of the way." Although spacecraft can be sped up by gravity assist - a celestial crack-the-whip around planets, such as the one around Saturn that flung Voyager 1 to the edge of the solar system - round-trip travel times between planets are still measured in years to decades. And a journey to the nearest star would take centuries if not millennia.

Worse yet, chemical rockets are just too fuel-inefficient. Think of driving in a gas guzzler across a country with no gas stations. You'd have to carry boatloads of gas and not much else. In space missions, what you can carry on your trip that isn't fuel (or tanks for fuel) is called the payload mass - e.g., people, sensors, samplers, communications gear and food. Just as gas mileage is a useful figure of merit for the fuel efficiency of a car, the "payload mass fraction" - the ratio of a mission's payload mass to its total mass - is a useful figure of merit for the efficiency of propulsion systems.

With today's chemical rockets, payload mass fraction is low. "Even using a minimum-energy trajectory to send a six-person crew from Earth to Mars, with chemical rockets alone the total launch mass would top 1,000 metric tons - of which some 90 percent would be fuel," said Bret G. Drake, manager for space launch analysis and integration at Johnson Space Center. The fuel alone would weigh twice as much as the completed International Space Station.