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May 4, 2007: On January 16, 2007, a dazzling blue flame blasted across the sands of the Mojave desert. In many respects, it looked like an ordinary rocket engine test, but this was different. While most NASA rockets are powered by liquid oxygen and hydrogen or solid chemicals, "we were testing a methane engine," says project manager Terri Tramel of NASA's Marshall Space Flight Center (MSFC).
Click on the image to play the movie:
Above: Test firing of a 7,500 pound-thrust LOX/methane engine. Image credit: Mike Massee/XCOR Aerospace. [Press release] 
The main engine, built and fired by the NASA contractor team Alliant Techsystems/XCOR Aerospace, is still in an early stage of development and isn't ready for space. But if the technology proves itself, methane engines like this one could eventually be key to deep space exploration.
Methane (CH4), the principal component of natural gas, is abundant in the outer solar system. It can be harvested from Mars, Titan, Jupiter, and many other planets and moons. With fuel waiting at the destination, a rocket leaving Earth wouldn't have to carry so much propellant, reducing the cost of a mission.
Perhaps surprisingly, this flammable gas has never powered a spacecraft before. But now scientists and engineers at Marshall, the Glenn Research Center and the Johnson Space Center are developing LOX/methane engines as an option for the future. "Several efforts are underway, including a rival LOX/methane main engine design by KT Engineering," notes Tramel.
"Methane has so many advantages," continues Tramel. "The question is, why haven't we done this before?"
Consider the following: Liquid hydrogen fuel used by the space shuttle must be stored at a temperature of -252.9°C—only about 20 degrees above absolute zero! Liquid methane, on the other hand, can be stored at the much warmer and more convenient temperature of -161.6°C. That means methane fuel tanks wouldn't need as much insulation, making them lighter and thus cheaper to launch. The tanks could also be smaller, because liquid methane is denser than liquid hydrogen, again saving money and weight.
Methane also gets high marks for human safety. While some rocket fuels are potentially toxic, "methane is what we call a green propellant," Tramel says. "You don't have to put on a HAZMAT suit to handle it like fuels used on many space vehicles."
But the key attraction for methane is that it exists or can be made on many worlds that NASA might want to visit someday, including Mars.
Although Mars is not rich in methane, methane can be manufactured there via the Sabatier process: Mix some carbon dioxide (CO2) with hydrogen (H), then heat the mixture to produce CH4 and H20--methane and water. The Martian atmosphere is an abundant source of carbon dioxide, and the relatively small amount of hydrogen required for the process may be brought along from Earth or gathered in situ from Martian ice.
Traveling further out in the solar system, methane becomes even easier to come by. On Saturn's moon Titan, it is literally raining liquid methane. Titan is dotted with lakes and rivers of methane and other hydrocarbons that could one day serve as fuel depots. Imagine, a methane-powered rocket could allow a robotic probe to land on the surface of Titan, gather geological samples, refill its tanks, and blast off to return those samples to Earth. Such a sample-return mission from the outer solar system has never been attempted.
Above: This false-color radar image shows what researchers believe are lakes of liquid methane on Titan. Credit: NASA/ESA/Cassini. [More]
The atmospheres of Jupiter, Saturn, Uranus and Neptune all contain methane, and Pluto has frozen methane ice on its surface. New kinds of missions to these worlds may become possible with methane rockets.
This first series of desert test firings of the 7,500 pound-thrust main engine was a success, but challenges remain before methane rockets will be ready for use in a real mission. "One of the big questions with methane is its ability to ignite," Tramel says. Some rocket fuels ignite spontaneously when mixed with the oxidizer, but methane requires an ignition source. Ignition sources can be hard to make in the outer solar system where planetary temperatures drop to hundreds of degrees below zero. Tramel and her colleagues at Marshall and Glenn are currently working to assure that the rocket will ignite reliably in all conditions.
Such challenges will be surmountable through NASA's continued efforts, Tramel says, and she believes LOX-methane engines will be used in rockets of the future. The blue flame in the desert was a beautiful first step.
NASA's Future: The Vision for Space Exploration