Robotic scouts chart gravity 'rivers'
Budgets are tight and probably will shrink as the world economy continues whatever tectonic shifting is underway, but the world's space programs are making their first tentative steps to send humans out of Earth's gravity well. After extended discussion, the International Space Station partnership has decided to send an astronaut and a cosmonaut to the station for a year to practice the kind of endurance space-travel future crews must undertake to reach an asteroid or cismartian space.
At the 63rd International Astronautical Congress in Naples, Italy, at the beginning of the month, there were indications that serious thought is being given to building a pressurized space habitat that can set off into the Solar System from “EML2,” the second Earth-Moon Lagrangian point (AW&ST Oct. 8, p. 26).human-spaceflight chief William Gerstenmaier, who has a knack for poetry unusual in engineers, says they could follow gravity “rivers” into the unexplored continent beyond Earth orbit. They would start from halo orbit around EML2, riding an Orion capsule linked to a spacecraft so new in concept that it doesn't really have a name.
“This is potentially our habitation vehicle that is going to be used to go to an asteroid or go to Mars,” Gerstenmaier says, drawing on work underway in Russia, Europe and Japan as well as in his own organization. “We're just going to place it in a gravity well to learn how it would operate, work on systems development, et cetera, and it may not be the actual [International Space Station] module, but it will be something along those lines. It's not a destination and it's not a space station.”
Lagrangian points are appealing because not much delta-v—propulsive change in velocity—is needed to escape them, which means you don't have to lift as much propellant through Earth's gravity. Mission planners sending probes to comets and asteroids—and, in the Voyager missions, toward the edge of the Solar System—have combined gravity and low-thrust, high-specific-impulse ion propulsion systems to save weight at liftoff. Now the human explorers are preparing to follow the trails they have blazed.
The state of the art in gravity navigation is's Dawn mission, which pulled away from orbit around the large asteroid Vesta (see photo) on Sept. 5 for a 2.5-year transit to the even larger asteroid Ceres. Once there, it will become the first spacecraft to orbit two bodies as its instruments and cameras soak up data scientists can use to refine their theories about the formation of the planets and the history of the Solar System.
“We have ion propulsion,” says Marc Rayman, chief engineer and mission director on the Dawn mission. “It's a totally reliable, flexible system. Without it, this mission would truly have been impossible.”
Like its targets, Dawn is orbiting the Sun. The operations team uses the xenon-ion propulsion system to change the spacecraft speed—and its heliocentric orbit—as it moves through space. With its low thrust, the spacecraft moves more like a sailboat than a powerboat, sending its thrust in the direction that works best with the ever-changing gravity conditions, instead of simply aiming at the target and blasting away. Rayman was project manager on the Deep Space 1 mission NASA launched in 1998 to an asteroid and a comet as a test of xenon-ion propulsion, and he is adding his experience to those of his colleagues in “bootstrapping” their way through the Asteroid Belt.
While there is a lot more empty space in the belt than is generally appreciated, there is a danger of colliding with an object too small to see with telescopes from Earth. On the approach to Vesta, the Dawn team used its cameras to search for previously undetected moons orbiting there that could complicate navigation.
More important is the need to collect a precise understanding of local gravity conditions. Controllers at theswitch off the ion engine periodically for a figurative finger in the wind, to see which way gravity is pulling the spacecraft. Optical navigation data from the spacecraft is combined with its thruster history and ground tracking, and the necessary adjustments are made.
“Once we actually got into orbit, as Dawn revolved around Vesta and Vesta rotated beneath it, we could measure the higher-order gravity terms, looking for in some sense the same kind of thing that was found on the Moon,” Rayman says, referring to irregularities in the two bodies' gravity fields that impact spacecraft orbiting at low altitudes.
Engineers of robotic spacecraft have been perfecting navigation techniques using gravity assists since the beginning of the space age. “We're trying to learn how to use these gravity lanes to maneuver around space with humans,” says Gerstenmaier. “It takes a little bit longer, but it's a much more effective way to move things along. So we're learning from the robotic community.”