For nearly 20 years, Kathleen Howell has been combing the solar system in search of specific points in empty space, called libration points. Using high-speed computers, she now has determined that spacecraft launched from Earth can practically "surf" toward a libration point by simply landing in a kind of gravitational "undertow" that naturally funnels a craft there.
Her efforts could help lower costs of planetary missions. Her work already is being utilized to support studies of the sun and its effects on planetary environments. Such missions may provide advance warning on solar activity that can cripple communications satellites orbiting our planet.
Howell, an associate professor of aeronautics and astronautics at Purdue, studies spacecraft trajectories in the vicinity of libration points, where the gravitational pull from two or more heavenly bodies, plus the centrifugal force from their rotation, cancel each other out. For example, there is a libration point between the Earth and the sun.
Theoretically, a spacecraft placed at a libration point would stay there indefinitely. However, very subtle gravitational tugs from distant planets or an errant asteroid could disturb a craft enough to cause it to drift away, Howell says. But place a spacecraft in orbit near a libration point and you've got a stable venue for making observations and taking data.
"In the late 1970s, when I was working on my dissertation at Stanford University, my adviser was involved in the discovery of orbits around libration points, and that's when the utility of these regions was first realized," Howell explains. "These orbits are very complicated, much more complex than the orbit of a planet around the sun, which is why we often refer to the orbit as 'near' a libration point instead of 'around' a libration point."
Howell will talk about her research on libration point orbits Feb. 10-12 at the American Astronautical Society Spaceflight Mechanics Meeting in Huntsville, Ala.
An orbit near a libration point is considered stable if a spacecraft in orbit needs little or no "station keeping," small nudges from positioning rockets to keep the craft on the right track. Howell has investigated many orbits near libration points throughout the solar system.
"Libration points are located at places in the solar system where very interesting things are happening, such as the environment between the Earth and the sun," Howell says. Spacecraft placed in such regions can gather data over a longer period of time than other missions, such as flybys, and can collect data from various regions of space, without being limited to the space near planets.
Getting into a libration point orbit can be half the fun.
Currently, space missions are planned by calculating a specific trajectory, or path, to a destination, a complicated mathematical process that has to be repeated for each new mission.
But by applying sophisticated mathematics to what is essentially an engineering problem, Howell, in the past three years, has discovered that a spacecraft could "surf" into a libration point orbit. She and her colleagues have found that the complicated gravitational fields in space can form two-dimensional "surfaces" around libration points, surfaces that extend out into space. These surfaces, some of which pass relatively close to the Earth, contain gentle curves and bends, like the surface of a sheet billowing in the wind. If you can get a spacecraft anywhere on the sheet, she says, it will naturally get into orbit around a particular libration point, without having to use fuel to steer the craft. And once you find a sheet, you don't have to perform the calculations to find it again.
"It's kind of like a marble rolling in a funnel," she says. "All I have to do is get on the sheet and I can 'ride' to a libration point. We can also 'shift' from one sheet to another to get to a destination, not necessarily a libration point. This is an entirely new way of planning a mission. It's a tremendous tool that if nothing else will clearly cut down on the computational misery associated with designing libration point mission trajectories."
Howell says that riding a surface would be slower than using high-powered engines to go directly to a destination, such as Jupiter. However, a mission utilizing the surfaces could be a "low-thrust" mission, meaning much less powerful engines would be needed on a craft, less fuel would be consumed on the journey and smaller spacecraft could be used.
Howell and her graduate students are looking for such surfaces in the solar system, determining where they go and then plotting them on a computer. They are among the first to use the surfaces to plan possible future missions being submitted to NASA. For example, they are putting together three surfaces to build a path for a spacecraft that will collect and bring back to Earth samples of material being swept out of the sun.
Eventually Howell would like to provide mission designers with a complete data base of surfaces, enabling them to click on the starting and ending points of a mission and have the computer quickly and automatically plot a trajectory using the sheets. She is working with engineers and mathematicians at the Jet Propulsion Laboratory in Pasadena, Calif., to determine how the surfaces can be used most effectively.
Libration point orbits also could help lower the cost of planetary and exploratory missions. "Instead of launching five individual spacecraft from Earth, which is expensive, we could launch five spacecraft all in one craft and send them to a particular libration point orbit," Howell says. "From there, it would take much less fuel to send each craft off in a different direction, to other libration point orbits or possibly to other planets."
The practicality of having spacecraft in libration point orbits was demonstrated just last month.
Between Jan. 6 and 10, two satellites in libration point orbits between the Earth and the sun, called SOHO and WIND, detected and recorded data from a major space "storm" that originated from a huge eruption on the sun. The disturbance hurled particles and radiation from the sun into space, sweeping past the Earth.
"Solar activity such as this and solar flares can interfere with power stations and disrupt satellites in Earth orbit," Howell says. Earth-orbiting satellites control everything from telephone and television signals to sensitive global positioning systems used extensively for air, land and sea navigation.
"Not only can satellites in this region help us better understand the environment around the sun and Earth, but they also can provide advance warning for possibly crippling communications and power disturbances," Howell says.
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