This artist concept shows Comet Hitchhiker, an idea for traveling between asteroids and comets using a harpoon and tether system. Image credit: NASA/JPL-Caltech/Cornelius Dammrich
Comet Hitchhiker Would Take Tour of Small Bodies
Catching a ride from one solar system body to another isn't easy. You have to figure out how to land your spacecraft safely and then get it on its way to the next destination. The landing part is especially tricky for asteroids and comets, which have low gravitational pull.
A concept called Comet Hitchhiker, developed at NASA's Jet Propulsion Laboratory, Pasadena, California, puts forth a new way to get into orbit and land on comets and asteroids, using the kinetic energy -- the energy of motion -- of these small bodies. Masahiro Ono, the principal investigator based at JPL, had "Hitchhiker's Guide to the Galaxy" in mind when dreaming up the idea.
"Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won't stop to pick you up. Instead of a thumb, our idea is to use a harpoon and a tether," Ono said. Ono is presenting results about the concept at the American Institute of Aeronautics and Astronautics SPACE conference on September 1.
A reusable tether system would replace the need for propellant for entering orbit and landing, so running out wouldn't be an issue, according to the concept design.
While closely flying by the target, a spacecraft would first cast an extendable tether toward the asteroid or comet and attach itself using a harpoon attached to the tether. Next, the spacecraft would reel out the tether while applying a brake that harvests energy while the spacecraft accelerates.
This technique is analogous to fishing on Earth. Imagine you're on a boat on a lake with a fishing pole, and want to catch a big fish. Once the fish bites, you would release more of the line with a moderate tension, rather than holding it tightly. With a long enough line, the boat will eventually catch up with the fish.
Once the spacecraft matches its velocity to the "fish" -- the comet or asteroid in this case -- it is ready to land by simply reeling in the tether and descending gently. When it's time to move on to another celestial target, the spacecraft would use the harvested energy to quickly retrieve the tether, which accelerates the spacecraft away from the body.
"This kind of hitchhiking could be used for multiple targets in the main asteroid belt or the Kuiper Belt, even five to 10 in a single mission," Ono said.
Ono and colleagues have been studying whether a harpoon could tolerate an impact of this magnitude, and whether a tether could be created strong enough to support this kind of maneuver. They used supercomputer simulations and other analyses to figure out what it would take.
Researchers have come up with what they call the Space Hitchhike Equation, which relates the specific strength of the tether, the mass ratio between the spacecraft and the tether, and the change in velocity needed to accomplish the maneuver.
In missions that use conventional propellant, spacecraft use a lot of fuel just to accelerate enough to get into orbit.
"In Comet Hitchhiker, accelerating and decelerating do not require propellant because the spacecraft is harvesting kinetic energy from the target," Ono said.
For any spacecraft landing on a comet or asteroid, being able to slow down enough to arrive safely is critical. Comet Hitchhiker requires a tether made from a material that can withstand the enormous tension and heat generated by a rapid decrease in speed for getting into orbit and landing. Ono and colleagues calculated that a velocity change of about 0.9 miles (1.5 kilometers) per second is possible with some materials that already exist: Zylon and Kevlar.
"That's like going from Los Angeles to San Francisco in under seven minutes," Ono said.
But the bigger the velocity change required for orbit insertion, the shorter the flight time needed to get from Earth to the target -- so if you want to get to a comet or asteroid faster, you need even stronger materials. A 6.2 mile-per-second (10 kilometer-per-second) velocity change is possible, but would require more advanced technologies such as a carbon nanotube tether and a diamond harpoon.
Researchers also estimated that the tether would need to be about 62 to 620 miles long (100 to 1,000 kilometers) for the hitchhiking maneuver to work. It would also need to be extendable, and capable of absorbing jerks on it, while avoiding being damaged or cut by small meteorites.
The next steps for studying the concept would be to do more high-fidelity simulations and try casting a mini-harpoon at a target that mimics the material found on a comet or asteroid.
Comet Hitchhiker is in Phase I study through the NASA Innovative Advanced Concepts (NIAC) Program. NIAC is a program of NASA's Space Technology Mission Directorate, located at the agency's headquarters in Washington. Professor David Jewitt at the University of California, Los Angeles, partnered in this research. JPL is managed by the California Institute of Technology in Pasadena for NASA.
For a complete list of the selected proposals and more information about NIAC, visit:
http://www.nasa.gov/niac
For more information about the Space Technology Mission Directorate, visit:
http://www.nasa.gov/spacetech
Media Contact
Elizabeth Landau Jet Propulsion Laboratory, Pasadena, Calif. 818-354-6425 elizabeth.landau@jpl.nasa.gov
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At Saturn, One of These Rings is not like the Others
The planet Saturn, viewed by NASA's Cassini spacecraft during its 2009 equinox. Data on how the rings cooled during this time provide insights about the nature of the ring particles. Image Credit: NASA/JPL/Space Science Institute
Fast Facts:
› A study suggests the particles in one section of Saturn's rings are denser than elsewhere, possibly due to solid, icy cores.
› The findings could mean that particular ring is much younger than the rest.
When the sun set on Saturn's rings in August 2009, scientists on NASA's Cassini mission were watching closely. It was the equinox -- one of two times in the Saturnian year when the sun illuminates the planet's enormous ring system edge-on. The event provided an extraordinary opportunity for the orbiting Cassini spacecraft to observe short-lived changes in the rings that reveal details about their nature.
Like Earth, Saturn is tilted on its axis. Over the course of its 29-year-long orbit, the sun's rays move from north to south over the planet and its rings, and back again. The changing sunlight causes the temperature of the rings -- which are made of trillions of icy particles -- to vary from season to season. During equinox, which lasted only a few days, unusual shadows and wavy structures appeared and, as they sat in twilight for this brief period, the rings began to cool.
In a recent study published in the journal Icarus, a team of Cassini scientists reported that one section of the rings appears to have been running a slight fever during equinox. The higher-than-expected temperature provided a unique window into the interior structure of ring particles not usually available to scientists.
"For the most part, we can't learn much about what Saturn's ring particles are like deeper than 1 millimeter below the surface. But the fact that one part of the rings didn't cool as expected allowed us to model what they might be like on the inside," said Ryuji Morishima of NASA's Jet Propulsion Laboratory, Pasadena, California, who led the study.
The researchers examined data collected by Cassini's Composite Infrared Spectrometer during the year around equinox. The instrument essentially took the rings' temperature as they cooled. The scientists then compared the temperature data with computer models that attempt to describe the properties of ring particles on an individual scale.
What they found was puzzling. For most of the giant expanse of Saturn's rings, the models correctly predicted how the rings cooled as they fell into darkness. But one large section -- the outermost of the large, main rings, called the A ring -- was much warmer than the models predicted. The temperature spike was especially prominent in the middle of the A ring.
To address this curiosity, Morishima and colleagues performed a detailed investigation of how ring particles with different structures would warm up and cool down during Saturn's seasons. Previous studies based on Cassini data have shown Saturn's icy ring particles are fluffy on the outside, like fresh snow. This outer material, called regolith, is created over time, as tiny impacts pulverize the surface of each particle. The team's analysis suggested the best explanation for the A ring's equinox temperatures was for the ring to be composed largely of particles roughly 3 feet (1 meter) wide made of mostly solid ice, with only a thin coating of regolith.
"A high concentration of dense, solid ice chunks in this one region of Saturn's rings is unexpected," said Morishima. "Ring particles usually spread out and become evenly distributed on a timescale of about 100 million years."
The accumulation of dense ring particles in one place suggests that some process either placed the particles there in the recent geologic past or the particles are somehow being confined there. The researchers suggest a couple of possibilities to explain how this aggregation came to be. A moon may have existed at that location within the past hundred million years or so and was destroyed, perhaps by a giant impact. If so, debris from the breakup might not have had time to diffuse evenly throughout the ring. Alternatively, they posit that small, rubble-pile moonlets could be transporting the dense, icy particles as they migrate within the ring. The moonlets could disperse the icy chunks in the middle A ring as they break up there under the gravitational influence of Saturn and its larger moons.
"This particular result is fascinating because it suggests that the middle of Saturn's A ring may be much younger than the rest of the rings," said Linda Spilker, Cassini project scientist at JPL and a co-author of the study. "Other parts of the rings may be as old as Saturn itself."
During its final series of close orbits to Saturn, Cassini will directly measure the mass of the planet's main rings for the first time, using gravity science. Scientists will use the mass of the rings to place constraints on their age.
The Cassini-Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington.
For more information about Cassini, visit:
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