[meteorite-list] Comet Hitchhiker Would Take Tour of Small Bodies

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4707

Comet Hitchhiker Would Take Tour of Small Bodies
Jet Propulsion Laboratory
September 1, 2015

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 at jpl.nasa.gov 

2015-287