Mapping Planets, Moons and Asteroids
Researcher Draws the Solar System’s Topo Maps
If you ever go hiking on an asteroid, you’ll want to pack one of Robert Gaskell’s maps.
Gaskell, a senior scientist at the Planetary Science Institute (PSI), is to our solar system’s frontier what Lewis and Clark were to the American West — the guy producing the most accurate and detailed maps available.
His maps of the asteroid Itokawa are "…the highest resolution description of an asteroid," according to his NASA Exceptional Achievement medal. It’s good to better than 40 cm, a lot better than you could do with your GPS.
Gaskell, who is based in Altadena, Calif., uses a method called stereo-photo-clinometry, or SPC. Just as stereophonic means sound from different directions, stereo-photo means light from different directions, and clinometry means that slopes, or inclines, are being measured. "SPC means finding slopes from the way the surface looks under different illuminations, and once we know the slopes we can find the heights," Gaskell said.
Gaskell has created sophisticated software that combines hundreds of spacecraft images of varying resolution to produce the maps. He’s been developing the software for nearly 25 years, and if you want to map a planet, moon, or asteroid, he’s the guy to ask.
His newest project, which was recently funded by NASA, will create highly accurate maps of the entire surface of Mercury based on images sent back by NASA’s MESSENGER spacecraft. MESSENGER flew by Mercury in January and is scheduled to go into orbit in 2011, after several passes of Earth, Venus and Mercury.
Currently Gaskell is combining images from that flyby with those taken by Mariner 10, which visited Mercury in 1973, to produce initial maps. But the sun angle for the Mariner 10 photos was the same for three flybys and so far there is only one flyby for MESSENGER.
"It won’t be until we get overlapping data from different sun directions that it will really start making a lot of sense," Gaskell said. "It does give a reasonable solution now, but I don’t completely trust it."
Gaskell’s maps not only give scientists useful information about a body’s surface, they also can be used for navigating spacecraft, calibrating spacecraft instruments, and gaining information about the geology, internal structure and past history of an object. The maps could be used to select landing sites and plan future robotic and human missions to locations in our solar system. Selecting the perfect landing site is important for scientific investigations, such as the search for life in our solar system. Having the most detailed information about a planet’s surface as possible is the first step in identifying the most scientifically useful places to land.
In addition to Mercury, Gaskell is mapping eight of Saturn’s moons, including Enceladus, a frigid world punctuated by icy geysers. In October, NASA may use those maps as navigational tools to plot — and possibly adjust — the Cassini spacecraft’s trajectory as it flies past Enceladus.
Lidar data from the NEAR spacecraft, which went into orbit around the asteroid Eros in 2000, is being compared to Gaskell’s maps of its surface, and an error of about 14 meters has been detected. Lidar is a range-finding technology that’s much like radar. But instead of using radio waves, lidar is based on laser light.
"We suspect that these residuals (errors) are primarily due to incorrect spacecraft position," Gaskell said. "In other words, the orbits are not exactly correct. We intend to solve for those orbits and then the new orbital data can be used to more accurately determine the body’s gravitational field."
Once Gaskell’s computers produce maps covering an entire body, they yield a very accurate image of the object’s shape. The moons of Saturn, for instance, have changed orbits during their history and gravitationally interact with one another. Once their shape became fixed, it recorded the tidal stresses at the time they froze, which gives scientists a way of determining the orbital history of the system.
For Io, Jupiter’s highly volcanic moon, mapping its shape provides planetary geologists with part of the data they need to determine what processes may be going on inside its fluid core, which is being heavily torqued by the giant planet’s intense gravitational field.
Despite the four computers in Gaskell’s office that grind out mapping data nearly 24/7, there are thousands of objects in the solar system, and "So far, I’ve barely scratched the surface, if you’ll pardon the expression," he said.
Describing himself as an evangelical stereo-photo-clinometrist, he is sharing his work with others and recruiting more researchers into the long-term effort to map the solar system. Some of those are at the Jet Propulsion Laboratory, The University of Arizona, the Johns Hopkins Applied Physics Laboratory, and USGS.
With so many planets, moons and asteroids to explore and map, "It’s like being in a big candy shop," Gaskell said.