Scientists Temper Ambitions for GRACE Follow-on Mission

SUMMERLAND KEY, Fla.–


With the twin satellites of NASA’s Gravity Recovery and Climate Experiment (GRACE) showing their age, U.S. scientists advocating a follow-on mission have backed away from their request for a more elaborate version that would use lasers instead of microwaves to map Earth’s dynamic gravity field.

Concerned about a data gap between GRACE and the proposed follow-on mission, the scientists are now pushing for something that more closely resembles the current satellite tandem, whose batteries are beginning to die. The scientists are concerned about losing track of processes such as glacial melting and desertification, which show up in GRACE data as changes in Earth’s gravity field. By not trying to develop the more complex laser system and potentially a new system of thrusters to compensate for atmospheric drag, NASA and potential European partners could have a new set of GRACE satellites ready to launch by 2013 if they decide to go forward with the mission, said Mike Watkins, GRACE project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Earlier this year Watkins led a team that studied options for a GRACE successor.

GRACE advocates were disappointed when the U.S. National Research Council’s most recent list of priorities for new Earth-watching satellites, called the decadal survey, recommended launching a laser-equipped GRACE successor mission between 2016 and 2020.

The five-year-old GRACE satellites are limping into their extended mission with a backup GPS receiver and some battery issues. One of 20 battery cells on one satellite has died and one cell on the other satellite is experiencing intermittent weakness. GRACE still has 19 cells left on each satellite, but no one expects them to last until 2016, and under the decadal survey’s proposed timeline, a gap in gravity readings from space would be all but inevitable.

“Of course, if you’re looking at things like the Greenland ice mass, that’s something you don’t want,” said R. Steven Nerem, an aerospace engineering professor at the University of Colorado, who was part of the NASA study team.

Since its launch in 2002, GRACE has documented a faster-than-expected melting of ice in Greenland and Antarctica. In 2006, glaciologist Isabella Velicogna, now of the University of California, Irvine, used GRACE to show that Antarctica had lost 152 cubic kilometers of ice in a single year – which after three years would add up to the equivalent volume of Lake Erie. Velicogna said her latest readings show that Antarctica is losing about 130 cubic kilometers a year. GRACE’s readings have helped inspire the current policy debate about how best to address global warming, GRACE scientists say.

Watkins, Velicogna and Nerem will appear at the American Geophysical Union meeting in San Francisco Dec. 10-14 to discuss GRACE’s findings. In hopes of making sure the readings continue, GRACE advocates have turned their sights away from the $450 million “Cadillac mission” they had pitched to the decadal survey. They believe they can build and launch new GRACE satellites with the microwave technology for under $300 million, with NASA bearing as little as $200 million in costs if an international partner is brought on board. Shifting to lasers is estimated to add another $30 million; adding a system along with the laser to compensate for atmospheric drag would bring the cost to $450 million, Watkins said.

“We are working closely with NASA and with our European partners to look for options for an earlier flight,” Watkins said.

The existing GRACE satellites fly about 200 meters apart. They pass microwave signals between each other to sense the increase in distance when the lead satellite encounters the gravity tug of a massive geographic feature such as Antarctica or Greenland: the more massive the object, the greater the acceleration, and the greater the change in distance between the satellites. The separation returns to normal when the second craft feels the tug and speeds up. The satellites naturally slow down again after they pass the feature.

GRACE takes up to 30 days to revisit any given geographical feature, and scientists crunch these readings into color-coded maps showing changes in gravity caused by swelling rivers, expanding deserts or shrinking ice cover. The drawback of GRACE is that it cannot measure the gravitational influence of features that are less than about 400 kilometers across, Watkins said.

Laser light has tighter wavelengths than microwaves and would, in theory, produce more accurate distance readings and thus more sensitive gravity measurements.

The experience with GRACE, however, suggests that some of the improvements associated with lasers might be overwhelmed by the effects of large weather features, such as cloud banks, storms and low or high pressure systems, which also have mass. GRACE scientists have not yet figured out how to subtract out weather formations entirely from their gravity readings so that they would be measuring only the mass of the ground or water below.

They try to do that by filtering their readings through daily weather models produced by the European Centre for Medium-Range Weather Forecasts based in Reading, England.

“There’s a limit to how well you can do that, especially in areas where there is not a lot of data, such as over Antarctica,” Watkins said. “Today, if someone said ‘is it worth it to build the laser,’ we would have to think twice. It’s not clear today how to get these other errors out.”