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From Issue #52 September 25, 2014

Fall from GRACE

Two satellites chase each other eternally, but will soon plunge to the ground they so closely monitor.

By Jennifer Mack Twitter icon 

Artist’s depiction of GRACE satellites.

A satellite low on fuel races over the horizon chased by its twin, which is also running on fumes. Flying blind, its purpose is to be buffeted by gravity from the planet below. The twin carefully monitors the orbit of its partner, relaying data to scientists studying the planet’s mass.

The GRACE (Gravity Recovery And Climate Experiment) satellite mission is a long-running NASA project to measure and study Earth’s gravity. The GRACE satellites launched in 2002 and had a five-year operational plan. Twelve years after launch, scientists still receive useful data. This data is the lifeblood of climate science and it’s at risk: the satellites have little life left and a new set of siblings may not launch before they burn up.

Variations in total water storage from normal in the Tigris and Euphrates river basins

A game of cat and mouse

“People have been trying to measure the average gravity field [of Earth] for a long time,” says Michael Watkins, the project scientist for the GRACE mission at NASA’s Jet Propulsion Laboratory (JPL). “The breakthrough was our realization that if we could measure the gravity field well enough, you could see the climate-related changes.”

Watkins notes that satellites are displaced in orbit by mass below, which perturbs their paths. As the mass in a given area changes over time — a river changes course or a basin drains — the differences can be measured slowly over thousands or tens of thousands of passes. But, Watkins says, a single satellite cannot be tracked with enough precision from the ground. His team “came up with the idea of tracking one satellite with another satellite.” GRACE is unique: the only satellites watching something other than the Earth or the sky.

The pair, nicknamed Tom and Jerry, measure the distance between themselves and report the variations back to JPL via a downlink station in Germany. JPL processes the raw data and then ships it to the University of Texas at Austin Center for Space Research (CSR). CSR transforms the gravitational changes into geographic data that can be used to analyze changes over time.

Both surface and ground water have a profound impact on surface mass. By measuring gravity, GRACE is also measuring the movement of water. Research professor Srinivas Bettadpur at CSR says that in an area with normal seasonal rainfall, gravity will increase in strength as the water accumulates, and then decrease with evaporation and runoff. Any one month’s worth of data is a snapshot. But by controlling for normal seasonal changes in the data, scientists can look for longer trends spanning years.

GRACE has delivered a remarkable amount of insight since it was inserted into polar oribt in 2002, with its measurements providing as much as 1,000 times more accuracy than comparable maps that preceded it. The satellites have shown sea-level rise, the thinning of ice sheets, and changes in ocean circulation; examined the aftermath of earthquakes and watched the earth rise as glaciers melt; and helped discover a 500-kilometer-wide impact crater in Antarctica.

That’s about to come to an end.

Earth’s gravity from space as measured by GRACE

The satellite that keeps going

The mission lifetime was designed to be five years, and this far out the team is concerned. Watkins says, “We overdesign a little bit just to make sure we get the guaranteed design life, and how much beyond that we get is a little bit of good design and a little bit of luck both.” With a laugh, he continues, “The fact we’re seven years beyond the design life is a little bit of luck.”

Even with the best planning, missions still depend on the weather. Bettadpur says, “The solar cycle has been relatively benign, so during the course of the GRACE mission we’ve experienced much less drag than we thought we would. That has partly helped [the satellite] stay up in orbit for this long.”

Now, GRACE is closing in on sounding its final notes. “We had enough fuel originally that we could adjust the orbit a little bit, but we’re trying to save that, and make the mission last as long as we can,” Watkins says. Fuel conservation is not the only worry, as GRACE is “having a little bit of battery aging…and we’re trying hard to manage the battery life.” If the batteries hold up, the team expects the fuel to last up to three more years. However, should solar activity increase, so too will drag, burning more fuel and revising that estimate.

GRACE’s replacement is already designed, most elements have been approved, and it is set to launch in August 2017. The new project is called GRACE Follow On (GRACE-FO), nicely avoiding the social-media disaster of a name like GRACE Follow Up. If all goes well, GRACE-FO’s satellites will be fired up and producing a stream of data before the original birds fail. Watkins of JPL says, “We believe that if we nurse the current GRACE along we can eliminate a data gap.”

GRACE-FO has nearly the same blueprints as the current mission, but the team is giving it a few updates in the form of faster computers and other components. GRACE-FO will also carry an experimental package consisting of a laser interferometer, which could increase measurement accuracy by 10 to 100 times over the current system, a microwave unit. Even at the low end, “It should be a very noticeable and significant increase,” Bettadpur says. If it fails to deliver, a duplicate of the original microwave equipment will carry the load.

But it has to get up there in time. While it’s romantic to consider Tom and Jerry chasing each other endlessly in orbit as they report on the world below, this is a critical time for tracking major and minor effects of climate change, and any gap in data could effectively blind scientists to what’s happening on the ground right around them.

GRACE satellites illustration and gravitational map courtesy NASA/JPL. Middle East water map courtesy NASA/UC Irvine/NCAR.

Jennifer Mack is a freelance writer from Phoenix, Arizona. When not writing, she can be found at local craft breweries and hacker spaces.

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