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Satellites measuring Earth’s melting ice sheets to go dark
By Paul
Voosen
Sep.
15, 2017 , 11:30 AM
A
sentinel of Earth’s climate is going dark. After running for a
decade beyond its planned life, the Gravity Recovery and Climate
Experiment (GRACE) is nearly out of fuel and will soon make its
final science run, NASA announced late
yesterday. The tandem of satellites—called GRACE-1 and
GRACE-2—measure minute shifts in Earth’s gravity to chart flows
of mass across the planet, such as the unexpectedly rapid melt of
polar ice sheets and the drawdown of underground water reservoirs
called aquifers.
Scientists
had hoped GRACE would operate until its successor, the $550 million
GRACE Follow-on (GRACE-FO) mission, reached orbit. But troubles
securing a ride to space have delayed GRACE-FO’s launch until early
2018. Meanwhile, the battery in GRACE-2 used to store solar power has
been deteriorating rapidly, forcing the satellite to burn through
fuel. Engineers turned off an accelerometer last year to keep it
running, but the satellite’s data have continued to degrade.
On
4 September, scientists lost contact with GRACE-2 after another of
its battery cells stopped operating. Four days of feverish work
followed, with scientists steeling themselves for the mission’s
end. But finally, engineers bypassed the satellite’s flight
software, successfully rebooting it. NASA has now put GRACE-2 on
standby until mid-October, when it will run until early November in
full sun on its final planned science collection.
However
small, a gap between the missions will make it more challenging to
stitch their records together into a seamless whole, says Eric
Rignot, a glaciologist at NASA’s Jet Propulsion Lab in Pasadena,
California. There are alternative ways to calculate some of the
measures GRACE provides, so stopgaps are possible. For example,
changes in the mass of the ice sheets can be estimated by using other
satellite data to compare discharges of peripheral ice to snowfall
accumulations. But there is no comparable method to monitor changes
in the mass of glaciers or ice caps, let alone the measures of
Earth’s groundwater and soil moisture that hydrologists derive from
the satellites. “It would be an impossible task to fill the gap,”
Rignot says.
A dynamic duo
A
joint U.S.-German effort, GRACE has provided an unprecedented view of
the planet’s water and ice since its launch in 2002. The experiment
relies on measuring changes in the tug of gravity as the two
satellites orbit the Earth. Flying 220 kilometers apart, the GRACE
satellites constantly monitor their distance from each other with
microwave pulses, down to microns. When the satellites approach a
more massive feature, such as an ice sheet, the enhanced gravity of
that region tugs a little bit more on the first satellite—briefly
widening the distance between the pair—before the second satellite
catches up. The changes in distance can be translated into mass.
This
data revolutionized entire disciplines, such as hydrology, allowing
scientists to document the loss of groundwater due to human
exploitation. GRACE showed that the melting polar ice sheets are
contributing more to sea level rise than the demise of mountain
glaciers. Greenland, it found, is losing 280 gigatons of ice a year
on average, while Antarctica is shedding 120 gigatons—rates that
both seem to be accelerating. GRACE also inspired a similar mission,
NASA’s Gravity Recovery and Interior Laboratory, which probed the
moon’s interior.
There’s
much that can still be done with GRACE’s archival data, says
Isabella Velicogna, a geophysicist at the University of California,
Irvine. For example, Velicogna and her colleaguesrecently
used GRACE data to observe for the first time a strange,
counterintuitive effect:
Melting ice sheets in Greenland and Antarctica are pouring water into
the oceans and adding to sea level rise. But the lost ice also means
lost gravity—and so sea levels in the immediate vicinity of the ice
sheets actually drop, while ocean levels half a world away are
goosed. The dynamic, called sea level fingerprints, had wide
acceptance in the field, but GRACE provided the first direct
confirmation that it was happening.
Although
the data gap is unfortunate, it was never a sure bet that GRACE would
hold out, Velicogna says. And GRACE-FO, essentially a replication of
the first mission, will provide finer mass resolutions by measuring
the distance between the two satellites not
just with microwaves, but with an experimental laser ranging
interferometer.
It’s the same technology that could one day help a planned
satellite constellation capture gravitational waves.
