22 SEPTEMBER 2017 • VOL 357 ISSUE 6357 1225 SCIENCE sciencemag.org
GRACE’s demise will leave gap before follow-on can launch
Asentinel of Earth’s climate is going dark. The Gravity Recovery and Cli- mate Experiment (GRACE) has moni- tored minute shifts in Earth’s gravity to reveal the unexpectedly rapid melt of polar ice sheets and the drawdown
of underground aquifers. But it has run a decade beyond its planned life, and one of its
tandem satellites, GRACE-2, is nearly out of
fuel. GRACE will soon make its final science
run, NASA announced last week.
Scientists had hoped GRACE would operate until its successor, the $550 million
GRACE Follow-on (GRACE-FO) mission,
reached orbit. But troubles with a planned
Russian booster delayed GRACE-FO’s launch
to next year at the earliest, on a U.S. rocket.
Meanwhile, GRACE-2’s battery, which stores
solar power while the satellite is in Earth’s
shadow, has deteriorated rapidly, forcing
the satellite to burn through fuel. Engineers
turned off an accelerometer last year to save
power, but the satellite’s data have continued
On 3 September, scientists lost contact
with GRACE-2 after another battery cell
failed; the next day its computer malfunctioned. Four days of feverish work followed
before engineers could reboot the satellite.
NASA has now put GRACE-2 on standby
until mid-October, when it will orbit in full
sun along the terminator, the line between
night and day, until early November—its final
planned science collection.
However small, the gap between GRACE
and GRACE-FO will make it hard to stitch
their data together into a seamless whole,
says Eric Rignot, a glaciologist at NASA’s
Jet Propulsion Laboratory in Pasadena,
California. “It would be an impossible task
to fill the gap,” he says.
A joint U.S.-German effort, GRACE has provided an unprecedented view of the planet’s
water and ice since its launch in 2002. Flying 220 kilometers apart, the twin satellites
constantly monitor their separation, down
to microns, by swapping microwave pulses.
When they approach a more massive feature,
like an ice sheet, its extra gravity 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.
The exquisite mass measurements revolutionized climate science, showing, for
example, that melting polar ice sheets contribute more to sea level rise than the demise
of mountain glaciers. Greenland, GRACE
found, is losing 280 gigatons of ice a year
on average, and Antarctica is shedding
120 gigatons—rates that both seem to
be accelerating. Much 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 colleagues recently used GRACE
to observe a counterintuitive effect of ice
loss in Greenland and Antarctica. The meltwater adds to sea level rise. But the lost ice
also means lost gravity. As a result, sea levels near the ice sheets are actually dropping
while ocean levels half a world away are
goosed. Oceanographers predicted the dynamic, but GRACE confirmed it.
The impending data gap is unfortunate,
but it was never a sure bet that GRACE
would hold out, Velicogna says. And GRACE-FO promises a bonus. It will measure the
distance between the two satellites not just
with microwaves, but also with a laser range
finder, providing even finer mass resolution—
and a sharper eye on a changing Earth. j
For the first time, scientists have used gene-editing techniques on human embryos to probe how they develop. The work suggests that a protein called OCT4 is active earlier in human embryos than in those of mice. But
biologists say the study is more important
as a proof of principle; previous human embryo–editing research has focused instead
on correcting faulty genes.
The new experiments, published this
week in Nature, are also a first test of the
United Kingdom’s carefully crafted embryo-editing research regulations, which require
that researchers undergo a review by a government authority and receive a license
before moving forward. Kathy Niakan, a developmental biologist at the Francis Crick
Institute in London, applied in 2015 to use
the CRISPR editing technique on human
embryos to learn more about the genes active in early development. The researchers
planned to focus first on OCT4, known as a
marker for pluripotent stem cells—cells that
can become all tissues in the body.
Niakan’s group used CRISPR to “knock
out,” or deactivate, the gene that codes for
OCT4 in 37 single-cell human embryos left
over after in vitro fertilization treatments
and donated by couples. Mouse embryos
lacking the protein form mostly placental cells; the cells destined to become the
fetus don’t appear. But in the human embryo knockouts, placental cells also failed to
form, indicating that in humans OCT4 plays
a role in the development of both cell types.
Niakan’s work shows that “you can
do [CRISPR] effectively enough and efficiently enough” to study development,
says Janet Rossant, a developmental biologist at The Hospital for Sick Children
and the University of Toronto in Canada.
Researchers have relied on mouse models
to understand early mammalian development, she adds, but to understand human
development and how it can go wrong, the
real thing may be best. j
CRISPR inactivation of gene
allows developmental study
By Gretchen Vogel
By Paul Voosen
Death watch for climate probe
GRACE’s tandem satellites
monitor melting ice sheets
and groundwater depletion.