Twin Spacecraft to Weigh in on Earth's Changing Water
A pair of new spacecraft that will observe Earth's ever-changing water cycle, ice sheets and crust is in final preparations for a California launch no earlier than May 19.
A pair of new spacecraft that will observe our
planet's ever-changing water cycle, ice sheets and crust is in final
preparations for a California launch no earlier than Saturday, May 19. The
Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a partnership
between NASA and the German Research Centre for Geosciences (GFZ), will take
over where the first GRACE mission left off when it completed its 15-year
mission in 2017.GRACE-FO will continue monitoring monthly
changes in the distribution of mass within and among Earth's atmosphere,
oceans, land and ice sheets, as well as within the solid Earth itself. These
data will provide unique insights into Earth's changing climate, Earth system
processes and even the impacts of some human activities, and will have
far-reaching benefits to society, such as improving water resource management.GRACE-Follow On (GRACE-FO) is a satellite mission scheduled for launch in May 2018. GRACE-FO will continue the work of the GRACE satellite mission tracking Earth's water movement around the globe. These discoveries provide a unique view of Earth's climate and have far-reaching benefits to society and the world's population."Water is critical to every aspect of life on
Earth -- for health, for agriculture, for maintaining our way of living," said
Michael Watkins, GRACE-FO science lead and director of NASA's Jet Propulsion
Laboratory in Pasadena, California. "You can't manage it well until you can
measure it. GRACE-FO provides a unique way to measure water in many of its
phases, allowing us to manage water resources more effectively."Like GRACE, GRACE-FO will use an innovative
technique to observe something that can't be seen directly from space. It uses
the weight of water to measure its movement -- even water hidden far below
Earth's surface. GRACE-FO will do this by very precisely measuring the changes
in the shape of Earth's gravity field caused by the movement of massive amounts
of water, ice and solid Earth."When water is underground, it's impossible to
directly observe from space. There's no picture you can take or radar you can
bounce off the surface to measure changes in that deep water," said Watkins. "But
it has mass, and GRACE-FO is almost the only way we have of observing it on
large scales. Similarly, tracking changes in the total mass of the polar ice
sheets is also very difficult, but GRACE-FO essentially puts a 'scale' under
them to track their changes over time."A Legacy of DiscoveriesGRACE-FO will extend the GRACE data record an
additional five years and expand its legacy of scientific achievements. GRACE
chronicled the ongoing loss of mass from the Greenland and Antarctic ice sheets
and mountain glaciers. That wealth of data shed light on the key processes,
short-term variability and long-term trends that impact sea level rise, helping
to improve sea level projections. The estimates of total water storage on land
derived from GRACE data, from groundwater changes in deep aquifers to changes
in soil moisture and surface water, are giving water managers new tools to
measure the impact of droughts and monitor and forecast floods.GRACE data also have been used to infer changes
in deep ocean currents, a driving force in Earth's climate. Its atmospheric
temperature profile data, derived from measurements of how signals from the
constellation of GPS satellites were bent as they traveled through the
atmosphere and received by antennas on the GRACE satellites, have contributed
to U.S. and European weather forecast products. GRACE data have even been used
to measure changes within the solid Earth itself, including the response of Earth's
crust to the retreat of glaciers since the last Ice Age, and the impact of
large earthquakes.According to Frank Webb, GRACE-FO project
scientist at JPL, the new mission will provide invaluable observations of
long-term climate-related mass changes."The only way to know for sure whether observed
multi-year trends represent long-term changes in mass balance is to extend the
length of the observations," Webb said.An Orbiting Cat and MouseLike its predecessors, the two identical
GRACE-FO satellites will function as a single instrument. The satellites orbit
Earth about 137 miles (220 kilometers) apart, at an initial altitude of about
305 miles (490 kilometers). Each satellite continually sends microwave signals
to the other to accurately measure changes in the distance between them. As
they fly over a massive Earth feature, such as a mountain range or underground
aquifer, the gravitational pull of that feature tugs on the satellites,
changing the distance separating them. By tracking changes in their separation
distance with incredible accuracy -- to less than the thickness of a human hair
-- the satellites are able to map these regional gravity changes.A GPS receiver is used to track each spacecraft's
position relative to Earth's surface, and onboard accelerometers record
non-gravitational forces on the spacecraft, such as atmospheric drag and solar
radiation. These data are combined to produce monthly maps of the regional
changes in global gravity and corresponding near-surface mass variations, which
primarily reflect changes in the distribution of water mass in Earth's
atmosphere, oceans, land and ice sheets.In addition, GRACE-FO will test an experimental
Laser Ranging Interferometer, an instrument that could increase the precision
of measurements between the two spacecraft by a factor of 10 or more, for
future missions similar to GRACE. The interferometer, developed by a
German/American instrument team, will be the first in-space demonstration of
laser interferometry between satellites."The Laser Ranging Interferometer is an
excellent example of a great partnership," said Frank Flechtner, GFZ's GRACE-FO project
manager. "I'm looking forward to analyzing these innovative
inter-satellite ranging data and their impact on gravity field modeling."GRACE-FO will be launched into orbit with five
Iridium NEXT communications satellites on a commercially procured SpaceX Falcon
9 rocket from Vandenberg Air Force Base in California. This unique "rideshare" launch
will first deploy GRACE-FO, then the Falcon 9 second stage will continue to a
higher orbit to deploy the Iridium satellites.GRACE-FO continues a successful partnership
between NASA and Germany's GFZ, with participation by the German Aerospace
Center (DLR). JPL manages the mission for NASA's Science Mission Directorate in
Washington.For more information on GRACE-FO, visit:https://www.nasa.gov/gracefoA
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