GRACE and GRACE-FO

GRACE and GRACE-FO
Name	GRACE and GRACE-FO
Mission type	Earth observation
Operator	NASA / DLR / GFZ
Mission duration	GRACE: 15 years, GRACE-FO: Ongoing
Spacecraft	2 identical satellites
Launch mass	~500 kg each
Launch date	GRACE: 17 March 2002, GRACE-FO: 22 May 2018
Launch rocket	Rokot / Falcon 9
Launch site	Plesetsk Cosmodrome / Vandenberg Space Force Base
Orbit regime	Low Earth orbit
Orbit inclination	89°
Orbit period	~90 minutes
Apsis	gee

GRACE and GRACE-FO. The Gravity Recovery and Climate Experiment (GRACE) and its successor, the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), are joint United States-Germany satellite missions that have revolutionized the measurement of Earth's gravity field. Led by NASA's Jet Propulsion Laboratory and Germany's DLR and GFZ, these twin-satellite formations provide unprecedented data on mass distribution and movement across the planet. Their observations are critical for tracking changes in groundwater storage, ice sheet mass, ocean currents, and the solid Earth.

Overview and Mission

The primary objective of these missions is to make detailed measurements of Earth's gravity field anomalies by precisely tracking the distance between two identical satellites flying in tandem. This partnership between NASA and the German agencies DLR and the GFZ represents a cornerstone of modern geodesy. The concept relies on detecting tiny changes in the separation distance caused by gravitational pulls from varying mass below, such as mountain ranges or aquifers. This data provides a global "scale" for weighing changes in water, ice, and land mass over time, filling a critical gap in the Earth observation system.

Satellite Design and Technology

Each mission consists of two nearly identical satellites, launched approximately 220 kilometers apart in a near-polar low Earth orbit. The core measurement is made by a highly precise microwave ranging system that monitors the satellite-to-satellite distance, accurate to within a few microns. GRACE-FO also carries an experimental technology demonstration: a laser ranging interferometer developed by a German-United States consortium, including the Max Planck Institute for Gravitational Physics, which provides even greater accuracy. Both spacecraft are equipped with SuperSTAR accelerometers from ONERA to measure non-gravitational forces like atmospheric drag and solar radiation pressure.

Scientific Measurements and Data

The missions produce monthly gravity field maps, revealing temporal variations in Earth's mass distribution. These datasets are processed by science teams at the University of Texas at Austin, the Jet Propulsion Laboratory, and other institutions within the GRACE Science Data System. Key measurements include the total water storage change across major river basins like the Amazon Basin and the Ganges, mass balance of the Greenland ice sheet and Antarctic ice sheet, and shifts in deep ocean currents. The data is archived and distributed by physical oceanography and hydrology data centers such as NASA's Physical Oceanography Distributed Active Archive Center.

Key Discoveries and Applications

These missions have provided landmark insights into the Earth system. They quantified the rapid ice loss from the Greenland ice sheet and West Antarctic Ice Sheet, directly contributing to understanding sea level rise. In hydrology, they have revealed the depletion of major aquifers, including the Ogallala Aquifer and those in North India and California, due to agricultural use. The data is also used to monitor drought severity, as seen in studies of the Murray-Darling basin, and to study post-glacial rebound and large-scale earthquake effects, such as those from the 2004 Indian Ocean earthquake and tsunami.

Mission Timeline and Operations

The original GRACE mission was launched on a Rokot rocket from Plesetsk Cosmodrome in March 2002 and far exceeded its planned five-year lifespan, operating until October 2017. GRACE-FO was launched in May 2018 on a SpaceX Falcon 9 rocket from Vandenberg Space Force Base, ensuring continuity of the critical data record. Flight operations are conducted from the German Space Operations Center in Oberpfaffenhofen and NASA's Goddard Space Flight Center. The end of the GRACE mission was precipitated by the failure of its final battery, while GRACE-FO continues nominal operations.

Future and Legacy

The legacy of these missions is profound, having established the monitoring of mass transport as a essential component of Earth science. They directly informed the design of planned future gravity missions from ESA, such as the Next Generation Gravity Mission. The long-term data record is crucial for climate models maintained by the Intergovernmental Panel on Climate Change and for water resource management by agencies like the United States Geological Survey. The technological heritage, particularly the laser interferometry, paves the way for even more precise future missions in geodesy and planetary science.

Category:NASA probes Category:Earth observation satellites Category:German space program Category:Satellite constellations