GRACE (spacecraft)

GRACE (spacecraft). The Gravity Recovery and Climate Experiment (GRACE) was a joint mission by the National Aeronautics and Space Administration (NASA) in the United States and the German Aerospace Center (DLR). Launched in 2002, this pioneering mission consisted of two identical satellites flying in tandem to make precise measurements of Earth's gravity field. Its data revolutionized the understanding of mass distribution and movement on and within the planet, providing critical insights into climate change, hydrology, and geophysics.

Mission Overview

The GRACE mission was a cornerstone of NASA's Earth System Science Pathfinder program and a collaborative effort with the German Research Centre for Geosciences (GFZ). Managed by the Jet Propulsion Laboratory, its primary goal was to map Earth's gravity field with unprecedented accuracy by measuring the minute changes in distance between the two satellites. This innovative approach allowed scientists to detect temporal variations in gravity caused by the movement of water, ice, and solid Earth. The mission far exceeded its planned five-year lifespan, providing a continuous 15-year dataset that became indispensable for global environmental monitoring.

Spacecraft Design

The twin GRACE satellites, built by Astrium in Friedrichshafen, were identical, each with a mass of approximately 432 kilograms. Their key instrument was a highly precise K-band microwave ranging system, which measured the satellite-to-satellite distance change to within a micron. Each spacecraft also carried a SuperSTAR accelerometer, provided by the French space agency (CNES), to measure non-gravitational forces like atmospheric drag and solar radiation pressure. The satellites maintained their formation using a cold gas propulsion system and were equipped with star trackers and GPS receivers for precise orbit determination.

Scientific Objectives

The core scientific objective was to produce monthly high-resolution models of Earth's gravity field, revealing how mass is redistributed. This enabled the tracking of changes in polar ice sheets like those in Greenland and Antarctica, large-scale groundwater depletion in aquifers such as the Ogallala Aquifer, and mass loss from mountain glaciers. GRACE data also contributed to studies of post-glacial rebound, ocean bottom pressure, and major climate oscillations like the El Niño-Southern Oscillation. By monitoring the movement of water, the mission provided essential data for the Global Climate Observing System.

Orbit and Operations

The satellites were launched from the Plesetsk Cosmodrome in Russia on a Rokot rocket into a near-polar, low-Earth orbit at an altitude of about 500 kilometers and an inclination of 89 degrees. They flew in formation approximately 220 kilometers apart, aligned along the track of their orbit. This configuration allowed the leading satellite to be pulled or pushed by gravity anomalies before the trailing satellite, creating a measurable change in their separation. The mission operations were conducted from the German Space Operations Center in Oberpfaffenhofen, with science data processing handled by the University of Texas at Austin's Center for Space Research and the GFZ.

Data Analysis and Legacy

Analysis of GRACE's ranging data, combined with GPS and accelerometer measurements, yielded monthly gravity maps that showed trends in terrestrial water storage, ice mass balance, and sea level change. Its findings were pivotal in documenting the accelerated melting of the Greenland Ice Sheet and the West Antarctic Ice Sheet. The mission's legacy is continued by the GRACE-FO (Follow-On) mission, launched in 2018. GRACE data remains a fundamental record for institutions like the Intergovernmental Panel on Climate Change and has influenced major scientific programs such as the World Climate Research Programme.

Category:NASA probes Category:Earth observation satellites Category:Spacecraft launched in 2002