GRACE (Gravity Recovery and Climate Experiment)
Overview
The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA (National Aeronautics and Space Administration) and DLR (German Aerospace Center). Launched in March 2002, the mission aimed to accurately map variations in Earth's gravity field. GRACE consisted of two identical spacecraft that flew about 220 kilometers apart in a polar orbit 500 kilometers above Earth.
Mission Objectives
The primary objectives of GRACE were to provide a global and high-resolution model of the static and time-variable gravitational field, and to provide a better understanding of Earth's climate, geology, and ocean currents. The mission's data has been used to study various aspects of the Earth system including ocean circulation, ice dynamics, and hydrology.
Satellite Design
The two GRACE satellites, GRACE-1 and GRACE-2, were identical in design. Each satellite was approximately 3 meters long and 1 meter in diameter, and weighed about 500 kilograms. The satellites were equipped with a microwave ranging system, a GPS receiver, and an accelerometer to measure non-gravitational forces such as atmospheric drag and solar radiation pressure.
Operations
The GRACE satellites operated in a nearly polar orbit, flying in tandem about 220 kilometers apart. The distance between the two satellites was precisely measured using microwave ranging. As the lead satellite passed over a region of slightly stronger gravity, it was pulled slightly ahead of the trailing satellite. By measuring these changes in distance, scientists were able to map Earth's gravity field with unprecedented accuracy.
Data Collection and Analysis
Data collected by GRACE has been used to create detailed maps of Earth's gravity field, which have provided new insights into the distribution of mass within the Earth. The data has also been used to measure changes in Earth's ice sheets, sea level, and groundwater reserves. GRACE's measurements have provided a unique view of the Earth system and have led to significant advances in our understanding of Earth's climate and its changes.
End of Mission and Follow-On
The GRACE mission officially ended in October 2017, after more than 15 years of successful operations. A follow-on mission, GRACE Follow-On (GRACE-FO), was launched in May 2018 to continue the legacy of GRACE. GRACE-FO carries the same type of instruments as GRACE and is expected to provide data of similar quality.
Impact and Legacy
GRACE has had a profound impact on our understanding of Earth's climate and its changes. The mission's data has been used in a wide range of research areas, from studying changes in ice sheets and sea level to monitoring changes in groundwater reserves. The mission's legacy continues with GRACE-FO, which is expected to provide valuable data for many years to come.