Editing GRACE and GRACE-FO

{{short description|Joint American-German space mission to map Earth's gravitational field}}
{{Infobox spaceflight |auto=all
| name                  = GRACE
| names_list            = GRACE-1 and GRACE-2<ref name="NSSDCA G1">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-012A |title=GRACE 1 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=17 August 2016}}</ref><ref name="NSSDCA G2">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2002-012B |title=GRACE 2 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=17 August 2016}}</ref> <br/> Tom and Jerry<ref name="NSSDCA G1" /><ref name="NSSDCA G2" /> <br /> ESSP-2A and ESSP-2B<ref name="gunters">{{cite web |url=http://space.skyrocket.de/doc_sdat/grace.htm |title=GRACE (ESSP 2) |work=Gunter's Space Page |access-date=10 December 2017}}</ref>
| image                 = GRACE spacecraft model 2.png
| image_size            = 275px
| image_caption         = Illustration of the twin GRACE satellites

| mission_type          = Gravitational science
| operator              = [[NASA]]{{·}}[[German Aerospace Center|DLR]]
| COSPAR_ID             = {{unbulleted list|2002-012A|2002-012B}}
| SATCAT                = 27391 and 27392
| website               = {{URL|http://www.csr.utexas.edu/grace}}
| mission_duration      = Planned: 5 years <br /> Final: {{time interval|17 March 2002 09:21|27 October 2017|show=ymd|sep=,}}

| spacecraft_bus        = Flexbus<ref name="gunters" />
| manufacturer          = [[Astrium]]
| launch_mass           = {{convert|487|kg|lb|abbr=on}} each<ref name="presskit-launch">{{cite web |url=https://www.jpl.nasa.gov/news/press_kits/gracelaunch.pdf |title=GRACE Launch: Press Kit |publisher=NASA |date=March 2002 |access-date=11 December 2017}}</ref>
| dry_mass              = 
| dimensions            = {{convert|1.942|xx|3.123|xx|0.72|m|ft|1|abbr=on}}<ref name="presskit-launch" />
| power                 = <!--end-of-life power, in watts-->

| launch_date           = {{start-date|17 March 2002, 09:21}}&nbsp;UTC<ref name="nssdc-orbit" />
| launch_rocket         = [[Rokot|Rokot-KM]] #2<ref name="gunters" />
| launch_site           = [[Plesetsk Cosmodrome|Plesetsk]] [[Plesetsk Cosmodrome Site 133|LC-133/3]]<ref name="gunters" />
| launch_contractor     = [[Eurockot Launch Services|Eurockot]]
| entered_service       =

| declared              = {{end-date|27 October 2017}}<ref name="end-of-science-mission"/>
| decay_date            = {{nowrap|GRACE-1: 10 March 2018,<br />{{in5}}06:09&nbsp;UTC<ref name="decay-1">{{cite web |url=https://www.space-track.org/basicspacedata/query/class/tip/NORAD_CAT_ID/27391/orderby/ID%20DESC/format/html/ |title=Decay Data: GRACE-1 |publisher=Space-Track |date=10 March 2018 |access-date=11 March 2018 |url-access=registration}}</ref>}} <br /> {{in5}}{{coord|-45.9|20.4}} <br /> {{nowrap|GRACE-2: 24 December 2017,<br />{{in5}}00:16&nbsp;UTC<ref name="decay-2">{{cite web |url=https://www.space-track.org/basicspacedata/query/class/tip/NORAD_CAT_ID/27392/orderby/ID%20DESC/format/html/ |title=Decay Data: GRACE-2 |publisher=Space-Track |date=24 December 2017 |access-date=13 February 2018 |url-access=registration}}</ref>}} <br /> {{in5}}{{coord|63.9|-160.9}}

| orbit_reference       = [[Geocentric orbit|Geocentric]]
| orbit_regime          = 
| orbit_semimajor       = {{convert|6873.5|km|abbr=on}}
| orbit_eccentricity    = 0.00182
| orbit_periapsis       = {{convert|483|km|mi|abbr=on}}
| orbit_apoapsis        = {{convert|508|km|mi|abbr=on}}
| orbit_inclination     = 89.0°
| orbit_period          = 94.5 minutes
| orbit_epoch           = 17 March 2002, 04:21&nbsp;[[UTC]]<ref name="nssdc-orbit">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=2002-012A |title=Trajectory Details: GRACE 1 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=23 May 2019}}</ref>
| apsis                 = gee

| instruments           = <!--a list of instruments on the satellite-->
}}

The '''Gravity Recovery and Climate Experiment''' ('''GRACE''') was a joint mission of [[NASA]] and the [[German Aerospace Center]] (DLR). Twin satellites took detailed measurements of [[Earth's gravity]] [[Gravitational field|field]] [[gravity anomaly|anomalies]] from its launch in March 2002 to the end of its science mission in October 2017. The two satellites were sometimes called Tom and Jerry, a nod to the [[Tom and Jerry|famous cartoon]]. The '''GRACE Follow-On''' ('''GRACE-FO''') is a continuation of the mission on near-identical hardware, launched in May 2018. On March 19, 2024, NASA announced that the successor to GRACE-FO would be Gravity Recovery and Climate Experiment-Continuity ('''GRACE-C'''), to be launched in or after 2028.<ref>{{Cite web |last= |date=March 19, 2024 |title=US, Germany Partnering on Mission to Track Earth’s Water Movement |url=https://www.jpl.nasa.gov/news/us-germany-partnering-on-mission-to-track-earths-water-movement |access-date=2024-03-21 |website=NASA Jet Propulsion Laboratory (JPL) |language=}}</ref>

By measuring [[gravity]] anomalies, GRACE showed how mass is distributed around the planet and how it varies over time. Data from the GRACE satellites is an important tool for studying Earth's [[ocean]], [[geology]], and [[climate]]. GRACE was a collaborative endeavor involving the Center for Space Research at the [[University of Texas at Austin]], NASA's [[Jet Propulsion Laboratory]], the German Aerospace Center and [[GFZ German Research Centre for Geosciences|Germany's National Research Center for Geosciences]], Potsdam.<ref>{{cite web |url=http://www.jpl.nasa.gov/news/news.cfm?release=2002-0174 |title=Grace Space Twins Set to Team Up to Track Earth's Water and Gravity |publisher=NASA/JPL |access-date=14 August 2009 |archive-date=5 June 2011 |archive-url=https://web.archive.org/web/20110605041318/http://www.jpl.nasa.gov/news/news.cfm?release=2002-0174 |url-status=dead }}</ref> The Jet Propulsion Laboratory was responsible for the overall mission management under the NASA ESSP (Earth System Science Pathfinder) program.

The principal investigator is Byron Tapley of the [[University of Texas]] Center for Space Research, and the co-principal investigator is Christoph Reigber of the [[GFZ German Research Centre for Geosciences|GeoForschungsZentrum (GFZ) Potsdam]].<ref name="overview">{{cite web |url=http://www.csr.utexas.edu/grace/overview.html |title=Mission Overview |publisher=University of Texas |date=19 November 2008 |archive-url=https://web.archive.org/web/20090515114041/http://www.csr.utexas.edu/grace/overview.html |archive-date=15 May 2009 |url-status=dead}}</ref>

The two GRACE satellites, GRACE-1 and GRACE-2, were launched from [[Plesetsk Cosmodrome]], Russia, on a [[Rockot]] ([[SS-19]] + [[Briz (rocket stage)|Briz]] upper stage) launch vehicle on 17 March 2002.  The spacecraft were launched to an initial altitude of approximately 500&nbsp;km at a near-polar inclination of 89°.  During normal operations, the satellites were separated by 220&nbsp;km along their orbit track. This system was able to gather global coverage every 30 days.<ref>{{cite web |url=https://earthobservatory.nasa.gov/Features/GRACE/page3.php |title=Gravity Anomaly Maps and The Geoid |work=Earth Observatory |publisher=NASA |access-date=14 March 2018 |date=30 March 2004}}</ref> GRACE far exceeded its 5-year design lifespan, operating for 15 years until the decommissioning of GRACE-2 on 27 October 2017.<ref name="end-of-science-mission">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=6984 |title=Prolific Earth Gravity Satellites End Science Mission |last=NASA |website=[[Jet Propulsion Laboratory]] |date=27 October 2017 |access-date=31 October 2017}}</ref> Its successor, [[#GRACE_Follow-On|GRACE-FO]], was successfully launched on 22 May 2018.

In 2019, a [[glacier]] in [[West Antarctica]] was named after the GRACE mission.<ref name="bbc20190607">{{cite news |url=https://www.bbc.com/news/science-environment-48547803 |title=Antarctic glaciers to honour 'satellite heroes' |work=BBC News |first=Jonathan |last=Amos |date=7 June 2019 |access-date=29 September 2019}}</ref><ref name="esa20190607">{{cite web |url=https://www.esa.int/Our_Activities/Observing_the_Earth/Space_for_our_climate/Antarctic_glaciers_named_after_satellites |title=Antarctic Glaciers Named After Satellites |publisher=European Space Agency |date=7 June 2019 |access-date=29 September 2019}}</ref>

== Discoveries and applications ==
{{multiple image |align=left |direction=vertical
 |image1=Gravity anomalies on Earth.jpg |caption1=[[Gravity anomaly]] map from GRACE
 |image2=GRACE ocean bottom pressure.jpg |caption2=Variations in ocean bottom pressure measured by GRACE
}}

The monthly gravity anomalies maps generated by GRACE are up to 1,000 times more accurate than previous maps, substantially improving the accuracy of many techniques used by [[oceanography|oceanographer]]s, [[hydrology|hydrologists]], [[glaciology|glaciologists]], geologists and other scientists to study phenomena that influence climate.<ref>{{cite web |url=https://www.jpl.nasa.gov/news/new-gravity-mission-on-track-to-map-earths-shifty-mass |title=New Gravity Mission on Track to Map Earth's Shifty Mass |publisher=NASA/JPL|access-date=March 1, 2023}}</ref>

From the thinning of [[ice sheet]]s to the flow of water through [[aquifer]]s and the slow currents of [[magma]] inside Earth, mass measurements provided by GRACE help scientists better understand these important natural processes.

=== Oceanography, hydrology, and ice sheets ===
GRACE chiefly detected changes in the distribution of water across the planet. Scientists use GRACE data to estimate ocean bottom pressure (the combined weight of the ocean waters and atmosphere), which is as important to oceanographers as [[atmospheric pressure]] is to meteorologists.<ref name="nasa20151101">{{cite web |url=https://grace.jpl.nasa.gov/news/77/nasa-finds-new-way-to-track-ocean-currents-from-space/ |title=NASA Finds New Way to Track Ocean Currents from Space |publisher=NASA/Jet Propulsion Laboratory |first=Carol |last=Rasmussen |date=1 November 2015 |access-date=14 March 2018}}</ref> For example, measuring ocean pressure gradients allows scientists to estimate monthly changes in deep ocean currents.<ref>{{cite web |url=https://www.nasa.gov/audience/foreducators/k-4/features/F_Measuring_Gravity_With_Grace.html |title=Measuring Gravity With GRACE |publisher=NASA |first=Dan |last=Stillman |date=16 April 2007 |access-date=14 March 2018}}</ref> The limited resolution of GRACE is acceptable in this research because large ocean currents can also be estimated and verified by an ocean buoy network.<ref name="nasa20151101" /> Scientists have also detailed improved methods for using GRACE data to describe Earth's gravity field.<ref>{{cite journal |title=Improved methods for observing Earth's time variable mass distribution with GRACE using spherical cap mascons |journal=Journal of Geophysical Research: Solid Earth |first1=Michael M. |last1=Watkins |first2=David N. |last2=Weise |first3=Dah-Ning |last3=Yuan |first4=Carmen |last4=Boening |first5=Felix W. |last5=Landerer |display-authors=1 |volume=120 |issue=4 |pages=2648–2671 |date=April 2015 |doi=10.1002/2014JB011547 |bibcode=2015JGRB..120.2648W|doi-access=free }}</ref> GRACE data are critical in helping to determine the cause of [[sea level rise]], whether it is the result of mass being added to the ocean – from melting [[glacier]]s, for example – or from [[thermal expansion]] of warming water or changes in [[salinity]].<ref>{{cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=1112 |title=NASA Missions Help Dissect Sea Level Rise |publisher=NASA/Jet Propulsion Laboratory |first=Rosemary |last=Sullivant |date=14 June 2006 |access-date=14 March 2018}}</ref>  High-resolution static gravity fields estimated from GRACE data have helped improve the understanding of global [[ocean current|ocean circulation]]. The hills and valleys in the ocean's surface ([[ocean surface topography]]) are due to currents and variations in Earth's gravity field. GRACE enables separation of those two effects to better measure ocean currents and their effect on climate.<ref name="oceanography">{{cite web |url=https://climate.nasa.gov/news/152/gravity-data-sheds-new-light-on-ocean-climate/ |title=Gravity data sheds new light on ocean, climate |publisher=NASA |first=Rosemary |last=Sullivant |date=26 August 2009 |access-date=14 March 2018}}</ref>

GRACE data have provided a record of mass loss within the [[ice sheet]]s of Greenland and Antarctica.  Greenland has been found to lose {{val|280|58|ul=Gt}} of ice per year between 2003 and 2013, while Antarctica has lost {{val|67|44|ul=Gt}} per year in the same period.<ref name="Velicogna et al. (2014)">{{cite journal |last1=Velicogna |first1=Isabella |author-link=Isabella Velicogna|last2=Sutterly |first2=T.C. |last3=van den Broeke |first3=M.R. |title=Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data |journal=J. Geophys. Res. Space Phys. |date=2014 |issue=119 |pages=8130–8137 |doi=10.1002/2014GL061052 |bibcode=2014GeoRL..41.8130V |volume=41 |hdl=1874/308354 |s2cid=53062626 |hdl-access=free}}</ref> These equate to a total of 0.9&nbsp;mm/yr of sea level rise.  Increases in [[ocean heat content]] resulting from [[Earth's Energy Imbalance]] of about 0.8&nbsp;W/m<sup>2</sup> were similarly found spanning 2002 thru 2019.<ref>{{cite journal |last1=Marti |first1=Florence |last2=Blazquez |first2=Alejandro |last3=Meyssignac |first3=Benoit |last4=Ablain |first4=Michaël |last5=Barnoud |first5=Anne |last6=Fraudeau |first6=Robin |last7=Jugier |first7=Rémi |last8=Chenal |first8=Jonathan |last9=Larnicol |first9=Gilles |last10=Pfeffer |first10=Julia |last11=Restano |first11=Marco |last12=Benveniste |first12=Jérôme |display-authors=5 |title=Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry |journal=Earth System Science Data |year=2021 |doi=10.5194/essd-2021-220 |doi-access=free}}</ref><ref>{{cite journal |last1=Hakuba |first1=M.Z. |last2=Frederikse |first2=T. |last3=Landerer |first3=F.W. |title=Earth's Energy Imbalance From the Ocean Perspective (2005–2019) |journal=Geophysical Research Letters |volume=48 |issue=16 |date=28 August 2021 |doi=10.1029/2021GL093624 |doi-access=free|bibcode=2021GeoRL..4893624H }}</ref>  

GRACE data have also provided insights into regional hydrology inaccessible to other forms of remote sensing: for example, [[overdrafting|groundwater depletion]] in India<ref>{{cite journal |last1=Tiwari |first1=V.M. |last2=Wahr |first2=J. |author-link2=John M. Wahr |last3=Swenson |first3=S. |title=Dwindling groundwater resources in northern India, from satellite gravity observations |journal=Geophysical Research Letters |date=2009 |volume=36 |issue=18 |at=L18401 |doi=10.1029/2009GL039401 |bibcode=2009GeoRL..3618401T|doi-access= }}</ref> and California.<ref name="Famiglietti et al. (2011)">{{cite journal |last1=Famiglietti |first1=J |title=Satellites measure recent rates of groundwater depletion in California's Central Valley |journal=Geophys. Res. Lett. |date=2011 |volume=38 |issue=3 |at=L03403 |doi=10.1029/2010GL046442 |bibcode=2011GeoRL..38.3403F |url=https://escholarship.org/content/qt8g651992/qt8g651992.pdf?t=n0u8a0|doi-access=free }}</ref>  The annual hydrology of the [[Amazon basin]] provides an especially strong signal when viewed by GRACE.<ref name="Tapley et al. (2004)">{{cite journal |last1=Tapley |first1=Byron D. |last2=Bettadpur |first2=Srinivas |last3=Ries |first3=John C. |last4=Thompson |first4=Paul F. |last5=Watkins |first5=Michael M. |title=GRACE Measurements of Mass Variability in the Earth System |journal=Science |date=2004 |volume=305 |issue=5683 |pages=503–505 |doi=10.1126/science.1099192 |bibcode=2004Sci...305..503T |pmid=15273390 |s2cid=7357519 |url=https://authors.library.caltech.edu/52043/7/Tapley.SOM.pdf}}</ref>  A [[University of California, Irvine]]-led study published in ''[[Water Resources Research]]'' on 16 June 2015 used GRACE data between 2003 and 2013 to conclude that 21 of the world's 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed." The most over-stressed is the [[Arabian Aquifer System]], upon which more than 60 million people depend for water.<ref name="NASA_GRACE">{{cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=4626 |title=Study: Third of Big Groundwater Basins in Distress |publisher=NASA |date=16 June 2015 |access-date=26 June 2015}}</ref>

=== Geophysics ===
[[File:15 Years of Freshwater Trends Seen by GRACE.webm|thumb|left|GRACE uses precise measurements of the motions of two spacecraft in Earth's orbit to track the movement of water through the oceans, land, and atmosphere.]]
[[File:Greenland+Antarctica Mass Loss.png|thumb|Change in mass of the Greenland and Antarctic ice sheets as measured by GRACE]]
GRACE also detects changes in the gravity field due to geophysical processes. [[Glacial isostatic adjustment]]—the slow rise of land masses once depressed by the weight of ice sheets from the last ice age—is chief among these signals.  GIA signals appear as secular trends in gravity field measurements and must be removed to accurately estimate changes in water and ice mass in a region.<ref>{{cite journal |last1=Tregoning |last2=Ramillien |last3=McQueen |last4=Zwartz |s2cid=15724840 |title=Glacial isostatic adjustment and nonstationary signals observed by GRACE |journal=J. Geophys. Res. |date=2009 |volume=114 |issue=B6 |pages=B06406 |doi=10.1029/2008JB006161 |bibcode=2009JGRB..114.6406T|doi-access=free }}</ref>  GRACE is also sensitive to permanent changes in the gravity field due to earthquakes.  For instance, GRACE data have been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami.<ref>{{cite news |url=https://www.nytimes.com/2006/08/08/science/08find.html |title=Before the '04 Tsunami, an Earthquake So Violent It Even Shook Gravity |work=The New York Times |first=Kenneth |last=Chang |date=8 August 2006 |access-date=4 May 2010}}</ref>

In 2006, a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the {{convert|480|km|mi|sp=us|adj=mid|-wide}} [[Wilkes Land crater]] in [[Antarctica]], which was probably formed about 250 million years ago.<ref>{{cite web |url=http://researchnews.osu.edu/archive/erthboom.htm |title=Big Bang in Antarctica—Killer Crater Found Under Ice |publisher=Ohio State University |url-status=dead |archive-url=https://web.archive.org/web/20160306140004/http://researchnews.osu.edu/archive/erthboom.htm |archive-date=6 March 2016}}</ref>

{{clear|left}}
=== Geodesy ===
Data from GRACE has improved the current [[Earth Gravitational Model|Earth gravitational field model]], leading to improvements in the field of [[geodesy]]. This improved model has allowed for corrections in the equipotential surface which land elevations are referenced from. This more accurate reference surface allows for more accurate coordinates of latitude and longitude and for less error in the calculation of geodetic satellite orbits.<ref>{{Cite web |url=http://www2.csr.utexas.edu/grace/gravity/geodesy.html |title=GRACE – Gravity Recovery and Climate Experiment |publisher=University of Texas Center for Space Research |access-date=21 March 2018}}</ref>

=== Other signals ===
GRACE is sensitive to regional variations in the mass of the atmosphere and high-frequency variation in ocean bottom pressure.  These variations are well understood and are removed from monthly gravity estimates using [[numerical weather prediction|forecast models]] to prevent [[aliasing]].<ref>{{cite web |title=GRACE AOD1B |url=http://www.gfz-potsdam.de/en/aod1b/ |website=gfz-potsdam.de |publisher=[[GFZ German Research Centre for Geosciences]] |access-date=11 June 2015}}</ref>  Nonetheless, errors in these models do influence GRACE solutions.<ref>{{cite book |last1=Ge |first1=Shengjie |title=GPS radio occultation and the role of atmospheric pressure on spaceborne gravity estimation over Antarctica |date=2006 |publisher=Ohio State University |url=https://etd.ohiolink.edu/ap/10?0::NO:10:P10_ACCESSION_NUM:osu1149070384 |access-date=11 June 2015 |archive-date=13 June 2015 |archive-url=https://web.archive.org/web/20150613134056/https://etd.ohiolink.edu/ap/10?0::NO:10:P10_ACCESSION_NUM:osu1149070384 |url-status=dead }}</ref>

GRACE data also contribute to fundamental physics. They have been used to re-analyze data obtained from the [[LAGEOS]] experiment to try to measure the relativistic [[frame-dragging]] effect.<ref>{{cite journal |last1=Ciufolini |first1=I. |last2=Pavlis |first2=E.C. |year=2004 |title=A confirmation of the general relativistic prediction of the Lense–Thirring effect |url=http://www.dm.unipi.it/cluster-pages/tommei/mc/Lensenature03007.pdf |journal=Nature |volume=431 |issue=7011 |pages=958–960 |doi=10.1038/nature03007 |pmid=15496915 |bibcode=2004Natur.431..958C |s2cid=4423434 |url-status=dead |archive-url=https://web.archive.org/web/20150613230457/http://www.dm.unipi.it/cluster-pages/tommei/mc/Lensenature03007.pdf |archive-date=13 June 2015}}</ref><ref>{{cite journal |last1=Ciufolini |first1=I. |last2=Pavlis |first2=E.C. |last3=Peron |first3=R. |year=2006 |title=Determination of frame-dragging using Earth gravity models from CHAMP and GRACE |journal=New Astron |volume=11 |issue=8 |pages=527–550 |doi=10.1016/j.newast.2006.02.001 |bibcode=2006NewA...11..527C}}</ref>

== Spacecraft ==
[[File:GRACE schematics.png|thumb|left|Diagrams illustrating the systems and instruments aboard the GRACE spacecraft]]
{{multiple image |align=right |direction=vertical
 |image1=Global Gravity Anomaly Animation over LAND.gif
 |image2=Global Gravity Anomaly Animation over OCEANS.gif
 |footer=Global gravity anomaly animations over land and oceans by GRACE
}}

The spacecraft were manufactured by [[Astrium]] of Germany, using its "Flexbus" [[satellite bus|platform]]. The microwave [[radio frequency|RF]] systems, and attitude determination and control system algorithms were provided by [[Space Systems/Loral]]. The star cameras used to measure the spacecraft attitude were provided by [[Technical University of Denmark]]. The instrument computer along with a highly precise BlackJack GPS receiver and digital signal processing system was provided by [[Jet Propulsion Laboratory|JPL]] in Pasadena. The highly precise accelerometer that is needed to separate atmospheric and solar radiation pressure effects from the gravitation data was manufactured by [[ONERA]].

=== Measurement principle ===

GRACE's key measurement, [[satellite gravimetry]], is not derived from electromagnetic waves. Instead, the mission uses a microwave ranging system to accurately measure changes in the speed and distance between two identical spacecraft flying in a polar orbit about {{convert|220|km|mi|sp=us}} apart, {{convert|500|km|mi|sp=us}} above Earth. The ranging system is sensitive enough to detect separation changes as small as 10 micrometers (approximately one-tenth the width of a human hair) over a distance of 220 kilometers.<ref name="presskit-launch" />  As the twin GRACE satellites circle the globe 15 times a day, they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a [[gravity anomaly]], it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.  By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from [[Global Positioning System]] (GPS) instruments, scientists can construct a detailed map of Earth's gravity anomalies.

=== Instruments ===

The two satellites (nicknamed [[Tom and Jerry|"Tom" and "Jerry"]]) constantly maintain a two-way, [[K band (IEEE)|K-band]] microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. This is made possible due to the onboard Ultra Stable Oscillator (USO) which produces the frequencies for the K-band ranging system.<ref name="grace-craft">{{Cite web |url=http://www.nasa.gov/mission_pages/Grace/spacecraft/index.html |title=Spacecraft |series=GRACE Mission |publisher=NASA |date=6 June 2013 |access-date=10 March 2019}}</ref> The micrometer-sensitivity of this measurement requires accordingly precise measurements of each spacecraft's position, motion, and orientation to be useful.  To remove the effect of external, non-gravitational forces (e.g., [[drag (physics)|drag]], [[solar radiation pressure]]), the vehicles use sensitive Super STAR electrostatic accelerometers located near their respective centers of mass.  GPS receivers are used to establish the precise positions of each satellite along the baseline between the satellites.   The satellites use star cameras and magnetometers to establish [[Spacecraft attitude control|attitude]]. The GRACE vehicles also have [[Corner reflector|optical corner reflectors]] to enable [[satellite laser ranging|laser ranging]] from ground stations using the Center of Mass Trim Assembly (MTA) which ensures the center of mass is modified throughout the flight accordingly.<ref name="grace-craft"/>

=== Data products ===

CSR, GFZ, and JPL process observations and ancillary data downloaded from GRACE to produce monthly [[geopotential model]]s of Earth.<ref name="PODAAC">{{cite web |title=GRACE PO.DAAC |url=http://podaac.jpl.nasa.gov/GRACE |website=JPL Physical Oceanography and Distributed Active Archive Center |access-date=11 June 2015}}</ref>  These models are distributed as [[spherical harmonics|spherical harmonic]] coefficients with a maximum degree of 60.  Degree 90 products are also available.  These products have a typical latency of 1–2 months.  These geopotential coefficients may be used to compute [[geoid]] height, gravity anomalies, and changes in the distribution of mass on Earth's surface.<ref>{{cite journal |last1=Wahr |first1=John |last2=Molenaar |first2=M. |last3=Bryan |first3=F. |s2cid=140194666 |title=Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE |journal=J. Geophys. Res. |date=1998 |volume=103 |issue=B12 |pages=30205–30229 |doi=10.1029/98JB02844 |bibcode=1998JGR...10330205W|doi-access=free }}</ref>  Gridded products estimating changes in mass in units of liquid water equivalent thickness are available at JPL's GRACE Tellus website.

== End of mission ==

Following an age-related battery issue on GRACE-2 in September 2017, it became apparent that GRACE-2's remaining battery capacity would not be sufficient to operate. Therefore, it was decided in mid-October to decommission the GRACE-2 satellite and end GRACE's science mission.<ref name="end-of-science-mission"/> Atmospheric reentry of GRACE-2 occurred on 24 December 2017 at approximately 00:16&nbsp;UTC;<ref name="decay-2" /> atmospheric reentry of GRACE-1 took place on 10 March 2018 around 06:09&nbsp;UTC.<ref name="decay-1" />

== GRACE Follow-On ==
{{Infobox spaceflight |auto=all
| name                  = GRACE-FO
| names_list            = {{unbulleted list|
* GRACE-FO 1<ref name="NSSDCA GFO1">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2018-047A |title=GRACE-FO 1 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=23 May 2019}}</ref>
* GRACE-FO 2<ref name="NSSDCA GFO2">{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2018-047B |title=GRACE-FO 2 |work=[[National Space Science Data Center]] |publisher=NASA |access-date=23 May 2019}}</ref>
}}
| image                 = GRACE-FO.jpg
| image_size            = 275px
| image_caption         = Illustration of the twin GRACE-FO satellites

| mission_type          = Gravitational science
| operator              = [[NASA]]{{·}}[[German Aerospace Center|DLR]]<ref>{{cite web |title=Twin Spacecraft Launch to Track Earth's Water Movement |url=https://www.nasa.gov/press-release/twin-spacecraft-launch-to-track-earth-s-water-movement |website=NASA |date=22 May 2018 |access-date=28 May 2019}}</ref>
| COSPAR_ID             = {{unbulleted list|2018-047A|2018-047B}}
| SATCAT                = 43476 and 43477
| website               = {{URL|nasa.gov/missions/grace-fo/}}
| mission_duration      = Planned: 5 years <br /> Elapsed: {{time interval|22 May 2018 19:47:58|show=ymd|sep=,}}

| spacecraft_bus        = Flexbus<ref name="gunters-FO">{{cite web |url=http://space.skyrocket.de/doc_sdat/grace.htm |title=GRACE-FO |work=Gunter's Space Page |access-date=23 May 2019}}</ref>
| manufacturer          = [[Airbus Defence and Space]] (formerly [[Astrium]])<ref>{{cite web |title=GRACE-FO |url=https://directory.eoportal.org/web/eoportal/satellite-missions/g/grace-fo |website=eoPortal |access-date=26 May 2019}}</ref>
| launch_mass           = {{convert|600|kg|lb|abbr=on}} each<ref name="presskit-launch-FO">{{cite web |url=https://www.jpl.nasa.gov/news/press_kits/grace-fo/download/grace-fo_launch_press_kit.pdf |title=GRACE-FO Launch Press Kit |publisher=NASA |date=May 2018 |access-date=23 May 2019}}</ref>
| dry_mass              = 
| dimensions            = {{convert|1.943|xx|3.123|xx|0.78|m|ft|1|abbr=on}}<ref name="presskit-launch-FO" />
| power                 = <!--end-of-life power, in watts-->

| launch_date           = {{start-date|22 May 2018, 19:47:58}}&nbsp;UTC
| launch_rocket         = [[Falcon 9 Block 4]] [[List of Falcon 9 first stage boosters#B1043|B1043-2]]
| launch_site           = [[Vandenberg AFB]], [[California]]
| launch_contractor     = [[SpaceX]]
| entered_service       =

| declared              = 
| decay_date            =

| orbit_reference       = [[Geocentric orbit|Geocentric]]
| orbit_regime          = 
| orbit_semimajor       = {{convert|6872.2|km|abbr=on}}
| orbit_eccentricity    = 0.00179
| orbit_periapsis       = {{convert|481.7|km|mi|abbr=on}}
| orbit_apoapsis        = {{convert|506.3|km|mi|abbr=on}}
| orbit_inclination     = 89.0°
| orbit_period          = 94.5 minutes
| orbit_epoch           = 29 September 2019, 15:36:45&nbsp;[[UTC]]<ref name="habove-gfo1">{{cite web |url=https://heavens-above.com/orbit.aspx?satid=43476 |title=GRACE-FO 1 – Orbit |website=Heavens-Above.com |date=29 September 2019 |access-date=29 September 2019}}</ref>
| apsis                 = gee

| instruments           = <!--a list of instruments on the satellite-->
}}

The GRACE-FO mission, a collaboration between NASA and [[GFZ German Research Centre for Geosciences|GFZ]], was launched on 22 May 2018 aboard a [[SpaceX]] [[Falcon 9]] rocket from Vandenberg AFB, California, sharing the launch with five [[Iridium satellite constellation|Iridium NEXT]] satellites.<ref name="FO launch">{{cite web |url=https://gracefo.jpl.nasa.gov/mission/overview/ |title=GRACE-FO Mission |publisher=NASA/JPL |access-date=19 November 2017}}</ref><ref name="space20180522">{{cite news |url=https://www.space.com/40672-spacex-launches-grace-fo-iridium-satellites.html |title=SpaceX Launches Twin NASA Probes to Track Earth's Water (and Satellites Hitch a Ride) |work=Space.com |first=Hanneke |last=Weitering |date=22 May 2018 |access-date=22 May 2018}}</ref> During in-orbit checks, an anomaly was discovered in the primary system component of the microwave instrument (MWI), and the system was temporarily powered down on 19 July 2018.<ref name="nasa20181101">{{cite web |url=https://www.nasa.gov/feature/jpl/grace-fo-resumes-data-collection |title=GRACE-FO Resumes Data Collection |publisher=NASA |last=Rasmussen |first=Carol |date=1 November 2018 |access-date=2 November 2018}}</ref> After a full investigation by an anomaly response team at JPL, the backup system in the MWI was powered up on 19 October 2018 and GRACE-FO resumed its in-orbit checks.<ref name="nasa20181101" /><ref name="nasa20180914">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=7236 |title=GRACE-FO Satellite Switching to Backup Instrument Processing Unit |publisher=NASA/JPL |first=Esprit |last=Smith |date=14 September 2018 |access-date=14 September 2018}}</ref>  GRACE-FO entered the science phase of its mission on 28 January 2019.<ref name="Webb2019">{{cite journal |url=https://media.gfz-potsdam.de/gfz/sec12/pdf/GRACE_FO_SDS_newsletter_No2.pdf |title=GRACE Follow-On Science Team & Highlights |journal=Science Data System Newsletter |first1=Frank |last1=Webb |first2=Frank |last2=Flechtner |first3=Felix |last3=Landerer |first4=Michael |last4=Watkins |first5=Himanshu |last5=Save |first6=Christoph |last6=Dahle |display-authors=1 |issue=2 |date=January–March 2019}}</ref>

The orbit and design of GRACE-FO is very similar to its predecessor.<ref name="gfo_brochure">{{cite web |url=https://www.jpl.nasa.gov/news/brochure/gracefo_brochure.pdf |title=GRACE-FO: Tracking Earth's Mass in Motion |publisher=NASA |date=2017 |id=NP-2017-4-002-GSFC |access-date=7 March 2019 |archive-date=26 January 2021 |archive-url=https://web.archive.org/web/20210126201514/https://www.jpl.nasa.gov/news/brochure/gracefo_brochure.pdf |url-status=dead }}</ref> GRACE-FO employs the same two-way microwave-ranging link as GRACE, which will allow for similar inter-satellite ranging precision. In addition, GRACE-FO employs laser-ranging interferometry (LRI) as a technological experiment in preparation for future satellites.<ref>{{cite press release |url=http://www.space-airbusds.com/en/press_centre/airbus-defence-and-space-to-build-two-new-research-satellites-for-nasa.html |title=Airbus Defence and Space to build two new research satellites for NASA |publisher=Airbus Defence and Space |date=29 November 2012 |archive-url=https://web.archive.org/web/20140720205747/http://www.space-airbusds.com/en/press_centre/airbus-defence-and-space-to-build-two-new-research-satellites-for-nasa.html |archive-date=20 July 2014 |url-status=dead}}</ref><ref>{{cite web |url=https://gracefo.jpl.nasa.gov/mission/spacecraft/microwaves-and-lasers/ |title=Spacecraft: Microwaves and Lasers |series=GRACE-FO |publisher=NASA/JPL |access-date=11 December 2017}}</ref><ref>{{Cite web |url=https://gracefo.jpl.nasa.gov/laser-ranging-interferometer/ |title=Laser Ranging Interferometer |series=GRACE-FO |publisher=NASA/JPL |access-date=29 September 2019}}</ref> The LRI allows for more accurate inter-satellite ranging due to the shorter wavelength of light, and additionally allows the angle between the two spacecraft to be measured as well as their separation via differential wavefront sensing (DWS).<ref name="gtel_gfo">{{Cite news |url=https://grace.jpl.nasa.gov/mission/grace-fo/ |title=GRACE Tellus: GRACE-FO |series=GRACE Tellus |publisher=NASA/JPL |access-date=18 April 2018}}</ref><ref>{{Cite web |url=https://earth.esa.int/web/eoportal/satellite-missions/g/grace-fo |title=GRACE-FO |work=eoPortal |publisher=European Space Agency |access-date=7 May 2020}}</ref><ref name="Abich2015">{{cite journal |last1=Abich |first1=Klaus |last2=Bogan |first2=Christina |last3=Braxmaier |first3=Claus |last4=Danzmann |first4=Karsten |last5=Dehne |first5=Marina |last6=Gohlke |first6=Martin |last7=Görth |first7=Alexander |last8=Heinzel |first8=Gerhard |last9=Herding |first9=Mark |last10=Mahrdt |first10=Christoph |last11=Müller |first11=Vitali |last12=Nicklaus |first12=Kolja |last13=Sanjuan |first13=Josep |last14=Schütze |first14=Daniel |last15=Sheard |first15=Benjamin |last16=Stede |first16=Gunnar |last17=Voss |first17=Kai |display-authors=1 |title=GRACE-Follow On Laser Ranging Interferometer: German contribution |journal=Journal of Physics: Conference Series |volume=610 |issue=1 |at=012010 |date=11 May 2015 |doi=10.1088/1742-6596/610/1/012010 |bibcode=2015JPhCS.610a2010A|doi-access=free |hdl=21.11116/0000-0003-655A-7 |hdl-access=free }}</ref> Using the LRI, scientists have improved the precision of the separation distance measurements by a factor of more than 20 relative to the GRACE mission.<ref name="gfo_brochure" /><ref name="physworld20190723">{{cite news |url=https://physicsworld.com/a/distance-between-spacecraft-measured-at-the-atomic-scale/ |title=Distance between spacecraft measured at the atomic scale |work=PhysicsWorld |first=Hamish |last=Johnston |date=23 July 2019 |access-date=29 September 2019}}</ref> Each laser on the LRI has about the same power as four laser pointers.<ref name=":0">{{Cite web |url=https://gracefo.jpl.nasa.gov/news/128/lasers-in-space-grace-fo-tests-new-technology |title=Lasers in Space: GRACE-FO Tests New Technology |website=GRACE-FO |publisher=NASA |date=8 May 2018 |access-date=5 March 2020}}</ref> These lasers must be detected by a spacecraft about {{convert|220|km|mi}} away.<ref name=":0" /> This laser approach will generate much more accurate measurements than the previous GRACE satellite mission.<ref>{{Cite web |url=https://gracefo.jpl.nasa.gov/mission/spacecraft/overview |title=Spacecraft Overview |website=GRACE-FO |publisher=NASA |access-date=5 March 2020}}</ref>

The GRACE-FO satellites obtain electricity from gallium arsenide solar cell array panels covering the outside of each satellite.<ref>{{Cite web |url=https://gracefo.jpl.nasa.gov/solar-cells-battery |title=Solar Cell Arrays |website=GRACE-FO |publisher=NASA |access-date=27 February 2020}}</ref>

GRACE-FO will continue to monitor Earth's gravity and climate. The mission will track gravitational changes in global sea levels, glaciers, and ice sheets, as well as large lake and river water levels, and soil moisture.<ref name="gtel_gfo" /> In addition, each of the satellites will use GPS antennas to create at least 200 profiles per day of atmospheric temperature distribution and water vapor content, a first for the GRACE mission.<ref name="gfo_brochure" />

GRACE-FO has a design life of 5 years.<ref name="gfo_brochure" /><ref name=":1">{{cite web |url=https://www.jpl.nasa.gov/news/fact_sheets/GracefoFactSheet.pdf |title=GRACE-FO |series=NASA Facts |publisher=NASA |access-date=29 September 2019 |archive-date=15 June 2021 |archive-url=https://web.archive.org/web/20210615035957/https://www.jpl.nasa.gov/news/fact_sheets/GracefoFactSheet.pdf |url-status=dead }}</ref>

== See also ==
{{Portal|Spaceflight}}
* [[CHAMP (satellite)|CHAMP]], an earlier single satellite mission using a similar "Flexbus" platform
* [[GOCE]], an ESA gravity mapping mission that used only a single satellite
* [[GRAIL]], a similar NASA probe pair that mapped the Moon

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Gravity Recovery And Climate Experiment}}
* [http://www2.csr.utexas.edu/grace/ GRACE website] by the University of Texas
* [https://grace.jpl.nasa.gov/ GRACE Tellus website] by the Jet Propulsion Laboratory
* [http://geoid.colorado.edu/grace/ GRACE Real-Time Data Analysis Portal] by the University of Colorado
* [http://isdc.gfz-potsdam.de/grace-isdc/ GRACE Information System and Data Center] by GFZ German Research Centre for Geosciences
* {{cite journal |url=http://www.gpsworld.com/government/natural-resources/instrument-grace-782 |title=Instrument of Grace: GPS augments gravity measurements |journal=GPS World |first1=Charles |last1=Dunn |first2=Willy |last2=Bertiger |first3=Yoaz |last3=Bar-Sever |first4=Shailen |last4=Desai |first5=Bruce |last5=Haines |first6=Da |last6=Kuang |first7=Garth |last7=Franklin |first8=Ian |last8=Harris |first9=Gerhard |last9=Kruizinga |first10=Tom |last10=Meehan |first11=Sumita |last11=Nandi |first12=Don |last12=Nguyen |first13=Tim |last13=Rogstad |first14=J. Brooks |last14=Thomas |first15=Jeff |last15=Tien |first16=Larry |last16=Romans |first17=Michael |last17=Watkins |first18=Sien Chong |last18=Wu |first19=Srinivas |last19=Bettadpur |first20=Jeongrae |last20=Kim |display-authors=1 |volume=14 |issue=2 |pages=16–28 |date=February 2003 |archive-url=https://web.archive.org/web/20120225162449/http://www.gpsworld.com/government/natural-resources/instrument-grace-782 |archive-date=25 February 2012}}
* {{YouTube|id=MaxBOvQ2a_o|title=For 15 Years, GRACE Tracked Freshwater Movements Around the World}} published on 16 May 2018 by [[NASA Goddard]]

{{Jet Propulsion Laboratory}}
{{Orbital launches in 2002}}
{{Orbital launches in 2018}}
{{Authority control}}

{{Use dmy dates|date=December 2020}}

[[Category:Earth observation satellites of the United States]]
[[Category:Spacecraft launched in 2002]]
[[Category:Satellites of Germany]]
[[Category:NASA satellites]]
[[Category:Spacecraft launched by Rokot rockets]]
[[Category:Twin satellites]]
[[Category:Satellite series]]
[[Category:Gravimetry satellites]]