Air & Space Smithsonian – September 2019

known as GRACE, for Gravity Recovery and

Climate Experiment, measure the changing force

of gravity at Earth’s surface.

The twin GRACE satellites fly about 220 kilo-

meters apart and use microwaves and lasers to

measurethedistancebetweenthemveryprecisely.

Whentheleadingsatellitepassesoveranobject

witha lotofmass,likea mountainrangeoran

icesheet,it is tuggedeversoslightlyforward,and

the distance between the two satellites increases.

When the trailing satellite approaches that same

mountain range or ice sheet, it too is tugged for-

ward and the gap gets shorter. GRACE can mea-

sure changes in distance as small as the width of

a human blood cell.

Thanks in part to GRACE, we know that

Greenland is losing about 280 billion tons of ice

a year, on average, while Antarctica is losing just

under 120 billion tons a year.

Meanwhile, NASA’s ICESat-2 satellite bounces

laser beams down from orbit that reflect off the

ice. By precisely measuring the return time,

ICESat-2 can build up a three-dimensional picture

of the crags and crevasses in the ice below. Over a

period of years, scientists can watch how the ice is

changing. “Now we’re going to have continuous

maps of elevation changes,” says Velicogna. “It’s

veryexciting.”

Geodesyiscrucialintrackingothertypesof

change,includinghowlanddropswhenfarm-

erspumpgroundwaterfortheirfields.TheU.S.

GeologicalSurveyhasbeenmeasuringthese

changesinplaceslikeCalifornia’sagricultural

valleysfordecades.AstheGPSmeasurements

getmoreprecise,researcherscanbetterseehow

pumpingwaterinonelocationchangesconditions

inanother,saysMichelleSneed,a hydrologistwith

theUSGSinSacramento,California.

Landsubsidenceisa bigproblembecauseit

damagesbuildingsandotherinfrastructure.Inthe

pastseveralyearsintheSanJoaquinValley,parts

oftheFriant-KernCanalhavesunkasmuchasa

meter,meaningthatwatermanagerscan’talways

deliverwatertoalltheplacesit’sneeded.While

someblamedtherecentCaliforniadroughtfor

thesubsidence,GPSmeasurementsshowedthat

somepartsofthelandcontinuedtosinkevenafter

thedroughtended.

It’simportantformanagerstounderstandhow

groundwaterpumpingcontributestotheproblem.

“Theyreallyneedtoknowwhat’shappening,in

reallyprecisemeasurements,”Sneedsays.

Moreglobally,geodesyishelpingofficials

understandnaturalhazardssuchashurricanes

andtsunamis.Precisesatellitemeasurementsof

sea-surfaceheight,forexample,cantellforecasters

whethertheremightbea poolofwarmwaterin

thepathofanadvancinghurricane—whichhelps

thempredicthowstrongthehurricanemightgrow.

Alltheseplanetarymeasurements,theroutine

aswellastheunusual,requirethatwekeepour

geodeticnetworksuptodate.“Weoftencallour-

selvestheinvisibleinfrastructure,”saysMerkowitz.

“It’skindofliketheelectricitygrid.Peopledon’t

thinkaboutit beingthere,butif youevertookit

away,it wouldbedisastrous.”

THANKS IN PART TO A PAIR OF

SATELLITES, WE KNOW THAT GREENLAND

IS LOSING ABOUT 280 BILLION TONS OF

ICE A YEAR, WHILE ANTARCTICA IS LOSING

JUST UNDER 120 BILLION TONS A YEAR.

September 2019 AIR&SPACE 55

NASA/JPL CALTECH

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