Air & Space Smithsonian – September 2019

INTHEDAVISMOUNTAINSOFFARWESTTEXAS,

at theUniversityofTexasMcDonaldObservatory,

astronomersspendtheirnightspeeringatthe

starsthroughsomeoftheworld’smostpowerful

telescopes.Soonthey’llbeaddinga moredown-to-

Earthjob.Withinsightofthegiantdomes,NASA

is installinga sprawlingnetworkofequipmentto

helpresearchersstudyplanetarychange.

LastJuly,engineersachievedfirstlightona

toweringradioantenna, 12 metersacross,that

watchesforsignalsflashingfromcosmicbeacons

calledquasarslocatedinthedistantuniverse.

Nearby,scientistshavesetupnewglobal-position-

ing-systemstations—tripodstoppedwithbulbous

headsthatcommunicatewithGPSsatellitesto

determinetheirpreciselocationonEarth’ssurface.

Ona neighboringmountain,techniciansplanto

builda powerfullasersystemthatcanzapa beam

oflightuptoa satellite,thenclockhowquicklyit

reflectsbacktoEarth.Together,allthishigh-tech

equipmentwillallowscientiststopinpointindi-

vidualspotsofgroundat McDonaldObservatory

towithinmillimeters.

Similaractivitiesaregoingonallovertheworld.

NASAishelpingtoupgradeEarth-measuring

observatoriesfromTahititoSouthAfrica.Bylink-

ingtheMcDonaldmeasurementswiththeothers,

researchersaimtobetterunderstandhowEarth’s

shape,rotation,andgravitychangeovertime.

Calledgeodesy,thisfieldofscienceunderlies

almosteveryaspectofmodernlife,whetherit’s

usingGoogleMapstofindthenearestcoffeeshop

ordetermininghowsealevelis risingastheplanet

warms.“Youwouldneverthinkthatnavigating

yourcarisdependentonourmeasurementsof

distantquasars,”saysStephenMerkowitz,an

astrophysicistatNASA’sGoddardSpaceFlight

CenterinGreenbelt,Maryland,whomanagesthe

agency’sSpaceGeodesyProject.“Butit is.”

Earthmaylooklikea perfectbluemarbleinphotos

takenfromspace,butit is notpreciselyround.It’s

squashedat thepoles,andit bulgesat theequator,

makingit about 43 kilometers(27miles)widerthan

it is tall.Partsoftheglobe,suchasScandinavia

andnorthernCanada,arerisingintotheair,still

reboundingaftertheweightofthegreatnorthern

icesheetsdisappearedthousandsofyearsagoat the

endofthelasticeage.Theplanetalsoshapeshifts

throughouttheyearasmassmovesaroundon

itssurface—aswhenicemeltsinthesummeror

watercyclesbetweentheoceanandatmosphere

duringanElNiño.

Allthisinformation,andmuchmore,feedsinto

somethingcalledtheInternationalTerrestrial

ReferenceFrame(ITRF),thesystemscientistsuse

tolocateterrestrialobservations.Thinkofit as a

globalwebofcoordinatesthatdefineswhereany

particularspotonEarthis atanygiventime.“If

youwanttodoanykindofpositioningornavi-

gatingonEarthorinspace,it’salldonewithina

referenceframe,”saysRichardGross,a research

scientistandgeodesyspecialistatNASA’sJet

PropulsionLaboratoryinPasadena,California.

Known as the

“Potsdam Gravity

Potato,” this

visualization of

variations in Earth’s

gravity fi eld is

produced by

Germany’s GFZ

Earth science

research center

based on satellite

and surface data.

Gravity “highs” are

red, “lows” are blue.

Thedifferencesare

duemostlytothe

differentdensityof

materialsaboveor

belowtheEarth’s

surface.

TRACKING THE

EFFECTS OF GLOBAL

CHANGE, DOWN TO

THE MILLIMETER.

BY ALEXANDRA WITZE

AIR&SPACE 51

GFZ POTSDAM

September 2019