All About Space - UK (2020-02)

Mercury

hichplanetisEarth’snearest

neighbourintheSolarSystem?The

obviousanswerisVenus,which

makestheclosestapproachtous

• butitspendshalfitsorbitontheothersideof
  theSun,whenit’sfurtherawayfromusthan
  Mercury.ItwasonlylastyearthatTomStockman,
  a graduateresearchassistantatLosAlamos
  NationalLaboratory,NewMexico,andcolleagues
  crunchedthenumberstoworkoutwhichplanet
  isactuallyclosestonaverage– andtheywereas
  surprisedasanyonebytheanswer:“When
  averagedovertime,Earth’snearestneighbour
  isinfactMercury,”theywrote.
  Despiteitsproximity,Mercuryhasalwaysbeena
  mysteriousplanetduetothedifficultyofobserving
  itthroughEarth-basedtelescopes.That’sdownto
  a combinationofitssmallsizeandthefactthatit
  nevergetsveryfarfromtheSuninthesky.The
  onlytimeitmakesa reallyspectacularsight,infact,
  iswhenitpassesdirectlyinfrontoftheSunduring
  a transitofMercury– liketheonethattookplacea
  fewmonthsagoinNovember2019.
  IfMercuryisa difficultplanettoobservefrom
  Earth,it’snotaneasydestinationforspacecraft
  either.That’spartlybecausea spacecraftspeedsup
  undertheeffectofgravityasitfallstowardsthe
  Sun– andthenitsrocketenginehastoworkhard
  tolosethatexcessspeedwhenitgetstoMercury.

W

Another problem is the extreme heat – well over

400 degrees Celsius (752 degrees Fahrenheit) –

in the vicinity of Mercury, which poses serious

challenges for spacecraft designers. Since the dawn

of the space age, only two space probes have been

to Mercury – Mariner 10 in the 1970s, followed by

MESSENGER more recently, both by NASA.

The photographs sent back by Mariner 10, which

made three close passes of Mercury in 1974 and

1975, revealed a desolate, crater-studded landscape

that looks a lot like our Moon. But the mission had

a surprise for scientists in its discovery of a well-

defined magnetic field around the planet. It’s a

hundred times weaker than Earth’s field, but Venus

and Mars don’t have internal magnetic fields at all,

and Mercury wasn’t expected to either.

A closer look at Mercury’s magnetic field was one

ofthekeyobjectivesofNASA’sfollow-upmission,

MESSENGER,whichenteredorbitaroundtheplanet

inMarch2011.Itremainedthereuntilitranoutof

manoeuvringpropellantfouryearslater– andin

thefinalfewmonthsthemissioncontrollersgot

increasinglybold.Theydippedthespacecrafttojust

15 kilometres(9.3miles)abovetheplanet,allowing

themtomeasurerelicmagnetisminthesurface

rocks.“Thesignalswedetectedarereallysmall,

andveryhardtomeasure,”explainsplanetary

geophysicistCatherineJohnson.“We’dneverhave

beenabletomeasurethemifnotforthesereally

Elementsthat

‘shouldn’t

bethere’

MESSENGER’s

instrumentsincluded

spectrometerstoanalyse

thechemicalcomposition

ofMercury’ssurface– they

discovereda surprisingly

highlevelofvolatile

elementssuchassulphur

andpotassium.

Mantle

Crust

Solidinner

core

Outer

moltencore

A giant

cannonball

Mercury’s structure is

dominated by its huge

iron core. Like Earth’s,

it’s molten in the outer

parts – which is where the

magnetic field originates –

and solid at the centre.

© Getty

© NASA