THE E-SAIL
In 1980, CarlSagan’s TV seriesCosmosintroduced
audiences to his ‘spaceshipof the imagination’. This
fantasticdesign resembledadandelion seedfloating
on the‘winds’ oftheUniverse.NowitseemsSagan’s
imagination may nothavebeen sofar-fetched.
Theelectricsail(e-sail)isthebrainchildofDr
PekkaJanhunen, at theFinnishMeteorological
Institute. He conceived the idea while studying the
Northern andSouthern Lights, whichare produced
when electrically chargedparticles from the Sun
interact withmolecules in our atmosphere.
Janhunen calculated how to build a virtual sail that
could harness theelectricalchargefrom this
continuous solar windandtransform it into
momentum to propelaspacecraft. He imagineda
central craft deployingbetween50and100wires,
eachsome 20kminlength.These wires would be
electrified to around 20,000 volts withpositive
electricalcharge, andso wouldrepelthepositively
chargedprotonsfrom thesolar windlikelight
bouncing off a mirror. This would transfer
momentumfrom theparticles to the spacecraft.
According to themaths, a one-tonne spacecraft
would beacceleratedby1mm/s every second.That
may not soundmuch,but this constant acceleration
would quicklyaddup.After a year, theterminal
speedwould be30km/s. NASA’shalf-tonne New
Horizons crafttooknine years to reachPluto; an
e-sailcouldtransport twice themassinhalfthetime.
A version ofthee-sailwill flyonthe Finnish
Aalto-1 satellite,whichisscheduledforlaunchin
October. Insteadofusing thesailto accelerate, the
satellite will deploy thesailat theendofitslifeto
slowdown andre-enter Earth’s at mosphere, to avoid
becoming spacedebris.
“Thee-sailtechnologyhas great potential.Itcan
provide much cheaper and much fasterpropulsion
for planetary missions, saving tens ofmillions of
dollarsinlaunchcost,”saysJaanPraks,principal
investigator ofAalto-1. “Themostimportantaspectof
this technology is that it is plausibletomanufacture
already. Itdoes not require any mysteriousfuture
inventions, just goodengineers and wisedecision-
makers are needed.”
NASAhasbecome interestedin this technology
too. Janhunen andUS colleagues won $500,000 of
funding in 2015 to continue testing in thelab.Asthey
studythe concept, those involvedare interested in
going muchfurther than Pluto.
Whileane-sailwouldtakejust10yearstoreach
theveryedge oftheSolar System, it wouldstilltake
thousandsof years to reachthe nearest stars.Charged particles
continuously stream
away from the Sun at
speeds of between 500
and 800 km/s – this is
known as the solar wind(^5) Because the thrust
is continuous, the spacecraft
constantly accelerates and
could reach the boundary of
interstellar space in about a
decade
NASA’s half-tonne
New Horizons craft
took nine years to
reach Pluto; an
e-sail could
transport twice
the mass in half
the time
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