http://www.skyandtelescope.com.au 11them to deduce what’s going on. The team
detectedsynchrotron radiation, polarised light
produced by electrons corkscrewing along
magnetic field lines. The polarised emission
varied, and quickly, on roughly15-minute
timescales.Thatmeansthemagneticfields
themselves, and the gas they interweave, are
movingalot.
These changes only appear very close to
theblackhole.Essentially,theastronomers
areseeingtangled,turbulentmagneticfields
cavortingrightnearwheregastakesitsfinal
plunge in past the event horizon.
Theobservationsrevealstructureson
ascalethat’sonlyahalfdozentimesthe
black hole’s radius, the team reports in the
December 4 issue ofScience. To be able to
detect these motions at this small scale is
“reallyjustamazing,”saysJohnson.
This result is great news for theorists, says
Chris Reynolds (University of Maryland), who
hasspenthislifeworkingonblackholes.It
hastakendecadestounderstandwhygas
intheaccretiondiskfallsintoablackholeat
all—itshouldjustorbitforever,becausethe
gasistootenuousforfrictiontoslowitdown.
Astrophysicistsrealisedthatmagneticfields
swirling around in the accretion disk would tug
on the gas, pumping up the turbulence and
robbingthegasofitsangularmomentum,
allowing it to fall into the black hole.
“Butit’sallbeenverymuchinatheoretical
domain,” Reynolds says. The new EHT
observations show that theorists have been
on the right track. “It’s honest-to-goodness
measurements of a real black hole that are
gettingtothecruxofthisissue.”
■CAMILLE M. CARLISLEIn this artist’s conception, tangled magnetic fields (blue) surround and emanate from Sagittarius
A*,theblackholeatthecentreofourgalaxy.ThefieldstheEventHorizonTelescopeteamfound
are either in the disk or the jet (if the black hole has one — astronomers don’t know).
M. WEISS / CFA
T
riumph is never sweeter than
when following defeat. At 23:
Universal Time on December 6,
Japanese Aerospace Exploration Agency
(JAXA) engineers executed an innovative
contingency plan to get the Venus
Climate Orbiter, named Akatsuki, to its
destination. The success came five years
to the day after a main-engine failure
caused the spacecraft to fly past Venus
instead of going into orbit.
The spacecraft ran into trouble in
20 10 when its main engine failed to
execute a planned 12-minute firing.
Telemetry analysis showed a pressure
drop caused by a faulty valve in the main
engine, which resulted in burning an
oxidizer-rich mixture beyond normal
limits. The failure overheated and
destroyed much of the engine.
With the primary means of propulsion
ruined, scientists and engineers
scrambled to recover the mission while the
spacecraft circled the Sun. Their solution:
fire the craft’s four reaction-control
thrusters for more than 20 minutes at the
next available opportunity to enter orbit
Japan’s Akatsuki finally reaches Venus
JAXAThe Ultraviolet Imager aboard Akatsuki
captured this image of Venus’s swirling
atmosphere on December 7, 2015, from an
altitude of 72,000 km.around Venus. Engineers ran a series of
short test burns in 2011, then placed the
spacecraft in electronic hibernation to
extend its life until the planned maneuver.
Akatsuki’s successful arrival marks
the first time the Japanese space agency
has put a spacecraft in orbit around
another planet.The track is slightly wider-ranging
than the one originally planned, which
would have carried the spacecraft around
Venus every 30 hours, with a closest
approach of 300 km. The December burn
instead placed Akatsuki in a 13.6-day
elliptical orbit that brought it as close as
400 km to the surface. As this issue went
to press, the JAXA team was planning a
series of adjustments that would shrink
Akatsuki’s elliptical orbit to a period
of 9 days, in time to start full science
operations in April.
The six instruments aboard will probe
Venus’s atmosphere, measuring its rotation
and convection. Researchers also hope to
detect evidence for lightning using a
high-speed imager. Viewing across radio,
infrared, visible and ultraviolet wavelengths,
the payload will also record heat radiated
from the surface — perhaps spotting active
volcanoes, if they exist. A series of
radio-occultation experiments will allow
researchers to probe the atmosphere’s
depths as the spacecraft makes successive
passes behind the planet as seen from Earth.
■ DAVID DICKINSON