http://www.skyandtelescope.com.au 19particles become accelerated via processes like those
involved with Earth’s aurorae. Yet despite seven flyby
spacecraft and Galileo’s 33 orbits around Jupiter, no
spacecraft has yet flown directly over Jupiter’s poles.
So Juno’s scientists and engineers loaded the
spacecraft with instruments that can measure charged
particles, magnetic fields and electric waves as it flies
for the first time through the magnetic field lines
where we think the auroral particles are generated.
At the same time, UVS, JIRAM and JunoCAM will
look down on the aurorae where those same field lines
intersect the planet.
I must admit that I’m a bit nervous for Juno —
those field lines carry millions of amps of electrical
current. It’s a scary place to explore.
Juno’s imminent arrival
For the first time, we have sent a spacecraft to
the outer Solar System that’s not powered by the
radioactive decay of plutonium. Instead, Juno
generates electricity via three huge solar-cell panels,
each 9 metres long. At Jupiter, where sunlight has^1 / 25
its intensity at Earth, these panels generate 400 watts
of power. Roughly half of this keeps the spacecraft
warm, while the rest powers Juno’s electronics, radio
transmitter and science instruments. The spacecraft
slowly cartwheels, once every 30 seconds. That’s a plus
for gathering charged-particle and electromagnetic
data, but even this slow rotation makes snapping
pictures tricky.
Juno rode an Atlas V rocket into space on August 5,
- After using a gravity boost from a flyby of Earth
in October 2013, it will finally arrive at Jupiter
on July 4. Firing its main engine for 30 minutes when
at its closest to the planet, the spacecraft will slow
enough to achieve a polar orbit. The first couple
of orbits are planned to last 53 days each, enabling
the Juno team to get used to operating the spacecraft
at Jupiter.
Then, in mid-October, more engine firings will
drop Juno into a sequence of 14-day-long orbits.
During each of these, the spacecraft will make a
2-hour dash over the north pole, duck under the potent
radiation belt as it speeds (at 60 km per second) just
4,200 km above the cloudtops, and then zoom back
out over the south pole. For the rest of each elliptical
circuit, which will carry the spacecraft out to about
2.7 million km, Juno will transmit the data gathered
during its most recent close-in dash and sample the
magnetospheric environment far from the planet.
Such an orbit would be just great if we could keep
it that way. But Jupiter’s 10-hour rotation makes its
equator 6% fatter than the poles. The bulge’s gravity
will continually tug on the spacecraft, altering its
trajectory and precessing (tilting) the orbit by about 1°
per circuit. Sooner or later Juno will plunge through
the equatorial radiation belt. We’ve protected the
electronics as best we can by encasing them in a
titanium vault. But quite likely bathing those sensitive
electronics in 10 MeV electrons will cause significant
damage. We’re hoping for 35 orbits — even a handful
would revolutionise our understanding of Jupiter —
and, who knows, perhaps the spacecraft will survive
much longer.
Get ready, Juno — the fun’s about to begin! ✦Fran Bagenal is a planetary researcher at the
University of Colorado, Boulder, and co-chairs Juno’s
Magnetospheric Working Group. She thanks Sushil
K. Atreya (University of Michigan) for assistance with
this article. For more information about Juno and its
mission, visit missionjuno.swri.edu and nasa.gov/
mission_pages/juno.What Juno’s scientific instruments will be looking for
Instrument Acronym Description
Gravity Science GS Detects Doppler shift of radio broadcasts from Juno to Earth
to derive small motions of Juno due to Jupiter’s uneven gravity
field and its internal mass distribution.Jovian Auroral JADE Measures the distribution, energy, and Distribution velocity
of ions (5 eV to 50 KeV) and Experiment electrons
(100 eV to 100 KeV) in auroral regions of Jupiter.Jovian Energetic JEDI Measures fluxes of high-energy ions (20 Particle Detector
keV to 1,000 keV) and electrons (40 keV to 500 keV) in the polar
magnetosphere of Jupiter.Jovian Infrared JIRAM Maps upper layers of the atmosphere to depths of 50 to
Auroral Mapper 70 km (5 to 7 bars); images aurorae at 3.4 μ (H 3 + ions);
detects CH 4 , H 2 O, NH 3 , PH 3.JunoCAM Visible-light telescopic camera to facilitate
education and public outreach.Magnetometer MAG Measures the strength and direction of the magnetic field;
Advanced Stellar Compass (ASC) monitors orientation of
MAG sensors.Microwave MWR Measures electromagnetic waves in six ranges (600 MHz
Radiometers to 22 GHz) to map H 2 O and NH 3 abundances to pressures
of up to 200 bars (depths of 500 to 600 km).Ultraviolet UVS 1024-by-256-channel detector provides spectral images of
Spectrometer the ultraviolet auroral emissions in Jupiter’s polar ionosphere.Radio & Plasma Waves Measures spectral energy distribution of
Wave Sensor radio and plasma waves in auroral regions.