Juno’s path is to thread a needle between Jupiter’s radiation belts
and its atmosphere. Starting at 11:18 pm EDT (8:18 pm PDT) on July 4’s
evening, the spacecraft will burn its main engine to slow it from its
165,000 mph speed with respect to Earth, one of the highest speeds ever
reached. The burn should last 35 minutes, until 11:53 pm EDT (8:53 pm
PDT). Eager scientists and engineers, joined by journalists and those
watching on NASA TV around the world, will listen for a beep at the
beginning of the engine burn and another beep at the end of the burn,
hoping that the exact timing indicates a successful insertion into a
polar orbit around Jupiter. All action actually took place 48 minutes
earlier, since the radio signals from Jupiter currently take 48 minutes
to travel to Earth at the speed of light. The last commands to Juno are
being sent today; the spacecraft will then be on its own for a few
days, though, of course, it will be monitored.
It will then
take a couple of days before the instruments are turned on. The
spacecraft has been made to spin faster than previously, 5 rpm, to
improve its stabilization before the burn; it will be slowed to 2 rpm
for its science observations. Not since the Pioneer spacecraft has
rotation been used for stabilization, but this spacecraft is in one of
NASA’s smaller (and less expensive) spacecraft series, following on the
very successful New Horizons spacecraft that flew by Pluto last year.
Most of the
instruments deal with Jupiter’s particles and magnetic field, which is
20,000 times more powerful than Earth’s. The main instruments, to
protect them, are in a vault made of 400 pounds of titanium to protect
them from high radiation. The Junocam, its imaging camera, is outside
that protection, and may not last as long as other instruments; further,
it will give images as it rotates that will have to be transformed to
the equivalent of steady views.
Juno’s instruments will be powered by huge solar panels; end to end,
the spacecraft would fill a basketball court from hoop to hoop. A boom
at the end of one arm extends even further, to put the magnetic-field
measuring equipment as far from the metal at the spacecraft’s center as
possible, to minimize distortions.
When the
planets of our solar system formed about 5 billion years ago, Jupiter
took up about half the leftover material besides the Sun. Juno’s main
goals include discovering what the inside of Jupiter is like, as part of
understanding how the planets formed. One specific goal is to discover
whether Jupiter has a rocky or ice core of elements heavier than
helium. It has been estimated as being 10 to 20 times the Earth’s mass;
though dense, it is not necessarily solid, even under the huge pressure
near Jupiter’s center.
Juno will be in
an extended elliptical orbit, coming as close as only 5000 miles above
the cloudtops at its lowest point. Its 14-day elliptical orbit will be
timed so that each time it comes close it passes a few degrees in
longitude around the planet, so that by the time it completes three
dozen orbits after about a year, it will have mapped the planet at high
accuracy completely. But unlike previous spacecraft to Jupiter, it is
concentrating on a layer under the visible cloud deck, where the
magnetic field is apparently formed. That layer stays much steadier
than does Jupiter’s atmosphere above it.
Jupiter’s
magnetosphere is huge; if we could see it, it would appear as big as our
Moon appears in our sky. We know from measuring radio signals from
Jupiter that the magnetic field rotates like clockwork, even though the
cloud deck rotates at different speeds at different latitudes. The
magnetic field generates a huge, fairly steady aurora, over twice as
large as Earth. The spacecraft has been traveling within Jupiter’s
magnetosphere, where the magnetic field dominates, for about a week.
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