They may
seem small and boxy, but five new
NASA probes have a lofty purpose: sifting through the Earth’s
magnetic field for the stormy beginnings of the planet’s most dynamic auroras.
Set to
launch in less than a month, NASA’s THEMIS spacecraft are designed to hunt down
the origin of substorm -- tempests of high energy particles that accumulate
somewhere in the Earth’s magnetic field, then flow back to the planet and
supercharge its aurora
borealis – also known as the Northern Lights [image].
“For
more than 30 years the source location of these explosive energy releases has
been sought after with great fervor,” said THEMIS mission principal
investigator Vassilis Angelopoulos, of the University of California, Berkeley’s Space Sciences Laboratory, which built the spacecraft quintet for NASA with Swales Aerospace. “Knowing the
trigger location will help us identify the correct physics in play.”
The
impact of substorm particles – accumulated from the Sun’s solar wind – with the Earth’s atmosphere are responsible for
the shimmering ripples and color changes that accompany some of the most
spectacular auroras to date, THEMIS researchers said [image].
But the seemingly aesthetic space weather
phenomena may also prove to be a building block for severe space storms capable
of endangering astronauts in
orbit, interfering with communications or harming satellites, they added.
A
United Launch
Alliance Delta 2 rocket is slated to launch the five THEMIS probes into
orbit from Florida’s Cape Canaveral Air Force Station during a 19-minute window
that opens at 6:07 p.m. EST (2307 GMT) on Feb. 15 [image].
The mission’s name – short for the hefty title Time History of Events and
Macroscale Interactions during Substorms – is also taken from the Greek goddess
of justice.
The
THEMIS family
Each about the
size of a dishwasher, the five THEMIS spacecraft are nearly identical, with any
one designed to fill in the orbital slot of its robotic brethren [image].
Only four probes are required to complete NASA’s planned two-year and $200
million mission – the fifth is a spare – but wrangling them all into launch
position is not without challenges.
“THEMIS is
the first NASA mission to launch five spacecraft on a single vehicle,” said UC
Berkley’s Peter Harvey, NASA’s THEMIS project manager, adding that the probes
are stacked atop one another in a sort of space probe wedding cake. “This
allowed to the probes to be independently carried into space.”
After
launch, the first probe – dubbed Probe A – is designed to pop free of from its
housing atop the wedding while the other four deploy like flower petals [image].
The spacecraft are then expected to extend a series eight booms that range from
just three-feet (one meter) to 65 feet (20 meters) in length [image]. Each THEMIS probes comes equipped with 11
instruments that include a pair of telescopes, six electric field instruments,
two different magnetometers and an electrostatic analyzer.
“Our
philosophy was to keep the probes as simple as possible and leave that
complexity on the ground,” Harvey said.
That added
complexity includes some 20 All-Sky camera stations and a host of ground-based
magnetometers scattered across Alaska and Canada to complement the THEMIS
probes’ findings.
Rooting
out substorms
In order to
scan the Earth’s magnetic field and pinpoint the origin of substorms, THEMIS
researchers plan to stagger their spacecraft in different orbits that range in
altitude from 10 to 30 times the radius of the Earth (the planet’s radius is
about 3,962 miles, or 6,378 kilometers) [image].
“The
problem has been that the substorm starts from a single point in space
somewhere near Earth and within minutes it progresses past the Moon’s orbit,” Angelopoulos said. “So a single satellite alone cannot identify
the substorm’s time and point of origin. Multiple spacecraft in tightly
choreographed orbits are needed.”
The spare
THEMIS probe and two others are slated for a one-day orbit some 10 Earth radii
out in space, while two others will take up stations about 20 Earth radii above
the planet. The final spacecraft is destined for a four-day orbit that reaches
out to a distance of 30 times that of Earth’s radius. Their flight path also
brings the five THEMIS spacecraft into alignment with themselves and Earth
every four days, when researchers hope will be a prime substorm hunting period.
“It is
amazing that these tiny probes will travel over halfway to the Moon,” Harvey said.
The
distribution allows the THEMIS probes to evaluate two candidate locations --
one near the halfway point between the Earth and Moon and another closer in --
for substorms. Mission scientists are hopeful they’ll record at least 30
substorm events during the two-year THEMIS mission.
“Right now,
we don’t have a real contender for which one is going to go in which position,”
Harvey said, adding that the near-identical probes will likely show their
differences once in space. “The best instruments then will go in the best
orbits.”
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