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June
10, 2008: For more than 400 years, astronomers have
studied the sun from afar. Now NASA has decided to go there.
"We
are going to visit a living, breathing star for the first
time," says program scientist Lika Guhathakurta of NASA
Headquarters. "This is an unexplored region of the solar
system and the possibilities for discovery are off the charts."
Right:
An artist's concept of Solar Probe Plus. [more]
The
name of the mission is Solar Probe+ (pronounced "Solar
Probe plus"). It's a heat-resistant spacecraft designed
to plunge deep into the sun's atmosphere where it can sample
solar wind and magnetism first hand. Launch could happen as
early as 2015. By the time the mission ends 7 years later,
planners believe Solar Probe+ will solve two great mysteries
of astrophysics and make many new discoveries along the way.
The
probe is still in its early design phase, called "pre-phase
A" at NASA headquarters, says Guhathakurta. "We
have a lot of work to do, but it's very exciting."
Johns
Hopkins' Applied Physics Lab (APL) will design and build the
spacecraft for NASA. APL already has experience sending probes
toward the sun. APL's MESSENGER spacecraft completed its first
flyby of the planet Mercury in January 2008 and many of the
same heat-resistant technologies will fortify Solar Probe+.
(Note: The mission is called Solar Probe plus because
it builds on an earlier 2005 APL design called Solar Probe.)
At
closest approach, Solar Probe+ will be 7 million km or 9 solar
radii from the sun. There, the spacecraft's carbon-composite
heat shield must withstand temperatures greater than 1400o
C and survive blasts of radiation at levels not experienced
by any previous spacecraft. Naturally, the probe is solar
powered; it will get its electricity from liquid-cooled solar
panels that can retract behind the heat-shield when sunlight
becomes too intense. From these near distances, the Sun will
appear 23 times wider than it does in the skies of Earth.
Above:
A simulated view of the Sun illustrating the trajectory of
Solar Probe+ during its multiple near-Sun passes. [Larger
image]
The
two mysteries prompting this mission are the high temperature
of the sun's corona and the puzzling acceleration of the solar
wind:
Mystery
#1—the corona: If you stuck a thermometer in the
surface of the sun, it would read about 6000o C.
Intuition says the temperature should drop as you back away;
instead, it rises. The sun's outer atmosphere, the corona,
registers more than a million degrees Celsius, hundreds of
times hotter than the star below. This high temperature remains
a mystery more than 60 years after it was first measured.
Mystery
#2—the solar wind: The sun spews a hot, million mph
wind of charged particles throughout the solar system. Planets,
comets, asteroids—they all feel it. Curiously, there is no
organized wind close to the sun's surface, yet out among the
planets there blows a veritable gale. Somewhere in between,
some unknown agent gives the solar wind its great velocity.
The question is, what?
"To
solve these mysteries, Solar Probe+ will actually enter the
corona," says Guhathakurta. "That's where the action
is."
The
payload consists mainly of instruments designed to sense the
environment right around the spacecraft—e.g., a magnetometer,
a plasma wave sensor, a dust detector, electron and ion analyzers
and so on. "In-situ measurements will tell us what we
need to know to unravel the physics of coronal heating and
solar wind acceleration," she says.
Right:
The re-designed Solar Probe+ spacecraft. [more]
Solar
Probe+'s lone remote sensing instrument is the Hemispheric
Imager. The "HI" for short is a telescope that will
make 3D images of the sun's corona similar to medical CAT
scans. The technique, called coronal tomography, is a fundamentally
new approach to solar imaging and is only possible because
the photography is performed from a moving platform close
to the sun, flying through coronal clouds and streamers and
imaging them as it flies by and through them.
With
a likely launch in May 2015, Solar Probe+ will begin its prime
mission near the end of Solar Cycle 24 and finish near the
predicted maximum of Solar Cycle 25 in 2022. This would allow
the spacecraft to sample the corona and solar wind at many
different phases of the solar cycle. It also guarantees that
Solar Probe+ will experience a good number of solar storms
near the end of its mission. While perilous, this is according
to plan: Researchers suspect that many of the most dangerous
particles produced by solar storms are energized in the corona—just
where Solar Probe+ will be. Solar Probe+ may be able to observe
the process in action and show researchers how to forecast
Solar Energetic Particle (SEP) events that threaten the health
and safety of astronauts.
Solar
Probe+'s repeated plunges into the corona will be accomplished
by means of Venus flybys. The spacecraft will swing by Venus
seven times in six years to bend the probe’s trajectory deeper
and deeper into the sun’s atmosphere. Bonus: Although Venus
is not a primary target of the mission, astronomers may learn
new things about the planet when the heavily-instrumented
probe swings by.
"Solar
Probe+ is an extraordinary mission of exploration, discovery
and deep understanding," says Guhathakurta. "We
can't wait to get started."
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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