[meteorite-list] No problem, they'll land at night

[Darren Garrison]

http://science.nasa.gov/headlines/y2008/10jun_solarprobe.htm?list1065474

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.

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.

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."