Johns Hopkins'
Applied Physics Laboratory is sending a spacecraft
closer to the sun than any
probe has ever gone — and what it finds could
revolutionize what we know about our star and the solar
wind that influences everything in our solar system.
NASA has tapped APL to develop the ambitious Solar
Probe mission, which will study the
streams of charged particles the sun hurls into space from
a vantage point within the sun's corona — its
outer atmosphere — where the processes that heat the
corona and produce solar wind occur. At closest
approach Solar Probe would zip past the sun at 125 miles
per second, protected by a carbon-composite
heat shield that must withstand up to 2,600 degrees
Fahrenheit and survive blasts of radiation and
energized dust at levels not experienced by any previous
spacecraft.
Experts in the United States and abroad have grappled
with this mission concept for more than
30 years, running into seemingly insurmountable technology
and budgetary limitations. But in February
an APL-led team completed a Solar Probe engineering and
mission design study at NASA's request,
detailing just how the robotic mission could be
accomplished. The study team used an APL-led 2005
study as its baseline but then significantly altered the
concept to meet challenging cost and technical
conditions provided by NASA.
"We knew we were on the right track," said Andrew
Dantzler, Solar Probe project manager at
APL. "Now we've put it all together in an innovative
package. The technology is within reach, the
concept is feasible, and the entire mission can be done for
less than $750 million [in fiscal 2007
dollars], or about the cost of a medium-class planetary
mission. NASA decided it was time."
APL will design and build the spacecraft, on a
schedule to launch in 2015. The compact solar-
powered probe would weigh about 1,000 pounds; preliminary
designs include a 9-foot-diameter, 6-inch-
thick carbon foam-filled solar shield atop the spacecraft
body. Two sets of solar arrays would retract
or extend as the spacecraft swings toward or away from the
sun during several loops around the inner
solar system, making sure the panels stay at proper
temperatures and power levels. At its closest
passes the spacecraft must survive solar intensity more
than 500 times what spacecraft experience
while orbiting Earth.
Solar Probe will use seven Venus flybys over nearly
seven years to gradually shrink its orbit
around the sun, coming as close as 4.1 million miles to the
sun, well within the orbit of Mercury and
about eight times closer than any spacecraft has come
before.
Solar Probe will employ a combination of in-place and
remote measurements to achieve the
mission's primary scientific goals: Determine the structure
and dynamics of the magnetic fields at the
sources of solar wind, trace the flow of energy that heats
the corona and accelerates the solar wind,
determine what mechanisms accelerate and transport
energetic particles, and explore dusty plasma
near the sun and its influence on solar wind and energetic
particle formation. Details will be spelled
out in a Solar Probe Science and Technology Definition Team
study that NASA will release later this
year. NASA will also release a separate Announcement of
Opportunity for the spacecraft's science
payload.
Robert Decker, Solar Probe project scientist at APL,
said, "Solar Probe is a true mission of
exploration. For example, the spacecraft will go close
enough to the sun to watch the solar wind speed
up from subsonic to supersonic, and it will fly through the
birthplace of the highest energy solar
particles. And, as with all missions of discovery, Solar
Probe is likely to raise more questions than it
answers."
APL's experience in developing spacecraft to study the
sun-Earth relationship — or to work near
the sun — includes ACE, which recently marked its
10th year of sampling energetic particles between
Earth and the sun; TIMED, currently examining solar effects
on Earth's upper atmosphere; the twin
STEREO probes, which have snapped the first 3-D images of
explosive solar events called coronal
mass ejections; and the Radiation Belt Storm Probes, which
will examine the regions of energetic
particles trapped by Earth's magnetic field.
Solar Probe will be fortified with heat-resistant
technologies developed for APL's MESSENGER
spacecraft, which completed its first flyby of Mercury in
January and will begin orbiting that planet in
2011. Solar Probe's solar shield concept was partially
influenced by designs of MESSENGER's
sunshade.
Solar Probe is part of NASA's Living with a Star
Program, designed to learn more about the sun
and its effects on planetary systems and human activities.
NASA's Goddard Space Flight Center,
Greenbelt, Md., manages the program for the Science Mission
Directorate at NASA Headquarters in
Washington, D.C.
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