WASHINGTON
- Spurred to action by Congress, NASA
is finally moving out on an ambitious mission to send a spacecraft closer to
the sun than any has ever gone before.
NASA directed the Johns Hopkins
University Applied Physics Laboratory (JHUAPL) to begin preliminary work on a
proposed $750 million Solar Probe mission last month, with plans to launch
around 2015 to fly through the sun's corona and study the stream of charged
particles it regularly blasts into space. At its closest approach, the Solar Probe would fly
within 4.3 million miles (7 million km) while being bombarded by radiation and blasted
by withering temperatures.
The Laurel,
Md.-based lab will receive $13.8 million from NASA this year to begin pre-Phase
A development work to address the mission's considerable technical risks, among
them designing a carbon composite heat shield capable of protecting the roughly
992-pound (450-kg) spacecraft from
temperatures that will reach as high as 2,552 degrees Fahrenheit (1,400
degrees Celsius).
And with NASA intending to solicit
instrument proposals for the Solar Probe mission this year, APL will use part
of the $13.8 million to support the instrument accommodation assessments that
must happen before NASA can select a spacecraft's science payload.
NASA is funding APL's pre-Phase A
work and the upcoming competition to pick the Solar Probe's science payload
with the help of $17 million Congress added to the space agency's 2008 budget
for the mission.
Sen.
Barbara Mikulski (D-Md.), chair of the Senate Appropriations commerce, justice,
science subcommittee told Space News in a written statement she was pleased
NASA tapped APL to begin work on Solar Probe.
"I fought
alongside the scientific community to start Solar Probe because of its
importance in understanding the effects of the sun on the Earth," Mikulski said
in a May 9 statement. "These effects are profound on everything from the health
and safety
of our astronauts, to civilian and national security satellites, our power
grid system and even international airline flights over the Earth's poles. I
will continue to fight to ensure that there is funding in the federal checkbook
for this important priority."
Mikulski's
pledged support could prove essential to ensuring that the Solar Probe work
getting under way this year continues into 2009. Although NASA intends to fund
Solar Probe instrument proposals starting next year, the agency has requested
no 2009 money for other aspects of the mission's development.
As a
result, after spending nearly $14 million this year on Solar Probe,
expenditures would drop to zero in 2009 unless Congress again decides to add
money. NASA's five-year budget projection, however, does include $3.4 million
for Solar Probe in 2010, $40.1 million in 2011, $74.2 million in 2012 and
$106.3 million in 2013. To keep Solar Probe on track for a 2015 launch, NASA
would need to come up with an additional $500 million.
Reaching
the Sun
The Solar
Probe mission concept has been incubating for more than 30 years, only to be
repeatedly stymied by budget and technical limitations, according to Walter
Faulconer, APL's business area executive for civilian space programs.
"In the
last incarnation, it was a $1.2 billion mission that required an RTG," he said,
referring to radioisotope thermoelectric generators that transform the heat
from decaying plutonium into electricity. "And that was a big improvement over
a previous version which was a multi-billion-dollar mission."
In 2007,
NASA's associate administrator for science at the time, Alan Stern, gave APL a
study contract worth $1 million to produce a Solar Probe mission concept that
could be accomplished for around $750 million, including launch, but not
require a nuclear power source.
With NASA
uncertain about how much plutonium-238 it will be able to obtain in the future,
the agency is trying to preserve the current inventory for a flagship-class
mission to the outer planets it intends to launch in the 2015-2020 timeframe.
Previous Solar Probe concepts entailed sending a fairly massive spacecraft out
to Jupiter for a gravity assist, an indirect route that added several years of
travel time and required a nuclear-power source to compensate for the dearth of
solar energy available so far from the sun.
Faulconer
said it was not easy getting the scientists and engineers to abandon some of
their preconceived notions and take a fresh look at what needed to be done to
send a probe so
close to the sun.
"We had to knock some heads since a
lot of people felt like they had studied this to death," he said.
By the time
the study was completed in March, the APL-led team, according to Faulconer, had
come up with "a very elegant mission that answered the mail - it's $750
million, it's non-nuclear and we did not give up any of the science."
The APL
team did, however, relax two long-established objectives for the mission. As
currently envisioned, Solar Probe would not go as close to the sun and it would
not fly over the sun's poles.
Previous
mission concepts called for flying within 1.8 million miles (3 million km) at
closest approach, closer than the current 4.3 million miles. But even at 9
solar radii, a distance of roughly 4.3 million miles, the probe still will be
well within the sun's outer atmosphere.
"It's still
hot but not as hot," Faulconer said.
Sending
Solar Probe over the sun's pole also turned out to be "a real cost driver" for
the mission, according to Faulconer. Dropping that requirement "really opened
things up."
"There was
a scientific reason why they wanted to go over the pole," he said. "But it
turns out that the phenomenon scientists once thought occurred only at the
poles we now know thanks to recent discoveries from
our Stereo mission that the same phenomenon can be observed over other parts
of the sun."
Instead of
flying out to Jupiter, Solar Probe would use seven Venus flybys over nearly
seven years to gradually shrink its orbit, Faulconer said. APL's Mercury
Surface, Space Environment, Geochemistry and Ranging mission is employing a similar
approach, flying past Venus in 2006 and again in 2007 as it seeks to settle
into orbit around the innermost planet in 2011.
The optimal
time to begin the Solar Probe's seven-year journey would be 2015, Faulconer
said, with less optimal launch opportunities in 2013 and 2018.
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