Image: NASA's Solar Probe spacecraft
NASA/JHUAPL
An artist's concept shows NASA's Solar Probe spacecraft making its daring pass toward the sun, where it will study the forces that create solar wind.
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updated 5/21/2008 2:49:45 PM ET 2008-05-21T18:49:45

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 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 spacecraft from temperatures that will reach as high as 2,552 degrees Fahrenheit.

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.

Revised objectives
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 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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