In the fall of 1958, a year after the Soviet Union had launched Sputnik, Americans wondered what NASA, their new space agency, would pick as its goal. The moon? Mars? A wayward asteroid? Few were prepared for the call that came in October, just weeks after the agency's formation, when the National Research Council's Space Science Board recommended that one of NASA's first priorities should be the sun.
But only now—almost 50 years after the board's report—is NASA seriously considering building a probe to send very close to the sun: a 2015 launch of a $750 million spacecraft. Why has it taken so long?
In the past, "nobody paid much attention to [the probe idea] because we didn't know how to do it," said Marcia Neugebauer, an adjunct research scientist at the University of Arizona's Lunar and Planetary Lab who spent most of her career at NASA's Jet Propulsion Laboratory (JPL). "We didn't have any rockets that could come anywhere close to [delivering] a solar probe. Then [Italian mathematician] Giuseppe Colombo came along and suggested that a Jupiter flyby could put you on a trajectory to the sun."
In the mid-1970s, an informal group at JPL started working on an unofficial effort to use a Jupiter gravity assist to send a probe on a suicide mission into the sun. The project, dubbed "an arrow to the sun," would have been a bare-bones effort, with no costly imagers and only minimal instrumentation. Its goal was to measure the properties of solar particles before they hit interplanetary space. Any data it would relay would have been sent back in real time; there would have been no second chances to gather more before the probe burned up as it approached the sun.
Critics of such suicide missions say that a wider range of information could be obtained through multiple close flybys. Spacecraft could measure the solar wind plasma near its origin and map how the corona, the sun's outer atmosphere, expands in the form of a hot, magnetized stellar wind. Although the Ulysses, Helios, SOHO, and Voyager spacecraft have explored the solar wind, the closest any spacecraft has come to the sun is 65 solar radii. In contrast, the proposed 2015 solar probe will come within 9 radii, well within the outer limits of the sun's corona. (One solar radius is some 431,000 miles, or almost twice the distance from Earth to the moon.) Learning about the genesis of the solar wind is crucial because satellite communications as well as future manned exploration missions are largely at its mercy. Coronal Mass Ejections (CMEs), huge bubbles of magnetized gas that burst from the sun over the course of several hours, whip up solar winds whose high-energy radiation can disable telecommunications satellites. Astronauts outside Earth's magnetosphere—on the moon, for instance—are also vulnerable.
Another part of the puzzle is astrobiological. Is our solar system and the star at its center somehow different from the rest of its stellar brethren? Our sun, on average only 93 million miles away, is considered a run-of-the-mill yellow dwarf, a typical yellow star that is in the process of converting hydrogen to helium by nuclear fusion in its core, and is roughly halfway through its estimated 10-billion-year lifespan. Do other planetary systems have yellow dwarf stars at their centers, and if so, do the planets orbiting them resemble Earth? "We deduce that other stars have coronas like ours, and we can see variations that can be interpreted [to be] solar cycles like ours," says Ralph McNutt, a space physicist at Johns Hopkins University's Applied Physics Laboratory and a member of NASA's science and technology definition team for the probe. "One of our questions is, if you have a yellow G dwarf star, do you always have a solar corona and a solar wind?"
For the past decade, engineers have had the elements needed to build a probe that could withstand such an extreme environment, but the hard engineering specifics were solved only recently at the Applied Physics Laboratory, which is designing the spacecraft. "The thermal environment is the bugaboo," says McNutt. "Solar arrays melt at that distance." Spacecraft designers must find another power source without using costly plutonium batteries. One option is to use retractable solar arrays, which would likely be continually unfurled and retracted throughout the mission on multiple close approaches to the sun.
The most probable flight trajectory will use multiple flybys of Venus to gravitationally enable the spacecraft to lose enough rotational energy to make increasingly closer approaches to the solar corona. The first approach would occur about six months after launch. "With four close passes per year in the final orbit, an extended mission of even just two years would give us over 10 close flybys," says Dave McComas, a space physicist at the Southwest Research Institute in San Antonio and chairman of the science and technology definition team.
The instrument package is expected to include an optical imager, and if that makes the final design cut, the resulting photographs would show the surface of the sun with a clarity unachievable from Earth. "These images would be on the evening news," says McNutt. But he cautions that each instrument's scientific value has to be weighed against the total mission budget. "Are pictures [worth] a quarter of a billion dollars?"
The team has already trimmed its initial cost projections from $1 billion in its 2005 incarnation to a target cap of $750 million. Last July, the Senate Appropriations Committee earmarked $20 million for fiscal year 2008 to fund design tweaks and development for the probe. Part of the craft's high cost stems from the extensive ground-based tests, McNutt notes, designed to simulate the solar system's most extreme thermal environment.
The team presented its research to NASA last February. "We've been able to prove that [the solar probe mission is viable]," said McComas. "Now there's no excuse. It just needs to be done."