Image courtesy JHU/APL
Published September 10, 2010
Last week NASA mission managers behind the planned Solar Probe Plus made their final picks for the scientific experiments that will hitch a ride on the compact car-size spacecraft.
Slated to launch no later than 2018, Solar Probe Plus will repeatedly plunge through the sun's superhot outer atmosphere, called the corona, coming as close as 4 million miles (6.5 million kilometers) from the star's fiery surface.
"This promises to be one of the most daring and dangerous space missions ever attempted," said Dave McComas of the Southwest Research Institute in San Antonio, Texas, one of the principal investigators on the mission.
"Our star is the dominant force in the solar system, with streams of charged particles blown out by its solar wind that create space weather that affects our planet—everything from auroras to knocking out our electrical grids and even satellites."
Over its three-year mission, Solar Probe Plus will swing by Venus on its way to orbit the sun, where it will swoop through the corona up to 24 times.
Carbon-based Heat Shield Key to Dive-bombing Probe
The probe's first pass by the sun will occur two months after launch, at a distance of 15 million miles (24 million kilometers) from the star.
Over the next several years, swing-bys of Venus will slowly tweak the probe's trajectory, so that Solar Probe Plus gradually gets closer to the sun during each successive orbit. By 2024 the probe's path will be eight times closer to the sun than the orbit of Mercury.
Flying so close to the sun means things will really heat up for the plucky probe, so NASA has outfitted the craft with a cutting-edge carbon-composite heat shield.
"You can think of it as a large chunk of material similar to the ceramic tiles found on the underbelly of the space shuttles, used to protect them from extreme heat experienced during reentry into Earth's atmosphere," McComas said.
(See a picture of the space shuttle Atlantis rolling over to dock with the International Space Station.)
The solar probe's heat shield will have to withstand temperatures exceeding 2,550 degrees Fahrenheit (1,400 degrees Celsius), as well as blasts of intense solar radiation.
When the spacecraft is at its closest, "the heat of the sun on the heat shield is actually more than 500 times more intense than would be felt at the Earth's orbit," McComas said.
"Therefore, to survive, the payload has to hide behind the shadow of the heat shield, where the space environment is pretty benign."
First Craft to Take Sun Samples
Ultimately, dive-bombing the sun should help scientists solve a number of mysteries about our nearest star.
The Solar Probe Plus mission "addresses the most fundamental processes that connect our star with space surrounding our planet," said Thomas Zurbuchen, a space sciences and engineering professor at the University of Michigan.
Zurbuchen is a project scientist on the recently approved Solar Orbiter mission, a joint project between the European Space Agency and NASA. Solar Orbiter will study the sun from a distance between Mercury and Venus' orbits.
(Related story and video: "Sun Erupts—Epic Blast Seen by NASA Solar Observatory.")
"Our star is active and varying on many time scales, and so is our space environment, with many consequences to our life here on Earth," Zurbuchen said. "With a successful [sun-touching] probe, we would learn about connections that have escaped our understanding so far."
For instance, it's still a mystery why the sun's inner atmosphere, the photosphere, averages about 11,000 degrees Fahrenheit (6,000 degrees Celsius) while the higher corona sees temperatures of well over 1.8 million degrees Fahrenheit (a million degrees Celsius). (See "'Corkscrew' Waves Seen on Sun—Keys to Solar Mystery?")
Scientists also don't know what processes accelerate charged particles in the corona to create the solar wind. (See "Sun's Mysterious Waves Found; May Be Solar Wind Source.")
In general, when it comes to the sun, "we have a lot of remote observations but no local samples," the Southwest Research Institute's McComas said. "This mission will actually go right down into the corona and make those measurements directly."
Sun Bombing a "Trip to Hell and Back"
For his part, McComas will lead an investigation that will inventory the elements in the sun's atmosphere.
The three other experiments will count and measure the most abundant particles in the solar wind, make 3-D images of the sun's corona, and take direct measurements of electric and magnetic fields, radio emissions, and shock waves coursing through the sun's atmosphere. (See sun pictures.)
NASA has budgeted $180 million (U.S.) total for the experiments' preliminary analysis, design, development, and tests—and the scientific payoff should be well worth the cost, experts say.
"The first near flyby of a star in the history of humanity is pretty important, no matter how we think about it," the University of Michigan's Zurbuchen said.
"I always think of it as a trip 'to hell and back' and marvel at the beauty of our star, as well as the technical solutions that enable such a mission."
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