Icarus (this guy) had nothing on Solar Probe Plus (SPP). The new solar spacecraft, planned for a 2018 launch, will fly closer to the Sun than any human-built object before it, sipping the Sun's corona. SPP won’t carry humans (or any other life forms), but it still needs protection from our star’s searing heat and damaging radiation.
So, how close could it really get? This might depend on what happens as it sneaks up on the Sun; the probe will take a circuitous, cautious path, like a trainer slowly approaching a wary lion. The short answer is, no one is really sure.
The Sun: It's Hot
If you’re fair-skinned like me and visited the beach this summer, you know what kind of damage the Sun can do here on Earth, despite our planet’s protective atmosphere and magnetic field. Another 200-odd million miles farther out in the solar system, the story isn't much better: This spring, fast-moving charged particles from the Sun interfered with the Mars rover Curiosity’s computer, causing engineers to switch to a backup system. Billions and billions of miles from the Sun, the Voyager 1 probe finally stopped hearing signals from the solar wind just last summer. The Sun has a quite large sphere of influence, and the closer you get, the more dangerous the environment becomes.
Shielding a spacecraft from solar radiation is one of the trickiest challenges in spaceflight, but it gets a lot more complicated when you get close to the source. There are three basic kinds of danger, although really only two affect spacecraft, and they offer different challenges.
Keeping Things Cool
A longstanding mystery of heliophysics, or the study of the Sun, is why the Sun’s atmosphere is hotter than its surface. Hotter really isn't a strong enough word — the surface is about 5,500 degrees C, while the corona reaches between one and three million degrees. Any spacecraft approaching this region will be subject to temperatures that can completely melt just about anything.
Solar Probe Plus will try to sample the corona directly, flying as close as four million miles to the surface, but it won’t be in the million-degree range (hopefully). Still, it’s hard to build something that can withstand 1,200 to 1,500-Celsius heat — and maybe hotter — while staring at the Sun, yet keep instruments functioning at room temperature. Space itself can help with this, explains Betsy Congdon, an engineer at Johns Hopkins University’s Applied Physics Laboratory, who's building SPP's heat shield.
The shield's coated sun-facing surface will heat up to 1,400 degrees Celsius, but the back of it will be just 400 degrees C. The shield, made of carbon foam and bonded with carbon glue to a titanium truss, also serves as a radiator, dissipating heat from its backside into space. It weighs 150 pounds, and is nine feet in diameter.
"We call it a giant Frisbee made of carbon. It’s like a ceramic mug, but on steroids," Congdon told DVICE. (Note: interviews were conducted before NASA employees and contractors were furloughed during the government shutdown.)
Christopher St. Cyr, an astrophysicist in the heliophysics division at NASA’s Goddard Space Flight Center, is working on a different mission called Solar Orbiter, which will study the generation of the heliosphere, the magnetic bubble the Sun blows around itself (which extends to the edge of the solar system). The spacecraft will spend its time at roughly the same distance as Mercury is to the Sun, where the solar disk appears three times bigger than it does on Earth. Obviously, it’s quite hot in sunlight, but luckily for spacecraft, space is really, really cold, so things can even out a bit, St. Cyr explains:
"Even for a spacecraft at 1 AU, which is the distance to Earth, the sunward side is hundreds of degrees. When you look out into the darkness of space, it gets really cold, so you have a problem keeping it warm enough," he told DVICE. "The whole game is moving heat from where it is, and trying to dribble some off to keep the instruments warm enough to work."
To survive, both Solar Orbiter and SPP will draw some inspiration from Messenger, the first probe to orbit Mercury, which was built with a thick ceramic sunshade to keep it cool. Congdon said the shield team also looked at space shuttle tiles, which were made of a carbon composite and designed to withstand the searing heat of atmospheric re-entry. It turned out they were denser and heavier than the SPP team could use. In space, however, there’s no oxygen to worry about, so pure carbon can withstand excruciating temperatures that would turn it into charcoal on Earth.
The probe will draw power from the Sun, but not too much — its solar panels will be cooled with water, which will circulate through tubes through the panels in the Sun and then back into the frigid shade.
Blocking High-Energy Particles
Along with the types of radiation we perceive as warmth, the Sun spews out charged particles like protons and electrons. These fast-moving particles pack a punch similar to the type of risk posed by X-rays and gamma rays. A coronal mass ejection, which we see as a solar flare, can spew charged particles billions of miles into the solar system.
"That radiation is what we worry about for human and robotic explorers," St. Cyr says. "Energetic particles can damage avionics and electronics, so you do a combination of things: you use radiation-hardened parts, you shield them the best you can by adding layers of thick material, and you try to design in redundancies."
Scientists think that charged particles were to blame for Curiosity’s computer glitch earlier this year. A redundant second computer let the mission stay on track while engineers worked on the problem.
Shielding a spacecraft from high-energy rays is more difficult than protecting it from heat. Simply put, you need a way to slow the particles down, and this requires a dense material like water, or maybe lead. The trouble is, dense materials like water and lead are heavy, and therefore expensive, to launch into space Energetic rays come from more than the Sun, too. Despite the semi-protection offered by the heliosphere (the kind of magnetic bubble the Sun blows around itself), cosmic rays from distant sources pose their own danger. Statistically speaking, spending a week in the cosmic-ray bombardment of space canshorten your life expectancy by a day, according to Goddard’s Imagine the Universe program. We should mention that it's fuzzy math; cosmic rays are unlikely to give you cancer, but they can, and if they do they’ll shorten your life by a lot.
These dangers are part of the motivation for SPP’s looping orbit around the Sun. SPP's planned trajectory uses seven Venus flybys to reach a distance of 9.5 solar radii in 6.4 years — meaning a distance equivalent to almost 10 times the Sun's radius. By making a gradual approach like this, scientists can be extra cautious, St. Cyr says. "If they had a problem at 15 or 18 solar radii, they could say, 'OK there’s a problem,'" he says, giving the team a chance to reevaluate their approach.
Protection from Ultraviolet Radiation
If you add humans into the mix, you have to account for not only heat and high-energy rays, but also the form of radiation most familiar to beach-goers: ultraviolet. In space, just like on Earth, unprotected astronauts would be exposed to ultraviolet rays, which can cause a nasty sunburn and various kinds of skin cancer.
Spacesuits are white to reflect sunlight, which dissipates heat. They come with a type of air-conditioning system and heaters in the gloves, so shade-facing hands don’t get too cold. And spacesuit helmets have dark coatings to protect astronauts’ eyes from harmful UV rays.
But in space, UV rays are just the beginning. With cosmic rays, solar gamma and X-rays, and high-energy solar particles, you've got much bigger problems. And it's very unlikely humans would ever venture as close to the Sun as Solar Orbiter or Solar Probe Plus — in mythology, or in real life.