Flying a spacecraft to the sun has been an objective of NASA since the very beginning.
In October 1958, just months after NASA was founded, a committee headed by physicists John Simpson and James Van Allen (for whom the radiation belts around the Earth are named) outlined some long-term goals for the new civil space agency. One of the principle missions identified by the committee was «a solar probe to pass inside the orbit of Mercury to study the particles and fields in the vicinity of the Sun.»
Additional missions were identified as well, such as a lunar spacecraft and probes to explore the outer planets. While most of these missions were accomplished within a few short decades, the solar spacecraft required a level of technological expertise that humanity just hadn’t achieved—until now.
«Solar Probe is the only one of those missions not to have launched,” says Andrew Driesman of the Johns Hopkins University Applied Physics Laboratory (APL). Driesman serves as the Project Manager of NASA’s Parker Solar Probe, a mission 60 years in the making, destined to fly faster and closer to the sun than anything ever conceived.
Fly Me to the Sun
At 3:48 a.m. EDT on Saturday, August 11, the Parker Solar Probe will begin its journey to the sun atop the second most powerful rocket in operation: the Delta IV Heavy. It may seem strange, but launching to the sun is as challenging, if not more, than launching to Pluto or beyond.
Despite being relatively light as far as spacecraft go, only about 700 kg including fuel, Parker Solar Probe will need to launch at an incredibly high velocity to make its rendezvous with the sun. The Earth itself is flying around the sun at immense speeds, so to avoid simply orbiting the sun at the same distance as the Earth, the rocket must launch against the velocity of the planet.
«Unlike going to the outer solar system, where you have to speed the spacecraft up, going into the inner solar system you have to slow it down—but from the perspective of the spacecraft it’s all changes in velocity,» Driesman says. «The Earth travels through the solar system at about 30 km/s… so we have to shed the Earth’s velocity in order to get close to the sun.»
Even the mighty Delta IV Heavy isn’t quite enough to get Parker on its way, so NASA has provided a third «kick stage» for the launch that will ignite its rocket engine in space after the primary three-core booster and second stage have burned through all their fuel. The spacecraft will then fly by Venus for a gravity assist, which will decelerate the probe further so it can fall in toward the sun.
Over the course of 24 orbits, the craft will swing by Venus a total of 7 times to continue shedding velocity and falling closer and closer to the sun, ultimately descending to within 4 million miles of the solar surface—about 7 times closer than a spacecraft has ever ventured before.
And yet, for all this slowing down, the Parker Solar Probe will «be the fastest human-made object in existence,» says Driesman. When Parker slings around the sun at closest approach, it will hit a blistering speed of 430,000 mph relative to the sun.
A Shield to Take on Helios
The Parker Solar Probe’s journey takes it into the sun’s corona, a bubble of superheated plasma stretching millions of miles from the solar surface. To survive in such a hellish environment, the spacecraft will carry a composite heat shield that would not have been possible to build even 10 years ago—light enough to launch, strong enough to withstand the launch, and with enough heat resistance to guard the spacecraft from the face of the sun.
«If you were a satellite in Earth orbit… and your face were a meter by a meter, it’s about 1,350 watts [of energy],» Driesman says. «Where we’re going, it’s about 500 times that. So it’s roughly 3 million watts of energy on the heat shield at closest approach.»
The spacecraft’s heat shield, known as the thermal protection system (TPS), is a sandwich of carbon-reinforced-carbon sheets with about 4.5 inches of carbon foam in between—a structure that will heat to about 2,500 degrees F.
«If you had tried to do this mission in the 60s or the 70s, I would even say the 80s, you’d be talking about flying high temperature metals,» Driesman says. «You’d be building heat shields out of some metals that have very high melting points, and you would never get it off the ground because it would be so heavy.»
Unlike most commercial carbon fiber, carbon-carbon structures are not held together by hardened resin—which would evaporate in the vicinity of the sun like gasoline on hot pavement. To build the heat shield, NASA manufactured sheets of carbon fiber and impregnative the substance with resin filled with «chopped up carbon fibers,» Driesman says, letting the resin harden before burning it off in a 3,000-degree oven and repeating the process four or five times.
«You’re left with these carbon fibers that are intertwined. … So when we talk about a carbon-carbon structure, it is pure carbon. There is no resin, there ain’t nothing else.»
Solar Panels—Up Close and Personal
The spacecraft will be powered by solar panels—there will certainly be plenty of solar energy where it’s going—but a robust cooling system is required to keep the panels from shattering in the heat.
«For every watt of electrical energy we generate, we have to deal with about 13 watts of heat,» Driesman says. «So the cooling system on the spacecraft has got about 6,000 watts of cooling capacity, which is enough to cool a living room in the average house.»
Some clever engineering was required to construct what is perhaps to most efficient liquid cooling system ever devised.
«Most solar panels are laid up on an aluminum or carbon or a honeycomb composite, and that’s what provides the structure for the glass cells,» Driesman says. «In this case, they’re laid up on a titanium platen, and the titanium platen has these very thin tubes built into them… where the water circulates… back through a manifold and then through a pump and up into radiators that pick up the heat from the solar panel and radiate that heat to deep space.»
Interestingly, the cooling system will use water, deionized to prevent corrosion and pressurized to raise the boiling point to about 150 degrees Celsius. The water, about a gallon of it, will be contained in something that looks like a «a very small scuba tank,» where the team has taken care to prevent it from boiling off near the sun or freezing in the frigid conditions immediately following launch. «There were something like 50 coolants looked at, and water turned out to be the best,» Driesman says. «It has a great heat capacity [and] it’s not toxic.»
Even with such a robust cooling system, the solar panels will need to hide from the direct fury of the sun as the spacecraft loops in for its 24 close passes. Mounted on actuators, the solar panels will fold behind the heat shield on closest approach, exposing only a small portion of the cells to the sun at a sharp angle—and that will be plenty of power to run the critical science and navigation equipment.
Kevlar Armor Against Micrometeorites and Orbital Debris
For all the hazards of flying in the vicinity of the sun, perhaps the least discussed is a barrage of tiny rocks waiting to shred any vulnerable object that ventures too close. So-called «sungrazing comets» are often broken to bits when they fly near the sun, ripped apart the by the immense gravity to leave behind a field of micrometeorites and orbital debris (MMOD).
«So in close to the sun, there’s hypothesized a hypervelocity dust environment where you’ve got these very small grains of dust moving at very high speeds, 200 or 300 km/s,» Driesman says. «It’s not uncommon for a spacecraft to have what they call micrometeorite protection, but in the outer solar system the velocities of the particles are much more tame… so [the spacecraft] has got this overwrap of Kevlar and blanket material that protects the cooling lines from punctures.»
As if the heat and the extreme velocities weren’t enough, NASA’s newest spacecraft will have to shield itself from impacts with rocky debris that is smaller, but «much more extreme» than debris in the outer solar system. Add up all the advanced defense mechanisms required, and it’s no surprise that it took NASA six decades after proposing a solar probe to finally «touch the sun.»
«There are still areas of the heliosphere that are unexplored,» Driesman says, referring to the entire solar system where solar wind holds influence. «And this is in many ways the last of them.»