In a story from Greek mythology, a clever craftsman named Daedalus was imprisoned in a tower for knowing too much. To escape, he fashioned a set of wings made of feathers and wax, one for him and one for his son Icarus. As they made their escape, he cautioned the boy to not fly too high, as his wings would melt. Icarus ignored his father, soared too close to the Sun, and fell to his death.
Luckily for astronomers and science enthusiasts everywhere, NASA has completely ignored this cautionary tale.
Sometime in the two weeks starting Saturday, NASA will launch the Parker Solar Probe, a spacecraft that will fly closer to the Sun than any previous space mission. Its objective is to pass through the Sun’s corona and study the complicated magnetic fields that surround it. The probe was named after legendary American astrophysicist Eugene Parker who, in the 1950s, contributed significantly to our understanding of the environment in space surrounding the Sun.
You might think that the Sun is well understood, given that we’ve been aware of it for millennia, but it is a coquettish beast, with some significant mysteries. The sun is a nuclear furnace, constantly shooting hot plasma — mostly protons and electrons from overheated hydrogen atoms — off into space. That hot plasma is the origin of the beautiful aurorae — known in the Northern hemisphere as the Northern lights — seen in the frigid nights of the polar regions.
While scientists understand pretty well how this plasma is emitted from the Sun’s surface, what isn’t known is how the plasma is accelerated to very high speeds. Somewhere in the Sun’s corona — the source of glow that you can see during a total solar eclipse — these particles can reach supersonic speeds and are collectively called the solar wind.
A related mechanism can accelerate protons from the sun to half the speed of light, turning them into projectiles that can damage spacecraft and satellites orbiting the Earth. The mechanism for both of these phenomena is almost certainly the Sun’s magnetic fields changing their strength over time, but the Parker Probe will help us understand it in detail.
Another very weird thing about the Sun’s corona is that it is much hotter than the surface of the Sun itself. The surface of the Sun is about 10,000 degrees Fahrenheit, but the corona can be as much as a hundred times hotter — with temperatures of about a million degrees. It’s the most peculiar thing; it’s like getting hotter as you walk away from a bonfire.
The mechanism for this is not understood, although the Sun’s changing magnetic fields is likely the core cause. There are several different ideas; one is magnetic induction, which is how an induction stove works. Another involves lines of magnetic fields coming together and neutralizing each other, with the magnetic energy heating the plasma. It is hoped that the Parker Solar Probe will teach scientists which idea is right.
The Parker Solar Probe will dive deeply into the solar corona, experiencing some of the most hellish temperatures imaginable. At its closest approach, it will pass within 3.8 million miles from the surface of the Sun. At that distance, the probe will encounter temperatures of about 2,500 degrees Fahrenheit, which is about the melting point of steel. However, NASA engineers have developed an amazing heat shield made of carbon composite, surrounding a carbon foam that is 97% empty space. This shield will keep the internal temperature of the probe at roughly room temperature, i.e. a comfortable 85 degrees Fahrenheit.
Another impressive feature of the probe is its advanced ability for autonomous operation. By operating so close to the Sun, it will be passing through an environment with enormous radio interference. Further, it will often be on the opposite side of the Sun from the Earth. The probe has a fault management system that will keep it oriented so the heat shield will protect it while it is not it contact with ground based controllers.
The Parker Solar Probe is quite small, about the size of a small car, and it weighs about 1,500 pounds. To give a sense of scale, the Curiosity probe sent to Mars was nearly six times heavier. Despite this relatively modest size, NASA will use the Delta IV Heavy rocket to launch it — one of the most powerful rockets in the world’s inventory.
It turns out that it is much harder to launch a probe to the Sun than it is to a distant planet like Pluto. The reason is that the Earth moves at around 18.2 miles per second as it orbits the Sun. The rocket has to be fired opposite the Earth’s motion so it can counteract the Earth’s orbital velocity. It takes 55 times as much launch energy to get to the Sun as it does to get to Mars and twice as much energy as it takes to get to Pluto.
Even with this powerful rocket, the probe will need some fancy orbital gymnastics to move into the desired orbit. The probe will pass by Venus seven times, losing a little energy with each pass to settle down into its final orbit around the Sun. A distance of 3.8 million miles from the Sun is impressively close. To give a sense of the scale, if the distance between the Earth and Sun were ten feet, the Parker Solar Probe would come within five inches of the Sun.
The probe’s mission is expected to take about seven years, during which time the world’s solar scientists will learn an unprecedented amount about the ins and outs of the life-giving star at the center of our solar system.
I just wish that Icarus was around to see it.