Justin Kasper, an astrophysicist in the at the Solar and Stellar X-Ray Group at the Harvard-Smithsonian Center for Astrophysics, is behind Solar Probe Plus, which will make the first ever trip to the sun—a NASA mission that will take begin in 2018. Kasper’s creation, SWEAP (Solar Wind Electrons Alphas and Protons) is one of the four instrument packages that will be on the spacecraft; it will help us learn why the sun is so hot and how the solar wind is accelerated to supersonic speeds.
Kasper is working with several institutions, including the University of California Berkeley Space Sciences Laboratory, NASA Marshall Space Flight Center, the University of Alabama Huntsville, NASA Goddard Space Flight Center, Los Alamos National Laboratory, University of New Hampshire and the Massachusetts Institute of Technology.
I recently talked with Kasper about the solar probe, why we want to get that close to the sun, and why the spacecraft won’t burst into flames.
Oh yeah. One astronomical unit (AU) is the distance between sun and Earth. Nothing has ever gotten closer than the orbit of Mercury—that’s 0.31 to 0.46 AU from the sun. In the ‘70s a pair of spacecraft called Helios used an encounter with Mercury to change their orbit, and their closest approach was 0.29 AU.
How did you break that barrier?
The biggest challenge in getting closer to the sun is the Earth's angular momentum. You leave Earth, and you have all that momentum, so you have to slow down so you can get closer. You can have a bigger rocket to slow you down more, or you can take advantage of other planets. So you send your spacecraft out using the biggest rocket you can afford and very carefully align things so it has an encounter with a planet.
What we did with Voyager was we swung by the outer planets-–Jupiter and Saturn—and it sped up. That’s how it was able to leave the solar system. In our case, we’re using encounters with inner planets, particularly Venus, to slow it down. As we get extremely close to Venus, it slows down because of the gravity. A couple months after the spacecraft launches, it will hit there. So every time the spacecraft orbits the sun it goes just past Venus and slows down.
So the idea is we launch in the summer of 2018, and it will have its first encounter with Venus just two months later. It will take seven encounters (gravity assists) with Venus over six years to direct to slow it down enough and direct the spacecraft into the sun’s atmosphere. At closest approach we will be 8.5 solar radii above the surface of the Sun (1/20th the distance between the sun and the Earth), where the sun is 500 times brighter than it is on Earth. This will happen in 2024, and at that point Solar Probe Plus will be the fastest moving object we have ever made.
That's a great question. Let me describe the atmosphere. People knew 100 years ago--they’d see a solar eclipse, and you could see the atmosphere of the sun. We’d call it the corona, which is Latin for crown—it was a like a crown surrounding the sun. So if you go back 100 years people understood they had a real problem—that the sun’s atmosphere either had to be 1,000 times hotter than the surface of the sun, or made up of a new form of matter, 1,000 times lighter than hydrogen.
Through a variety of different measurements, people realized that [the corona] was heated up to this really high temperature. The corona is also unstable. It expands. The further you get from the sun, the faster it expands, creating a supersonic solar wind.
So the idea is the corona’s really hot, and it turns into this wind that’s flowing faster and faster. The density is very low compared to air on Earth. So when the spacecraft is closest to the sun, it will be surrounded by the corona, which might have a temperature of several million degrees, but its density is extremely low so that plasma (the corona and solar wind) will not heat the spacecraft. What will heat it is the sun being 500 times brighter when you’re that close. If you had a pot of boiling water, you could stick your hand in the water and you’d burn yourself, but if you held your hand in the steam—which is just as hot as the water, but the density is lower—you won’t burn your hand.
So that’s why if you see any artists sketches of the spacecraft, the front is a gigantic heat shield. We need to keep the sunlight and heat from the sun from getting into the spacecraft. But the instruments in the spacecraft won’t really notice they are 10 solar radii from the sun; it will be like it’s operating near Earth.
We have a lot of different questions:
- How is the corona being heated?
- How are particles accelerated to high energies?
- How is the solar wind accelerated? We talk about space weather these days. One of the ways the sun can produce the biggest disturbances in space is when you have a coronal mass ejection (CME)—a chunk of the sun’s atmosphere that breaks loose and explosively expands outwards into space. It can reach speeds of 2,000 kilometers per second, five to 10 times faster than the solar wind itself. CMEs are crazy. When they reach Earth, if they do, they can crate all sorts of disturbances: They can disrupt GPS, radio communication, radars. CMEs are also able to make radiation. From a scientific point of view, this is fascinating. We’d love to figure out why this is so efficient. When that radiation reaches us, it can damage spacecraft and is dangerous to humans in space.
There will be four instrument packages on the spacecraft.
- SWEAP measures solar wind. We have a whole set of sensors on the spacecraft that sample the corona on the solar wind and determine the temperature and velocity (and direction).
- FIELDS has antennas that measure the eclectic field and magnetometers.
- ISIS measures particle radiation.
- WISPR is a white light imager.
What’s the significance of all this—the ability to get this information and study the sun so closely?
It was a real surprise to see the sun’s corona is able to heat up by a factor of 1,000. That has to do with plasma, and how that happens is a very fundamental question. Since then we’ve learned through x-ray observatories that you can see this happening on all different stars--materials heated up to extremely high temperatures. We don’t understand the underlying physics very well, and the sun is an excellent opportunity to study it directly. With the Solar Probe, we can actually send an object into that atmosphere. That’s one reason this is really exciting.
Another practical application of that stuff—the physics going on—it has to do with how a magnetic field can confine and trap a plasma. We need to understand that to make confined, controlled fusion on Earth possible. You can go to conferences where people are trying to work on fusion reactors and talking to us about spacecraft operations. We can take what we observe and share it with those other communities.
One of the things I study—what mechanisms are capable of doing that kind of perpendicular heating that’s happening the corona. Then you ask, how far can you push that? These are the kinds of things we’re studying. The problem is you can‘t reproduce this in the lab yet. Computer simulations are only able to simulate a couple seconds worth of this plasma when it goes unstable. So that’s why I’m fascinated by this stuff; this is a way to get in there and measure it directly.
You can’t simulate the sun in a lab, but how about testing materials before you get up there?
The front of the heat shield will get up to 1,400 degrees Celsius. What materials do you use? And you can--because we’ve done it in our testing--find companies in the U.S. that have really good ovens, and you can heat something to 2,000 degrees, no problem, even sit for 10 hours at 2,000 degrees. But that’s not really what the environment near the sun is like. It’s something getting hot from sunlight coming from just one direction while the other sides are facing deep space. So we thought we’d have to build materials that could survive that environment.
We went to the world’s largest solar furnace--Procedes, Materiaux et Engerie Solaire (PROMES)--in the Pyrenees in southern France. Imagine a mountainside covered with giant mirrors that track the sun and reflect it onto a 10-story tall building shaped by a parabola and covered by mirrors so all the sunlight is focused down to a small door. The sunlight goes through the door, and there’s a window made of quartz. About a megawatt of sunlight gets concentrated on that quartz window, and there’s a vacuum chamber on the end of the window. So we went there to test some of our materials. We have to come up with novel ways to recreate what it’s like close to the sun. You can set a rock on fire there. That’s actually hotter than we need to worry about.
I understand you secured a $67 million grant for this project. What’s it like trying to round up that much money?
It’s pretty amazing. You have to do a lot of groundwork before you submit a proposal for something like that. Once you’re selected you have 50 to 100 people working on the project. But we had about as many people helping prepare our proposal over the last few years.
It’s a NASA program, which selected Johns Hopkins Applied Physics Lab to build the spacecraft. So the money goes from NASA to APL, and NASA told APL, here are the teams selected to build the instruments. So they send funding to us. You don’t get a check all at once.
Right now we’re making our plans on how we’re going to build the instruments, the standards we’re going to use, then we’ll transition to design. We’ll spend two-and-a-half to three years in the design phase, then a few years building the flight hardware.
Solar Probe Photos: Johns Hopkins University/Applied Physics Laboratory