Fiona Harrison is a professor of physics and astronomy at the California Institute of Technology and scientific leader for a NASA space telescope, NuSTAR, that’s set to launch Wednesday. The $170 million mission will study high-energy X-rays from black holes, supernova explosions and our very own sun. Harrison, the second woman to lead a NASA mission, conceived the telescope in the early 1990s but had to wait for technology to catch up.
Most people think of X-rays in terms of a dental visit, but for astrophysicists, these high-energy waves open a window on strange objects in the universe. Why did you get interested in them?
Studying black holes was the catch for me. They seemed to live somewhere between science fiction and reality. They sounded exotic and really interesting, and they’re also related to this whole question of how the universe came about and evolved. That little question seemed fascinating, and I feel incredibly lucky I get to spend my time thinking about it.
Other space telescopes, like NASA’s Chandra, can already spot X-rays. Why do we need NuSTAR?
NuSTAR works at higher energies. Technically, it has not been possible to focus or concentrate X-rays above a certain energy. In this energy range, NuSTAR can make images that are 10 times crisper and 100 times more sensitive than [those from] any previous mission.
Why do we want to do that?
There are many reasons. The regions very near black holes emit high-energy X-rays, and by studying them, there’s a lot we can tell about how the matter is falling into the black hole — how black holes feed themselves. Also, high-energy X-rays, the same [kind of] energy your doctor and dentist use, are very penetrating. So you can [use them to] see through dust and gas and penetrate into the heart of distant galaxies.
What objects will you study?
We’ll first aim it at Cygnus X-1, the black hole nearest Earth. [It’s 6,100 light years from Earth.] Then we’ll survey our Milky Way galaxy to find the remnants of massive stars that have exploded and left behind small black holes and neutron stars, which are the mass of the sun compressed into the size of Manhattan.
We’ll examine the remnants of supernovae, stars that exploded in the past 500 years that are still glowing and radioactive, like supernova 1987A, which exploded in the Large Magellenic Cloud [a relatively nearby galaxy]. The light that’s still decaying, we think, is powered almost entirely by radioactive titanium. That’s what theorists expect to see, and now we can look for it.
A supernova that exploded last year caused a lot of excitement among astrophysicsts. Do you wish you could have watched it?
That was a real bummer for us. If we had been on orbit, we definitely would have looked at it. We’re hoping for another one.
What else will you study?
We’ll be looking at strange objects in nearby galaxies called ultra-luminous X-ray sources. These are very, very bright in the X-ray energies. It’s mysterious how they can be so bright. They’re probably black holes, but they challenge our theories about how black holes form.
And you’ll look at the sun?
Yes. We still don’t understand what heats the outer solar atmosphere, which is much hotter than the surface of the sun. There must be some energy pumping into there, what we call “reconnection events” with the surface of the sun. NuSTAR should be able to see those. In just a day pointing at the sun, we should be able to rule in or rule out that mechanism for heating the solar atmosphere.
This mission is relatively inexpensive for NASA. Did that force you to innovate?
Absolutely. We needed a telescope 10 meters [about 30 feet] long, but we didn’t have a budget to build it that big. And we had to launch on a small rocket [a Pegasus, which takes off from the underside of an airliner]. To be on a small rocket, we had to fold the whole thing, and then have it telescope out in orbit. It’s kind of a like a jack-in-the-box.
The big missions are very important. There are things Hubble can do, you can’t do any other way. But with the budget reality, those missions are going to be few and far between. The smaller missions, then, become even more important. And NuSTAR fits right in. It is a success story, I think. It is on budget, on schedule.