Since 1977, Dr. Martin Weisskopf of NASA's Marshall Space Flight Center has served as the project scientist for the Chandra X-ray Observatory, which is scheduled for launch on July 20. Chandra will be the most powerful and most sensitive X-ray telescope ever launched, and it will do for the field of X-ray astronomy what the Hubble Space Telescope has done for the field of astronomy in the visible and near-visible portions of the spectrum.
Earlier in his career, Weisskopf was a co-investigator on the second High-Energy Astronomy Observatory (HEAO-2, also called the Einstein Observatory) launched in 1978, a forerunner of Chandra.
Recently, Science@NASA sat down with Weisskopf to interview him about the pending launch and the new science that Chandra will make possible.
Science@NASA: How long have you been working working on Chandra and how does it feel being so close to getting it off the ground?
MW: I started in 1977 [when it was still called the Advanced X-ray Astrophysics Facility, or AXAF], so you can do the arithmetic. It has been a long time. My feelings are hard to describe. I know that I am getting very excited despite the fact that I am a veteran of the space program and I know we have still a few hurdles to go through successfully before we start getting data. These are incredibly complex systems that we are launching and we've done everything possible to make sure that they will operate perfectly, but I will be much happier when I see the X-ray data coming through the system. So I am extremely excited and and at the same time extremely nervous.
Science@NASA: More so than on HEAO-2?
MW: Oh, yes. On HEAO-2, I was one of many co-investigators. I played a role in the formation and concept of HEAO-2, and helped write the original proposal. But I had not spent every day of my life breathing it until the time of its launch. For the Chandra X-ray Observatory, I am "the" Project Scientist, although I am very ably helped by a staff of scientists both here at Marshall Space Flight Center, colleagues at the University of Alabama in Huntsville -- a handful who have been with the program even longer than I have -- and a large number of outstanding scientists at the Smithsonian Astrophysical Observatory.
Chandra is the product of so many talented people. Thousands of people have worked on various aspects of this observatory throughout these many years and all of them have played an important role, from the person who does the inventory of the spare parts to the scientist who dreamed up a clever way of making a charge-coupled detector work reliably in a radiation environment. All have contributed and it is impossible to list all their names and you always get nervous that you left somebody out. I would like to note the major contributions of Harvey Tannenbaum [director of the Chandra Science Center], Leon van Speybroeck [the Telescope Scientist], and Riccardo Giacconi who is the father of X-ray astronomy and whose vision and insight this mission represents.
But I am the one who has the responsibility for the scientific integrity of this great observatory and at times you really feel the awesomeness of that burden and privilege. It is a burden and a privilege all at the same time.
Science@NASA: Briefly, what is a project scientist? Is it a sort of point man in the science community?
MW: I think that is fairly accurate. Let me try to define point man. I am the person responsible for the scientific integrity of the program. Accomplishing this requires making sure this entire observatory is responsive to the scientific program and objectives set out. That can be everything from chairing the science working group that debates issues of science policy and discusses requirements. It's making sure that the requirements are understood by the engineers and managers. It is making sure that when requirements could be changed by a little bit and save the taxpayer a lot of money that it is brought to everyone's attention. It is doing detailed calculations, it is staying up late at night during the calibration of the observatory. It is going to Washington to help NASA Headquarters explain to congressmen why is it that this observatory is important -- after all they have already bought one observatory -- and why does this country need another one? All of these things are tied up in the role and responsibility of what it is to be a project scientist.
Science@NASA: That brings up the next question. Why do we need another observatory? What will CXO give us that the Hubble Space Telescope and Compton Gamma Ray Observatory do not? How will it go beyond what HEAO-2 and some of the other X-ray observatories?
MW: Clearly that is an important question. The Chandra X-ray Observatory is one of NASA's Great Observatories for Astrophysics. It's the one that will observe X-ray emissions (light at extremely high energies) of the universe. Chandra is designed to complement the Hubble (which looks at the visible light emissions from the universe), the Compton (which looks at even higher energies than Chandra is capable of), the planned Space Infrared Telescope Facility (SIRTF), and the ground-based radio observatories.
We discovered years ago with the launch and operation of the Einstein X-ray Observatory (HEAO-2), the forerunner of Chandra, that X-ray emission was not just an oddity but is, if I can use a fancy word, ubiquitous: it's everywhere. Every known major class of astronomical objects emits X-rays. In many cases this was an outstanding discovery, and a very surprising discovery. We X-ray astronomers brought astrophysics the first unique evidence that black holes exist based on X-ray observations of Cygnus X-1. With radio observations and subsequent optical observations, we helped nail down the location, identify the companion -- a normal star -- and nail down the period, so we could use Kepler's laws and determine that the compact object emitting the X-rays was a black hole.
It is an important piece of scientific capability that Chandra is providing, so important that the National Academy of Sciences said twenty years ago that we absolutely need to have it in order to further our understanding of the universe. There are some things it can do that no other observatory at other wavelengths can do, and there are some things it cannot do where we need the other wavelengths. I am not saying, "We're the best discipline and everybody else should go away." We need a multiwavelength view to understand what makes the universe tick. NASA and the National Science Foundation (for the ground-based work) are providing the scientific community with the tools to do that.
Science@NASA: X-ray astronomy has been a series of surprises. In 1949, a simple spectrometer carried by a U.S.-launched V-2 rocket detected X-rays coming from the Sun. Even so, for years afterwards the conventional wisdom was, "Well, maybe stars and galaxies are emitting, but they're so weak that you will just see a black sky."
MW: A wonderful compliment to X-ray astronomy is that every mission has brought such incredible surprises, starting from the first one, where no one thought we would see anything. The Sun was known to be an X-ray source, although no one understood exactly how this 6,000-degree sun could produce a million-degree outer skin. It takes a temperatures on the order of millions of degrees to produce X-rays, you move the sun back to the distance of the nearest stars you would never see it with the equipment of the time in 1962.
It turned out that was not true. We could see lots of bright X-ray sources. They turned out to be black holes and neutron stars orbiting more normal stars throughout in our galaxy. The next missions found the incredible time variability of these objects, then missions like Einstein found X-rays coming from quasars to normal stars to hot gas pervading the space between the clusters of galaxies, and many extra-galactic sources. It's a wavelength-chauvinistic fact, but most of the mass that we see in the universe is tied up in hot X-ray emitting gas, even more than we see from the stars in the galaxies.
Every X-ray mission has provided a dramatic surprise. The most recent one was the Rossi X-ray Timing Explorer (RXTE). RXTE increased the size of its detectors (not the ability to resolve objects) to be able to look at time variability more carefully. It has brought beautiful surprises. Researchers here at Marshall Space Flight Center, in fact, have made some discoveries with Rossi about magnetars, which are X-ray sources with huge magnetic fields on the order of 1014 to 1015 Gauss. On the surface of the Earth, the field is half a Gauss, so this is a huge difference. Just as any technology advances in sciences have brought enormous surprises, I am confident that Chandra will also, because it is more powerful.
I hate to use superlatives because we have superlatives in our culture today that are so overvalued for our athletes, our cars, etc. -- but this observatory is a technological marvel. It has the capability to distinguish objects -- we call that the "angular resolution" -- to almost a factor of ten better than anything that has been done before. It has the sensitivity to look for distant and faint objects at a factor of 20 to 50 times better than anything that has been done before. It brings to the fore, at a practical level, spectroscopy -- the ability to dissect the energy spectrum and look very carefully at the different energies emitted by the X-ray sources. We can study the line features and their characteristics -- what we call plasma diagnostics. We'll be studying plasmas under conditions you can't reproduce on Earth.
We can also study plasmas in the vicinities of black holes -- not in them because we could not see anything -- and near the surface of neutron stars. We can also study hot gases and clusters of galaxies where we, the "Geographers of the Universe," see that gravity is holding these gases in the clusters, and that gravity has to do with some type of matter. Right now we call it the dark matter because we have never seen it, and so we don't know what it is. It could be anything from rocks to superconducting cosmic strings that some theorists talk about, an exotic relic from the Big Bang. Perhaps we X-ray astronomers will not discover what the dark matter is, but we will surely map where it is. Maybe it emits x-rays and we will find it, or perhaps some of the other Great Observatories will find it.
Science@NASA: "Geographers of the Universe" can be taken as a little grandiose?
MW: The reasoning goes like this: both in the clusters of galaxies and even in certain galaxies, the giant elliptical galaxies, there is very hot X-ray emitting gas. We see this hot gas and gravity holds the hot gas in these large concentrations.
Gravity requires matter and so if you map where the gas is, you are mapping where the matter is. In that way you might discover where the dark matter is. Much in the same way as we use X-rays, the optical and radio astronomers, by measuring galaxy motion, for example, have discovered supermassive black holes that are at the centers of galaxies.
Science@NASA: You mentioned black holes earlier. They became very popular when they were proposed and possibly observed in the '70s. The public kind of took for granted that they existed but the scientific community stood back and said that they probably were there but we just don't have the observational evidence yet.
MW: In the last decade we astrophysicists, both observers and theorists, have come to the conclusion that there is no question as to the existence of black holes. There are two categories that are quite unambiguous. The first is the category of black holes with 10 to 12 times the mass of our Sun. We typically find these in binary systems where there is a normal star and such a stellar-mass black hole -- a small black hole -- orbiting each other in a binary system.
Then there is a second category of black holes which weigh from a million to a billion times as much as the Sun. We typically find these in the centers of galaxies, especially active galaxies, that can have a lot of X-ray, radio, and optical emissions. It seems to be a property of the universe that a galaxy can have a massive black hole at its center. That also, astrophysically speaking, is incontrovertible.
We once thought of the universe as kind of quiet and peaceful and soothing. It was assumed to have no beginning or end. We now know that is not correct. There was a revolution in our understanding -- with the discovery of red shifts, the expansion of the universe, the cosmic microwave background and so on. But I think black holes are here to stay. This is no longer just science fiction. We believe in them: the mathematics is there, and the physical evidence is there. If it looks like a rose and smells like a rose, then it must be a rose.
Science@NASA: What other cosmic creatures may be out there like black holes that have been proposed, there's evidence, a lot of us in the public take it as a fact that they are there, but you, the scientific community, are waiting for Chandra to supply the supporting data?
MW: Two possibilities quickly come to mind. One, the question and identity of dark matter in the universe is certainly an interesting question, and one where I think it is reasonably fair to say there is not universal agreement, but lots of theories floating around.Gravity is holding these clusters together, and if you add up what is in the stars and galaxies, and what is in the hot X-ray emitting gas, you find there is not enough matter to hold that stuff together.
So something else is there. And that "something else" could be anything from rocks to brown dwarves [very cool, small stars], and perhaps more exotic objects. As I noted previously, Chandra can play a significant and unique role in helping us unravel this puzzle.
Another area that I think will be very fruitful and very interesting to us -- and perhaps surprising to the theorists -- follows the recent discovery of the afterglow of gamma-ray bursts. Gamma-ray bursts are another two-hour topic of discussion. These mysterious bursts of energy in X-rays and gamma rays -- first discovered in the gamma rays -- have been shown to be at cosmological distances. These are not local phenomena, hence the amount of energy output is humongous! These systems will be studied in all wavelength bands. I anticipate that observations with Chandra will be very exciting!
The origin of the gamma-ray bursts is a very interesting question and observatories such as Chandra have such great sensitivity they will be able to look at the afterglows long after the bursts have taken place. We will have the sensitivity to study those objects and help determine what they are. Right now people are talking about neutron star-neutron star collisions, neutron star-back hole collisions and who-knows-what. Whenever there's a phenomenon, there are new theories, and I think we will make some very interesting progress over the next few years with Chandra and the other observatories -- Hubble, Compton, SIRTF, and radio telescopes -- to see what is really producing the gamma-ray bursts.
A third topic, interestingly enough, is the study of baby quasars. By taking an extremely long exposure -- a deep survey to see much more faintly than anything done before -- you will detect very distant, very faint X-ray emitting objects. Then we will use optical and radio telescopes to see what those objects look like in those wavelengths. Lots of them will turn out to be stars, lots of them will turn out to be quasars, and lots of them will turn out to be galaxies or clusters of galaxies. But one thing we know already is that some have to be very different than they are today. From previous missions we know if you take a less sensitive telescope and look, there is a glow, and that glow is made up of unresolved objects.
If you had a telescope, with the sensitivity and resolution of Chandra, you would see three or five or ten objects. What is terrifically exciting is that we already know that some of these objects must either be very different in their physical properties than their older counterparts, or that there are other types of objects waiting to be discovered. This is because we cannot simply account for this glow, the diffuse X-ray background, without appealing to some differences. The deep exposures will be very interesting. Either we will we discover new classes of objects or discover something interesting about the old classes of objects. Baby quasars didn't do what you thought they did -- or perhaps there is something very different in the early universe waiting for us to discover.
Science@NASA: We have mentioned Hubble and Compton and SIRTF several times with Chandra. These are NASA's four Great Observatories for orbiting astrophysics. Comparisons to Hubble are going to be inevitable. What should the public expect to be different about the images from Chandra, and what should they expect to be magnificent about the images that Chandra produces?
MW: Some of the most interesting astronomical pictures are those not of what we call point objects but of extended objects, because there's this very interesting pattern. Many types of X-ray sources are extended: the debris of exploding stars as shock waves heat the interstellar medium, dense regions of bright X-ray stars, the hot gas in galaxies, and clusters of galaxies, the largest known structures in the universe. In my opinion, the pictures will be as spectacular as any of the pictures from Hubble.
I think in part that the pictures will be beautiful in their own right, aesthetically pleasing. As with Hubble, we are trying to understand the same universe and we can't do it without each other. We are certainly not in competition with any other wavelength bands, rather, we are trying to work together to do multiwavelength observations to understand how the universe ticks.
Science@NASA: All eyes right now are focused on the launch, but how long after that before we get these wonderful images?
MW: Several weeks after the launch, assuming that everything goes nominally and smoothly. Quite frankly, I will be very surprised if it does. Not that I think anything will go terribly wrong, but I've been through launches before, and it always takes us a little bit longer to get everything going than we thought. Certain events cannot take place immediately. We want to make sure that the vacuum of space pumps out all the water vapor and gases that have collected inside the various pieces of equipment. We have to do certain calibrations which have to wait until the contamination covers open. So sometime in the third or fourth week we will start to actually take some data. You can imagine - for me, the nervousness and excitement just begin with the launch. They don't end with a successful launch, it just starts, until we have checked out all the instruments and see the first X-rays.