October 30, 2009
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Platt: A space mission that's wise about finding asteroids and other space objects. I'm Jane Platt with NASA's Jet Propulsion Laboratory in Pasadena, California. Our guest is Dr. Amy Mainzer of JPL, deputy project scientist for NASA's Wide-field Infrared Survey Explorer, or WISE, scheduled for launch in December. Amy, thanks for joining us, and can you just give us a brief introduction to the WISE infrared mission-what it is and what it will do?
Mainzer: Sure, thanks, Jane. It's great to be here, and WISE is a NASA telescope that's what we call a medium-class Explorer, so it's a fairly modest-sized spacecraft, and it contains a relatively small telescope. The telescope is about 40 centimeters in diameter, so that's basically a smallish size that would kind of fit under your arm. And this small telescope is going to go into orbit around the Earth. With it we will survey the entire sky in four infrared wavelengths. One of the goals of WISE is to basically just complete a census of the entire sky in these unique wavelengths, which are very different than visible light or even near-infrared light. And the advantage that the mid-infrared wavelengths give us is that they're sensitive to really fundamentally different properties than some of these other wavelengths. They give us new information that we would not otherwise have.
Platt: Such as what kind of information?
Mainzer: Sure, one of the great benefits of mid-infrared is that it's very sensitive to cool objects, so in other words, the radiation that comes from cold objects, so you're sensing things that are particularly cold. Like asteroids or clouds of dust or very distant galaxies that have been red-shifted far beyond their rest wavelength.
Platt: Red-shifted, for the layperson?
Mainzer: Basically it just means that they are far away and they're moving very fast and the light has been stretched out so that the light actually turns redder from these galaxies that are moving very fast and are very far away. So even though the light from this galaxy might be normal visible light if you were sitting right next to it, by the time it reaches us, it's the wavelengths of this light from these galaxies have been stretched so much that they appear in the mid-infrared instead.
Platt: Okay, and to see these very cool objects, WISE itself has to be very cool, literally and figuratively.
Mainzer: Right, right, since we're looking for things that are very cold, we actually have to keep our telescope very cold as well, along with our detectors. You might think of it as trying to observe the stars in broad daylight, if you don't cool your telescope when you're trying to look at things in the infrared. It's pretty hard. So in order to get the telescope cold enough to be really sensitive, what we do is we actually encapsulate the entire telescope inside what's called a cryostat. And you can think of this as basically a giant thermos bottle. It's full of solid hydrogen. So this is hydrogen in a fairly unusual state. It's cooled down to about seven degrees above absolute zero. I think that's minus 400- and-something Fahrenheit, very cold indeed. And that's what keeps our telescope at a cold enough temperature -17 degrees Kelvin - that we can see the sky in infrared light.
Platt: And let's talk about some of the objects WISE is looking for. Asteroids, let's start with that.
Mainzer: Sure, right, one of the things that WISE is going to be very good at will be finding new asteroids. And that's because asteroids tend to be sort of room temperature and therefore they emit a lot of infrared light. Since they absorb heat from the sun, they re-radiate this heat in the infrared, and WISE will be sensitive to that. One of the things we'll be doing we'll be doing with WISE is mining the WISE catalogues for new asteroids that we don't already know about.
Platt: We already have some tools for tracking asteroids. What will WISE bring to the table that's new or complementary?
Mainzer: Right, that's a good question. So most of the surveys that we have right now for asteroids operate in visible light only. So meaning that's the wavelength that your eyes are sensitive to. And what those surveys are seeing is sunlight that's being reflected by the asteroid. OK. But as I mentioned, asteroids absorb heat from the sun and they re-radiate that heat in mid-infrared wavelengths. So WISE is going to be doing a survey in these mid-infrared wavelengths that our ground-based surveys can't easily access. And what that tells us is more information about the asteroid size, and if you combine the infrared measurements that WISE is going to make with visible light measurements, you can get an indication of the asteroid's true reflectivity, or what we call albedo. That gives us some clue as to what the asteroid is made of.
Platt: So that will be helpful, I mean people are fascinated by asteroids, and most of them are harmless things out there, but if there are some that are a potential hazard to Earth, then you would be able to find those with WISE, or many of them.
Mainzer: We'll be finding some potentially hazardous asteroids with WISE, although WISE doesn't operate for long enough to find a very large, large number of them. In fact, we expect to find a few hundred new asteroids that get close to Earth, only a small fraction of which might be in potentially hazardous orbits. So it's not that WISE is going to find all of these asteroids, it won't do that. But it will find a large sample that will allow us to characterize the population. And by making measurements in mid-infrared, we'll get good measurements of these asteroids' sizes. That allows us to give a good prediction about how many potentially hazardous asteroids there really are. Platt: OK, let's talk about some of the other objects-brown dwarfs, and you're looking for dust clouds in space.
Mainzer: Right, one of WISE's primary science goals is to find something called brown dwarfs. And these are essentially failed stars. The ones that WISE is most interested in are very, very cold ones that have temperatures even as low as the same temperature as Earth. So a few hundred degrees Kelvin, or sort of 70 degrees Fahrenheit. Now these interestingly cool brown dwarfs we like because they to us would represent the real link between giant planets and stars. So far the coldest brown dwarfs that people have found are maybe sort of 6 or 7 hundred degrees Kelvin, maybe even a little bit cooler than that. But you have Jupiter, which is about 125 degrees Kelvin, and remember when I say Kelvin I mean degrees above absolute zero. So there's this missing temperature regime between Jupiter at about 130 Kelvin and the coldest currently known brown dwarfs at around 600 Kelvin. And this is the temperature regime that WISE is going to explore.
Platt: In addition, WISE is going to be searching for objects that are called ULIRGS.
Mainzer: Right, so that's NASA acronym speak for Ultra-luminous Infrared Galaxies. These are galaxies that are extremely bright in mid-infrared light compared to say a normal galaxy like our own. And we think this is because they have a large amount of star formation going on in them, so maybe even about 100 times as much star formation going on in one of these very unusual galaxies compared to our own. And they tend to be very distant on average, much farther away than a normal galaxy. So what we would like to do with these is try to find them and see how many there are and how bright they are, and we may actually be able to find the most luminous galaxies in the entire universe with WISE.
Platt: OK. And we've talked about some of the objects that you know you're hunting for with WISE. I know with some other missions, like the Spitzer Space Telescope, there have been some unplanned, or serendipitous discoveries. We've had recent examples. Can you talk a little bit about whether that will be part of the WISE mission?
Mainzer: Right, well one of the most exciting things about Spitzer is that it has been used for all kinds of things that the creators of it just never envisioned originally. I actually worked on Spitzer, so I get a real kick whenever I see one of the press releases coming out, like the one about finding the new giant ring around the planet Saturn. Totally incredible and something that none of us ever thought would happen. So in the same way, we expect with WISE the unexpected, I guess you could say. One of the great benefits of doing a survey mission like this, where we go out and map the entire sky, is that we'll probably find things that we have no idea about today. One good example of this is that the predecessor of WISE, which was the Infrared Astronomical Satellite, that launched in 1983, it also went out and did a survey in infrared. But it's now 25 years or so out of date. But IRAS discovered many, many, many new types of things that its creators never envisioned, and people are still using the IRAS catalogue today. In fact, the publication rate of papers that cite the IRAS measurements is about the same as it was when it launched, almost 25 or 30 years ago now. So I'm really hoping to retire using WISE data and looking at all the great things we will find.
Platt: Ah, that's a great thought, but of course it'll be a while for you, we hope. Just, final question, is there anything else you wanted people to know abou WISE, or any thoughts as we sort of count down to launch?
Mainzer: Well, it's an exciting mission and I do hope people will tune in and see what we find. We'll be covering a large chunk of sky every day, so keep checking our Web site for new updates and new pretty pictures, and yeh, we'll be launching in a couple of months.
Platt: OK, and I'll let you give the url for that Web site.
Mainzer: That's http://wise.astro.ucla.edu .
Platt: OK, Amy, thanks so much for joining us.
Mainzer: Thank you, Jane.
Platt: And you've been listening to a podcast from NASA's Jet Propulsion Laboratory.
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