November 3rd: WISE - Mapping the Sky in Infrared Light.
Podcaster: Bryan Mendez
Organization: Center for Science Education at UC Berkeley's Space Sciences Laboratory
The Wide-field Infrared Survey Explorer (WISE) is a NASA-funded Explorer mission that will capture invisible, infrared light from the universe to create a catalog of hundreds of millions of astronomical objects. WISE will find the nearest stars to the Sun, brown dwarfs too cool to be detected in visible light. It will also study objects ranging from near-Earth asteroids, star and planet forming regions in our Milky Way galaxy, distant quasars, to the most luminous galaxies in the universe. In this podcast, WISE mission lead Ned Wright, Amy Mainzer and Peter Eisenhardt will discuss the upcoming WISE launch and the remarkable astronomical discoveries expected to follow.
Interviewer: Bryan Mendťz, UC Berkeley Space Sciences Lab
Interviewees: Ned Wright Professor of Astronomy at UCLA and Principal Investigator of the WISE mission.
Peter Eisenhardt, Astronomer from the Jet Propulsion Laboratory and Project Scientist for WISE.
Amy Mainzer Astronomer from the Jet Propulsion Laboratory, and she is the Deputy Project Scientist for WISE, and she is the leader of the NEOWISE project.
Bryan: Hello! Iím Bryan Mendez from the Center for Science Education, at the UC Berkeley Space Sciences Laboratory. Iím talking today with members of the science team for NASAís newest space telescope, the Wide-field Infrared Survey Explorer, or ďWISEĒ for short.
With us here today are Ned Wright, Professor of Astronomy at UCLA and Principal Investigator of the WISE mission.
Bryan: We also have Peter Eisenhardt, Astronomer from the Jet Propulsion Laboratory and Project Scientist for WISE.
Peter: Hi Bryan.
Bryan: Hi Peter. And finally, we have Amy Mainzer, who is also an astronomer from the Jet Propulsion Laboratory, and she is the Deputy Project Scientist for WISE, and she is the leader of the NEOWISE project. Welcome Amy.
Amy: Good to be here. Thanks.
Bryan: Alright, so people want to know, theyĎve heard that WISE is a space telescope and they want to know why is it different, or how is it different than, say the Hubble or the Spitzer Space Telescopes that are up right now?
Ned: Well, as its name implies, WISE is basically a wide-angle camera. Itís like having a wide angle lens on your camera so you can take a picture of a large part of the scene at once. And thatís very useful if you want to photograph everything around you; make a panoramic image, and thatís what WISE is going to do. But the Hubble and the Spitzer Space telescopes are, in effect, telephoto lenses that concentrate on the small part of the scene so they need to know where to point that large telescope and thatís what WISE will do Ė provide that information.
Bryan: So this is an all-sky survey then. Why do an all-sky survey? Whatís special about getting the whole sky?
Amy: Well as Ned mentioned, you need an all-sky survey so that you know where to point your telephoto lens, so to speak. You need to know where to point our Hubbles and our Spitzer Space Telescopes. And, the best way to do that is to have a map of the whole sky. Also, if you are trying to find the most interesting and unique objects that we donít already know about, then you need to cover the whole sky. If you want to find the biggest or the brightest sorts of things in the universe, you know, like the brightest galaxies, and the closest planets or the closest brown dwarfs, you need to cover the whole sky. So there really is no substitute for that all-sky map.
Bryan: Okay, so since you are going to be looking for maybe some of these special objects, and I noticed that this is an infrared telescope implied in the name, so whatís interesting about infrared? Why look around the sky in infrared light?
Peter: Oh, Bryan, infrared is wavelengths that are a little bit redder than our eyes can see. We can see from what we call blue light up to red light and then there are wavelengths that are longer than that and thatís what we call infrared. Itís often talked about as heat radiation. All objects put out a whole spectrum of light. The Sun is bright in blue and red lights and is also bright in the infrared, so as objects get cooler their radiation shifts out into the infrared. So, by looking in the infrared, we can see cooler objects that might not be shining at all in light that we can see with our eyes, or with telescopes like the Hubble. So we can see things that are cold and dark by our normal standards. We can also see things that might be hidden behind dust. Most people probably notice that the setting Sun looks pretty red and thatís because the redder the wavelengths, the better it can get through the long paths of dust and dirt that are in the air that we see at sunset. So, when we look in the infrared we can see cooler objects and we can see through clouds of dust to things that are hidden. So we see, kind of, we like to say sometimes, that we see the dark side of the universe.
Bryan: Okay, thatís interesting. Since this is an all-sky survey, I assume that means we will be seeing things that are both near and far, right? So, thinking about the nearby universe, what about Solar System objects, what is WISE going to find there?
Amy: Well, it turns out that WISE will be an exceptionally good telescope for finding some of the closest things to the planet Earth and right here in our solar system: the near earth objects, as we call them. So these are asteroids and comets that have orbits similar to Earthís and they get them sometimes very close to Earth. So, it turns out that, as Peter mentioned, since WISE is very good at picking up heat radiation in infrared light, weíll actually be very sensitive to asteroids because most near-Earth objects , most near-Earth asteroids, tend to have temperatures that are sort of similar to Earthís. And that means that theyíre going to radiate a lot of their energy in infrared light. And thatís the kind of light that WISE is really optimized to see. So, it turns out that WISE is actually going to be quite a powerful detector for finding new asteroids and this project that we call NEOWISE is set up to do exactly that. We are going to be mining the WISE database to look for new moving objects in our solar system. And, weíll do a pretty good job at finding them. WISE will find something like, you know, hundreds of near-Earth objects that are new and hundreds of thousands of new main belt asteroids in our solar system, in addition to seeing lots of ones that we already know about.
Bryan: Oh, thatís pretty impressive. So thatís the very near, how about inside the Milky Way Galaxy? What will WISE see there?
Peter: Well, moving out beyond our own solar system, there is a very interesting class of failed stars that are called brown dwarfs. These are stars that donít quite have enough mass, enough gravity to keep the fusion reaction going that keeps the Sun hot. The Sun is powered by nuclear fusion, and if you get less than about 8 percent of the mass of the Sun that fusion reaction just canít get going. So a star thatís smaller than that will gradually cool off as it radiates and eventually it will become cool enough that itís not shining in visible light, but it will shine in infrared light. And whatís interesting about these brown dwarfs that canít sustain fusion is that there may be as many, or maybe more of them, as ordinary hydrogen fusing stars. And within 25 light years of the Sun, we know of about a hundred stars or so right now and about a half dozen of those are brown dwarfs. But we think there may be as many as a hundred or maybe even more brown dwarfs within 25 light years of the Sun. And, so we havenít found those yet. Where are they? Well youíve gotta look in the infrared and you gotta look everywhere because they could be in any direction. So WISE expects to find the nearest brown dwarfs and even thereís a chance that we could even find a brown dwarf thatís closer than any other star we know about which is Alpha Centauri system which is about 4 light years away. There may well be a brown dwarf system and thatís even closer than that and that would be pretty neat and pretty cool in more senses than one.
We move out a little bit beyond the immediate neighborhood of the Sun and the galaxy, out say a few hundred light years now. We could get out in the regions where other stars are forming. I mentioned earlier that infrared lets you see through dust and regions where stars are forming are very dusty and we canít see very well whatís going on unless we go out into the infrared. So, weíll be able to map all of the star formation activity thatís going out to hundreds of light years away by looking in the infrared.
Bryan: How about beyond the Milky Way, what will WISE see outside?
Ned: Well, WISE is very sensitive to star formation, as Peter just mentioned. And when the universe was younger, galaxies were much closer together and as a result there is a fairly frequent process of galaxies colliding and merging. And in this process you set up shock waves that cause a huge burst of star formation and so often you get a thousand solar masses per year of new stars being formed. Whereas in the Milky Way, itís only about one solar mass per year of new stars being formed. And so, these objects, just after a collision, are what is known as Ultra Luminous Infrared Galaxies because they are forming so many stars, they are very luminous and theyíre full of dust because stars form in gas and dust. So the Ultra Luminous Infrared Galaxies are really the most distant objects that WISE can see and theyíll be seen out to distances up to 10 -- you know -- 11 billion light years. In a sense the light is going to take us take that long to reach us so these will be when the universe was about 3 or 4 billion years old weíll be able to see them. In addition to that, weíll be able to just map the old starlight from normal galaxies. So, the Milky Way primarily has old stars. Itís forming a few stars, but most of the stars in the Milky Way are old and other galaxies are -- you know -- typically very much like the Milky Way. And the old stars radiate mainly in the infrared because they tend to be cooler than the Sun. And as a result, WISE will be an excellent tool for mapping the distribution of matter in the old stars. This is ordinary matter, not mysterious, dark matter or anything, out to several billion light years of distance.
Bryan: Okay, well it certainly sounds like thereís some very interesting science that WISE is going to be doing. What kind of telescope is WISE?
Amy: Well, the WISE telescope is a fairly small telescope. Itís about 40 centimeters in diameter, so that means it can kind of fit under your arm, if you held your arm out to your side, so itís a fairly petite telescope. However, itís extremely powerful and that is because itís in space and it is going to be in low Earth orbit and the key thing about this is because itís away from Earth and we can use cryogens to get this telescope extremely cold, we can get it down to only 17 degrees above absolute zero. The reason for that is because weíre looking in infrared light, as Ned and Peter have talked about, weíre looking for objects that are cool. Thatís one of our main goals of the WISE Survey is to find these very cool types of objects like asteroids and cool stars. So we need to cool our telescope down so that the thermal emission, the heat from our own telescope doesnít blind us. So we have to use solid hydrogen to get us very, very cold and we can really only do that in very easily in space. Thatís why we can get away with using such a small telescope to achieve such extraordinary results. Thatís because we can get it cold enough to get the background down. If we werenít able to do that, it would be sort of like trying to observe the stars during broad daylight. The background from the sky itself would be so high you just canít see the stars in visible light. So with infrared, in order to get the background down in the same way, we have to cool the telescope.
Bryan: Okay, so you are using solid hydrogen, thatís pretty interesting stuff, how long will that coolant last?
Peter: Weíve done some tests on ground here where we filled WISE up with frozen hydrogen and saw how fast it evaporated away and taking those test data and correcting for the effects of being in orbit, we estimate the WISE hydrogen will last 10 months, and thatís going to be in the low Earth orbit which you mentioned will be in. Thatís an orbit thatís a few hundred miles up Ė actually about 500 kilometers or 300 miles above the surface of the Earth. Thatís a little bit higher than the space station and a little bit lower than the Hubble Space Telescope.
Whatís interesting about that orbit is actually fundamental to our plans to map the whole sky is that we are actually going to be orbiting over the Earthís poles, essentially over the day-night line on the Earth, and so what happens is that we sweep out a map on the sky in a strip and then as the Earth goes around the Sun, that day-night line always faces the Sun and so that strip that we map on the sky always faces, the axis of that strip always faces the Sun and so that strip sweeps around the whole sky in 6 months. And so in the 10 months of hydrogen lifetime that we expect to have will more than sweep around the entire sky. It will only take 6 months to sweep around the sky. So we will get a bit extra coverage there which is really going to be good.
Bryan: Okay, has anybody done an infrared survey like this before?
Ned: Yes, infrared surveys have been done several times before. Thereís two satellite surveys that were done in the 1980s. The first one was on IRAS, which was an orbit very similar to what WISE is going to do and in 6 months it was able to survey the sky. And then there was another satellite called COBE, which also did an infrared survey and it was in an orbit very similar to the WISE and IRAS orbits. So, these surveys have been done before, but the COBE infrared survey had one pixel per infrared band, and the IRAS infrared survey had about fifteen pixels per infrared band. And WISE has a megapixel array in each band. And itís this large increase in the number of pixels that you can have in a digital infrared ray that makes it possible for WISE to be much more sensitive than earlier infrared surveys.
Bryan: Okay. So, Iíve heard on the Spitzer mission, its coolant ran out recently and it went into a so called ďwarm mission.Ē Is that going to be a possibility for WISE?
Peter: Bryan, we just started thinking about that. We expect the hydrogen to last about ten months, then after itís gone, because weíre fairly close to the Earth, it will warm up to a temperature that is considerably hotter than Spitzerís, but still very cold by normal terrestrial standards Ė about 120 degrees Fahrenheit above absolute zero. Something like that which is about minus 300 degrees Fahrenheit, which is pretty cold and probably cold enough so we could still do maps in our short wavelength infrared band, the ones that are sensitive to a little hotter temperatures. And, so now we are looking at the possibility of continuing the survey in those shorter bands and those would be very useful for continuing to search for these nearby cool brown dwarfs. The possibility of extending that brown dwarf surveying is quite interesting because these nearby objects will actually move appreciably over a yearís time, and so we can look for that motion on the sky, and thatís an additional way we can identify which are the very nearest of the Brown Dwarfs. And maybe the nearest star, who knows.
Bryan: Okay, great. Well so when is WISE going to launch?
Ned: Well, right now we have a launch date of December 7th. And of course, any space mission has a launch date there are very often delays and re-trys, so December 7th is the first day that we could launch and we hope that we will get off either on that day or close to it.
Bryan: So when the WISE mission has finished doing the survey, when could the general public expect to see the data from this mission?
Amy: Well, our plan right now is to release the first part of the catalogue and the atlas in the spring of 2011, and about a year after that all of the data products Ė the whole catalogue and the image atlas will be released to the public at large. The great thing about that is you can be anywhere in the world and access the WISE database Ė all of our images, all of our catalogue sources from anywhere. So, you could be sitting in your living room on your couch and get access to this great dataset.
Bryan: Okay, well it certainly sounds like an exciting mission and we look forward to hearing all about the fantastic discoveries that Iím sure will come from it. Thank you all for talking to us about it today. (Everybody says Thank you! Back)