"WISE Guys"

I met with two WISE guys on Tuesday during my trip to JPL. That's a joke that will probably get real old real fast as the launch date of the Wide-field Infrared Survey Explorer (WISE) approaches, but this was the first time I'd heard it!

WISE is scheduled to launch from Vandenberg Air Force Base on December 7 of this year. (That's new information, by the way.) It is an astronomy mission that will perform a survey of the entire sky in four infrared wavelengths. It's similar to past all-sky survey missions like COBE (the Cosmic Background Explorer, whose DIRBE instrument imaged the whole sky in near-infrared wavelengths) and IRAS (the Infrared Astronomical Satellite, which imaged the whole sky in mid-infrared wavelengths). However, WISE has hundreds of times the sensitivity of IRAS, and a whopping 500,000 ties the sensitivity of COBE DIRBE.

Here's the key: the combination of WISE's great sensitivity, the choice of infrared wavelengths, and the fact that they obtain overlapping coverage of the same spot in the sky eight times in quick succession means that the mission will be able to detect hundreds of thousands of asteroids and measure their diameters very accurately, much more accurately than you get from optical observations; they should also perform the first complete survey of solitary brown dwarfs in the region of space near our Sun. In fact, if there are stray Jupiters or even Neptunes wandering around particularly nearby, WISE will find them.

I spent nearly an hour on Tuesday with WISE principal investigator Ned Wright and project manager Bill Irace, and asked lots of questions about the work they'll be able to do on asteroids and brown dwarfs. First, Wright explained to me the basics of how the mission will work. WISE will be in a polar orbit, rotating once per orbit so that its telescope always points away from Earth. So on each orbit it maps a complete north-to-south strip across the sky at two longitudes -- one longitude running from north to south and then the longitude 180 degrees across from that as it returns from south to north. Thus it will take six months to produce the all-sky map. There are a few wiggles as it has to avoid looking at the Moon, but they'll compensate by adjusting the angle of the telescope slightly to the east or west of the Moon and will still manage to achieve complete coverage.

The images on each orbit overlap significantly, so that each spot in the sky gets imaged eight times on successive orbits. The images will be run through an automated process that identifies all the point light sources and compares them to existing maps of the sky, including the asteroid database maintained by Ted Bowell. There will be some point sources in each image that are unknown. The automated process will look at the overlapping coverage of these point sources and try to identify "tracklets" -- sets of observations of objects that appear to move between successive frames.

There may be a role in here for humans -- members of the public -- to look at the automatically identified tracklets and see if they appear to be real or just fortuitous alignments of cosmic ray hits. Good-looking tracklets will be submitted to the Minor Planet Center, with the hopes that professional and amateur astronomers will perform followup observations to get longer observational arcs and thus better orbits. The objects are mostly going to be very faint -- magnitude 22, give or take a couple -- so relatively big scopes will be required.

WISE will be able to do more than just detect the asteroids, however. For most of them -- the ones for which they can get good orbits, giving the distance from the asteroid to the Sun -- it will also be able to calculate diameters, fairly accurately, using the "radiometric technique" for determining the diameters of asteroids. I'm accustomed to the highly uncertain diameter estimates that astronomers arrive at from optical observation. They're uncertain because they depend on albedo, that is, how much sunlight the surface of the asteroid reflects, and the albedoes of asteroids are generally small numbers (that is, asteroids are dark) but they can range over more than an order of magnitude. Wright explained to me that the thermal emission of an asteroid is basically all the solar energy input minus the sunlight that gets reflected away. So it's sensitive to "1 minus albedo," not the albedo itself. So this technique actually benefits from the low albedo of asteroids -- very little sunlight gets reflected away, so the thermal emission gives you a very good estimate of the diameter, and is fairly insensitive to inaccuracy in albedo estimate.

They should detect hundreds of thousands of asteroids, most of them main-belt asteroids. They should detect all of them larger than two or three kilometers in diameter, and possibly as small as 1 kilometer when they start doing tricky things like summing frames. That could be as many as 700,000 asteroids. Wright told me that there's 100 times as many main-belt asteroids as near-Earth asteroids, but any way you look at it that's still a lot of near-Earth asteroids that they should find. They'll do a particularly good job of locating and tracking asteroids observed at high latitude relative to the ecliptic, because their polar orbit means there's a lot of overlap at high latitude. Objects observed at high ecliptic latitude are mostly near-Earth objects, for geometrical reasons that I decided I couldn't explain clearly here.

I asked about the four infrared wavelengths they'll be using, and why they were chosen. Here's the deal:

• Band 1 - 3.4 microns -- is a "big broad filter" intended to see "light from stars and things made from stars," that is, galaxies.
• Band 2 - 4.6 microns -- "brown dwarfs radiate there." So it's thermal radiation from things that are colder than stars but which do have internal heat sources.
• Band 3 - 12 microns -- is where they'll see thermal radiation from asteroids.
• Band 4 - 22 microns -- is where they'll see "really cold stuff" like dust in star-forming regions.

Moving on to the brown dwarf detections, I asked how they came up with the numbers of brown dwarfs that they expect to find. Wright told me that the DIRBE instrument on COBE was really, really insensitive, but a consequence of that was that unlike most other infrared instruments it could look directly at Jupiter without saturating. Using the DIRBE measurements of Jupiter's infrared radiation, they determined that WISE would be able to detect another Jupiter, if there was one, out to one light year, which is quite far but still within our Sun's zone of gravitational control. An object with two to three times Jupiter's mass -- a lightweight brown dwarf -- should be observable at a distance of two or three parsecs (seven to ten light years).

On top of that, Wright has done some observational work on a small patch of sky with the Spitzer space telescope. Using a wavelength similar to WISE's Band 2, Wright and his coworkers detected 18 brown dwarfs in their one Spitzer observation. Now, Spitzer is a much much more powerful telescope than WISE is, but WISE's all-sky map will observe 4,000 times as much area as the one Spitzer observation. Assuming the Spitzer patch of sky is representative and accounting for its greater sensitivity, Wright calculated that WISE should find 700 cold brown dwarfs in its all-sky survey.

There's a few ways to determine the distance to these brown dwarfs. One way is to get another wavelength, most likely an optical wavelength through a larger telescope like Keck II or Hubble. Another is simple parallax observations -- these things are so close that their positions relative to background stars will shift as Earth orbits the Sun. Wright said there should be 100 brown dwarfs closer than 6 parsecs away, which is comparable to the number of stars known to be closer than 6 parsecs away. In other words, WISE will double the density of the objects we know to exist in our local neighborhood. It could even detect some really small objects (small, that is, compared to stars) if they are close enough: it could find a Neptune, if one existed, out to 700 AU.

How about Kuiper belt objects? Unfortunately, anything that doesn't have its own internal heat source will be too cold for WISE to detect. The objects need to be 70 to 100 Kelvin to be detectable; even another Earth, if there was such a body in the Kuiper belt, would be a frigid 35 Kelvin, too cold to spot.

I asked if there were any other neat things WISE can do within the solar system, and Wright told me that WISE should see comet trails -- thermal radiation from the cold dust left behind in the orbits of comets, the same stuff that's responsible for annual meteor showers like the Perseids and Leonids. Even IRAS saw this, so the more-sensitive WISE should see a lot of them. It may see trails along orbits of dead comets -- things that look like asteroids because they've had too many trips close to the Sun, turning things that we once called asteroids into comets.

I asked Bill Irace how things were going in preparation for the launch. He said the spacecraft has arrived at Vandenberg and it appears to be working as well as it was before it was shipped, which is always good news. The challenge that they face before launch is that in order for these sensitive thermal infrared detectors to work, they must be kept incredibly cold. Their coolant will be a 15-kilogram block of SOLID hydrogen. Yes, solid hydrogen. I did a double take and remarked that I didn't even know hydrogen could be made solid! Wright told me I was thinking of helium -- but in fact they will be using ultracold liquid helium to cool their reservoir of hydrogen to 6 or 7 Kelvin, at which temperature it will be in its solid state. I can't really fathom what solid hydrogen must be like -- it's only 11% the density of water. Weird.

I asked if this had been done before, and Irace said yes, it was done on one science mission, the Wide-field Infrared Explorer or WIRE, which sadly failed shortly after launch, and it's been done on other missions as well.

The process of generating the solid hydrogen will take six weeks, beginning September 23. They'll cool the tanks with liquid helium, then inject gaseous hydrogen as they keep running liquid helium around the tanks; ultimately it will freeze. After it freezes they'll test and calibrate their detectors one last time. They'll cart the whole cooling assembly out to the launch pad and will have to keep running the liquid helium coolant through it until launch. Irace said there's a dedicated team of people whose sole purpose is to run tanks of liquid helium up and down the launch tower until the day of launch.

Solid hydrogen!! Cool. Cold, in fact.

So I'm looking forward to this mission! I'll keep you all posted on its status as it prepares for launch.