Long ago, shortly after what was then loving called the "Great 200-inch Telescope on Mt. Palomar" began its historic decades of dominance in astronomy in 1948, the companion "48-inch Wide Field Schmidt Telescope" began its epochal survey of the northern sky in a series of about 2000 photographic plates in the visual and red, each about 6° by 6° on a side, reaching to a depth of roughly 20th magnitude. Ever since, a reproduction of the Palomar Sky Survey (since extended to the south by a parallel Southern Sky Survey with the UK Schmidt Telescope) has been a valuable resource at almost every major astronomical center around the world. For the last few years this wonderful resource has been available in the form of the Digital Sky Survey (DSS), made at the Space Telescope Science Institute by digitizing the original plates, both on CD ROM, and lately on-line, free to all.
With the opening of the infrared sky longward of 7000 Å, and especially with NASA's plan for SIRTF, the 85 cm aperture Space Infrared Telescope Facility (SIRTF), scheduled for launch in less than 4 years, there is an urgent need for a similar survey of the infrared sky. For several reasons the heavens are even richer (in terms of number of stars brighter than a given magnitude) in the infrared than in the visual bands: first, about 95% of normal stars are less massive than the Sun, and hence cooler and redder. Second, the Galaxy has a huge number of bright red giant stars, which can be seen for great distances. Third, because absorption by interstellar dust and gas drops rapidly at longer wavelengths, the Galaxy is much more transparent in the infrared than visually. Combining all these factors, there are at least ten times more stars, on the average, in the near infrared sky than in visual wavelengths.
The Two Micron All Sky Survey, which is just entering its full operational phase, will survey the entire sky with two identical dedicated 1.2 m telescopes, one at CTIO in Chile and one on Mt. Hopkins in Arizona. Instead of 1-2 hour exposures on 6° by 6° plates, 2MASS will use three HgCdTe arrays, basically identical to those installed in the NICMOS camera on the Hubble Space Telescope last year. Each has a field of view just 8' (arc min) on a side, but is so sensitive that it need be exposed for under 1.50 s at a time. The special cameras built for the survey obtain simultaneous images in the J (1.25 µ), H (1.65 µ) and Ks (2.17 µ) bands. (1 µ = 10,000 Å = 1/10,000 cm.) Each array has a relatively small format, just 256 X 256 pixels, each pixel 2" (arc sec) square. To improve both sensitivity and resolution, six exposures are taken in rapid succession to give small, sub-pixel offsets, thereby oversampling in such a way that the images can be added to reach a pixel size of 1", with an effective resolution of 4". The entire process happens almost continuously at high speed, under the control of a computer, while the data stream to high-density magnetic tape cassette.
As a result of all this, in about 4 years we shall have a set of images forming a digital Atlas of the the Sky in the three wavelength bands, plus a Star Catalog expected to contain about 300,000,000 stars down to below mag 16 in J and 15 in Ks, with positions measured to better than 0.5", and brightnesses to about 5%. This catalog should include vertually all the red giants in the entire Galaxy (except for those in extremely obscured regions), plus all the normal stars (including the swarms of M dwarfs, 1000 to 10000 time fainter than the Sun) out to a distance of beyond 100 light years. In addition, there will be a Catalog of Galaxies, including the "zone of avoidance" (the region, including about half the sky within ±30° of the galactic equator) previously obscured from our view, with probably 500,000 galaxies and nebulae. The mass region between Jupiter (0.001 solar masses) and the faintest normal stars (0.08 solar masses) has been almost unexplored until now, but such objects could make a significant contribution to the total mass of the Galaxy, depending on their numbers. 2MASS should find many, probably hundreds, of such "brown dwarfs" -- objects with masses below the limit needed to achieve the stable hydrogen-burning fusion of normal main-sequence stars.
2MASS is under the leadership of Dr. Mike Skrutskie at the University of Massachusetts, with scientific collaborators from a number of US institutions. The data, many terabytes, are being reduced at IPAC, the Infrared Processing and Analysis Center at Caltech. Funding is from NASA and the NSF. I will no doubt have more to say about 2MASS in future columns: it is also my own primary scientific activity at present.
Information about 2MASS can be found at http://www.ipac.caltech.edu/2mass/. The DSS can be accessed at http://archive.eso.org/dss/dss.
Four months after launch, the huge Cassini spacecraft glides quietly on its way to a 26 April encounter with Venus. As I write, Cassini has just passed into the dawn sky between Earth and Sun, and is now nearer to Venus than to Earth. About the time this appears in print, it will be moving inside the orbit of Venus, which it will approach from behind. To survive the heat of the Sun, it must hide in the shadow of its high- gain antenna, which therefore cannot be pointed freely during this phase of the mission. Details can be followed wonderfully at http://www.jpl.nasa.gov/cassini/today/.
Following an article in the Houston Chronicle, rumors are circulating that Lunar Prospector has found ice on the Moon, which is critical for the economic future of lunar exploration. An announcement from NASA is promised for 5 March, but the official line is "don't believe any rumors you hear". It has been expected that the question would be settled within the first month or two of the Lunar Prospector mission, so some kind of news (Prospector was launched on 5 January) is about due in any case.
Wes Huntress, Associate Administrator for Space Science at NASA for the past 5 years, announced his plan to resign within the next several months. It is my impression that Huntress has done a pretty good job in bringing the Agency's science program through very difficult times in a remarkably robust condition. All of us with a stake in NASA's science programs will be watching with interest to see who is his successor.