he greatest sky survey ever undertaken has made it to the Physics Top Ten, with a technical summary appearing as #10. An astronomical survey is a way of taking a picture of the whole sky, recording everything that’s out there down to some limit of flux intensity. In the 2nd century BC, Hipparchus became one of the first people to map the sky, and this led him directly to the discovery of precession. The next survey came 1,700 years later by Tycho Brahe, who took celestial surveying into the realms of big science by constructing a major observatory. His data enabled Kepler to deduce the rules of planetary motion. Time and again astronomers have found that accurate maps or surveys are essential tools for understanding the structure of the universe.

     The Sloan Digital Sky Survey (SDSS) is the most ambitious astronomical survey ever undertaken. Started in 2000, the survey will take five years to complete. In #10 the SDSS Collaboration led by Donald York presents a technical description that will be required reading for all those astronomers using SDSS data. And what a treasure trove the database will be, comprising photometry, astrometry, and spectroscopy of 10⁸ stars, 10⁶ galaxies, and 10⁵ quasars, almost sufficient to fill the Library of Congress as printed books. In a nutshell, SDSS is an international collaboration which is cataloging the heavens to an unprecedented depth, area, and accuracy.

     SDSS will cover one-fourth of the northern sky using a dedicated 2.5-m telescope equipped with a large-format mosaic CCD camera in five optical bands as well as two digital spectrographs. The imaging array is made of 30 CCDs, and with data taken in five colors it is possible to categorize almost all objects into well-recognized types such as star types, galaxies, and quasars. A second set of 20 CCDs is dedicated to position determination, with an accuracy of 50 milli-arcseconds in both coordinates. As Science Watch went to press, SDSS had completed 50% of the mapping.

     To get the distances to objects, SDSS scientists use fiber-fed spectrographs. In each survey field up to 640 galaxies are selected for redshift measurement (candidate galaxies are screened on the basis photometric data from the five channels). Each fiber is manually located on the plug platte so that the fiber ends achieve a one-to-one matching with the distribution of galaxy images in the focal plane. This advanced technology enables SDSS to measure the distances to about 600 galaxies in less than one hour, and up to 5,000 in a good night. That’s a level of productivity that will yield 10⁶ galactic redshifts within five years.

     The prime task for SDSS is to determine cosmic structure, and this cannot really happen until the survey is completed. However early science results, the sizzle rather than the sausage, are enticing, and this may be why the citation rate has reached the Most Wanted status. Five times now SDSS has broken the record for the most distant quasar. The survey’s results on stellar distances are altering structural models of the Milky Way galaxy, because unexpectedly large numbers of blue stars being found within 20° of the galactic plane. These are perhaps a relict population from a dwarf galaxy torn apart long ago in a close encounter with the Milky Way. And nearer home, a scrutiny of asteroids has led to the discovery that the two main types of asteroid are spatially separated. And, reassuringly, the SDSS data show that the number of asteroids smaller than 4 km in size is fewer than previously thought, which lowers the risk of a major collision with Earth.

     The universe in its infancy is the topic of paper #8, which describes results of the MAXIMA-1 balloon flight to measure the lumpiness of the cosmic microwave background. The angular power spectrum (the amplitude of fluctuations as a function of multipole moment) of this radiation is a powerful diagnostic of the cosmological parameters. The sizes of lumps in the MAXIMA thermal maps peak in a size range about 1° across, which immediately indicates that the universe follows high-school flat geometry. Flatness is an important prediction of inflationary models of the universe. By looking at balloon cosmology and supernova cosmology results side by side, researchers now conclude that only 5% of the matter in the universe is baryonic. About 30% is dark matter, and 65% a negative pressure (the cosmological constant) driving the acceleration of the universe. The 5% value for baryons ties in beautifully with independent estimates from the theory of nucleosynthesis in stars.