is thought to be the best accessible site in the world for millimeter and submillimeter astronomy. It is likely to attract other facilities.

The NRAO-ESO agreement was for a $650 million budget, with construction shared between the two continents. Following recent increases in commodity costs, it has become necessary to contemplate a descope of the project. An investigation by the project and the ALMA Science Advisory Committee (ASAC) found that the only descope option that could lead to significant savings without catastrophic loss of scientific capability was a reduction in the number of antennas.

The committee has read the ASAC reports dated September 2004 and March 2005, the relevant chapters of the ALMA Project Book, the ESO document “Science with ALMA,” and the 1999 Report of the Antenna Size Committee. It has also listened to presentations by Wayne van Citters on behalf of the National Science Foundation, Ewine van Dishoeck, representing ESO, and Jean Turner, ASAC chair. Mark Holdaway of NRAO kindly explained aspects of the ALMA imaging philosophy.

As the premier instrument in the world for the exploration of planets, stars, galaxies, and the unknown for several decades, ALMA will be a long-term investment for frontier research by the U.S. astronomy community. One salient feature of the ALMA project is the array’s planned accessibility to the broader U.S. astronomy community. Previous radio and millimeter arrays have required users to be highly proficient in interferometric techniques. In contrast, ALMA is designed to be an observatory accessible to astronomers who normally observe at other wavelengths. ALMA’s unprecedented number of antennas is the key driver for this accessibility, and the potential descope will significantly curtail the number of astronomers who will be able to use the observatory.

Observatory facilities have a long active life. The Very Large Array (VLA) is 24 years old and will likely operate for twice as long. ALMA is currently expected to have a 30- to 50-year productive lifetime. From this perspective, the incremental cost of com-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement