The quest to explore extrasolar planetary systems is one of the most exciting and rapidly growing areas of astronomy. We are at the threshold of being able to answer basic questions about our place in the cosmos: I s our Solar System unique? Are we alone in the universe? These are questions that capture the imagination of the public ( as demonstrated by reports that regularly appear in newspapers and magazines).

The United States is the world leader in this field, both in scientific discoveries and in the development of the next generation of instruments. In his “Vision for U.S. Space Exploration” U.S. President George W. Bush included “advanced telescope searches for Earth-like planets and habitable environments around other stars” as a key component of space exploration. The NASA Vision for Space Exploration showed on its schedule of missions: the Kepler spacecraft and the Space Interferometry Mission (SIM) PlanetQuest, followed by James Webb Space Telescope and the Terrestrial Planet Finder mission.

However, it appears that NASA is distracted from its commitment to this historic exploration. NASA’s flagship mission in this field — SIM PlanetQuest — has been delayed by at least four years, due to pressure elsewhere in the space science budget. In addition, the Terrestrial Planet Finder missions have been effectively canceled, with zero funding from October 2006 onward. Only the Kepler (Discovery class) mission to find Earth-size planets in the habitable zones of other solar systems remains relatively unaffected.

As members of the SIM Science Team, we are dismayed at this dramatic change in NASA’s vision of and commitment to this exciting area of human endeavor. In 2005 SIM completed, and subjected to external peer review, the last of eight milestones mandated by NASA as a precondition for flight development. SIM’s technology development is already a great astronomy success story.

SIM represents an entirely new capability in astronomy. This may appear to be a big claim, but it is warranted by SIM’s expected improvement in astrometric accuracy over all other existing or proposed ground or space observatories by a factor of 100 to 1,000. It i s the kind of breakthrough that astronomers dream of, and it is why the SIM science program has been endorsed by two National Research Council Decadal committees, Bahcall (1990) and McKee/Taylor (2000).

SIM’s unique ability to directly measure planet masses sets it in a class by itself. It will be able to detect planets less than three Earth masses (in the ‘habitable zone’) around every one of the closest 45 solar type stars, down to five Earth masses around the nearest 250 stars, and Neptune-sized planets around 2,000 stars.

But SIM is not just about planets. The Science Team Key Projects, selected by NASA in 2000, span broad areas of astrophysics, from exotic binary systems, stellar astrophysics, galactic structure and dark matter, to the dynamics of the local group and internal motions in quasars. In addition, SIM will define a reference frame 100 times more accurate than the current international standard, the International Celestial Reference Frame.

Recently, there have been some reports in the press claiming that SIM’s costs have increased in the past year. NASA’s plans to delay SIM has less to do with cost growth or with technical issues, but result from pressures elsewhere in agency’s budget. The p roject went through a redesign in 2005, with essentially no degradation in its planet-finding capabilities, and it met the NASA-imposed cost cap for a planned launch in 2011.

Now that the project is mature and ready for launch the SIM Science Team has recommended that NASA invite astronomers to come up with proposals to take advantage of the capabilities of SIM — microarcsecond narrow-field astrometry and four microarcsecond global astrometry.

We are ready to build this amazing instrument. We urge NASA to renew its commitment to the field of extrasolar planet research, its commitment to flying innovative but technically mature instruments, and to providing U.S. astronomers with a new precision tool to take a measure of our galaxy and local universe.

This Commentary was submitted by S. R. Kulkarni, of the California Institute of Technology and the SIM PlanetQuest Science Team ( Co-signers included: R. Allen, Space Telescope Science Institute; C. Beichman, Michelson Science Center; B. Chaboyer, Dartmouth University; A. Gould, Ohio State University; T. Henry, George State University; K. Johnston, United States Naval Observatory; S. Majewski, University of Virginia; G. Marcy, University of California; S. Shaklan, Jet Propulsion Laboratory; M. Shao, Jet Propulsion Laboratory; E. Shaya, University of Maryland; A. Wehrle, Infrared Processing & Analysis Center and G. Worthey, Washington State University.