21 July 2003 John Bahcall, Chair NASA Blue Ribbon Panel on HST Termination and Transition to JWST Dear John: I recommend that the HST mission be continued until after JWST launch, as long as there are no major failures not amenable to routine servicing missions. My reasons are, first, HST's unique scientific capabilities and, second, the technical risk associated with JWST. Astronomy today makes routine use of almost the entire accessible EM spectrum. NASA in particular is placing much emphasis on new capabilities in the high-energy and infrared domains outside the conventional UVOIR (100-1000 nm) band. But the fact remains that the UVOIR band contains the most information about astrophysical sources. UVOIR identifications/observations are almost always prerequisite to a thorough understanding of cosmic sources discovered in other EM bands, as vividly illustrated by the avalanche of research on gamma ray bursts following their optical identification. The best performing UVOIR telescopes are therefore always critical resources for astronomy. HST will remain the premier UVOIR instrument for high spatial resolution observations and access to the vacuum UV (100-320 nm) for the foreseeable future. HST-quality imaging at wavelengths below 600 nm (and probably 1000 nm) will not be available on JWST. Nor is it likely that ground-based telescopes will be able to duplicate HST imaging performance below 1000 nm within 15 years. Thus, the progression from HST to JWST is not analogous to upgrading from, say, a 5-m to a 10-m ground-based telescope where the 10-m offers improved capabilities across the board. JWST will not supersede HST's key capabilities. HST can be expected to play a central role in exploiting the rich harvest from the unprecedented array of deep surveys and new facilities (e.g. ALMA) in progress for this decade. It is the only instrument capable of GALEX or FUSE follow-on. It would remain a key asset well into the JWST mission. Of course, it's difficult to predict how much essential science will emerge from HST after 20 years of operations. But the confirmation of a Jupiter-like planet in the globular cluster M4 and the demonstration of multiple star-formation epochs in the (ostensibly ancient) halo of M31 are recent examples of science that could not have been done by another telescope. HST remains uniquely capable and productive after 13 years. I think the HST mission should be extended, with regularly scheduled servicing missions and upgrades, at least past JWST launch. I believe the overlap with JWST is important because JWST is a considerably more risky enterprise. I am no expert here and have no special insights. But we must acknowledge the finite probability that JWST will not materialize because of a launch or deployment failure or for other reasons. The systematic descopes to which it has been subjected appear to threaten its ultimate capabilities even if all goes well technically. In balancing costs and benefits, we should recognize that investments in a robust, well-understood, and superbly performing bird-in-the-hand like HST are less risky than those in nascent missions. HST is now regarded by the public and Congress as NASA's most successful scientific endeavor, second only to Apollo as NASA's signature mission. If HST suffers a major failure, so be it. But if HST remains healthy, it would be very shortsighted to shut it down. Because HST's special stature sets it apart, your committee would be justified in asking the NASA Administrator to fund a continuing HST mission through a separate line which is not in direct budgetary conflict with other science missions. Sincerely --- Bob ------------------------------------------------------------------ Robert W. O'Connell Hamilton Professor of Astronomy Email: rwo@virginia.edu Astronomy Dept, UVA Phone: [434] 924-7494 P.O. Box 3818 FAX: [434] 924-3104 Charlottesville, VA 22903-0818 USA www.astro.virginia.edu/~rwo/ ------------------------------------------------------------------