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Hard X-Ray telescope up for final NASA review; project will be led by Caltech's Fiona Harrison

PASADENA, Calif.--If all goes well with a technical study approved by NASA for this year, an innovative telescope should be orbiting Earth by the end of the decade and taking the first focused high-energy X-ray pictures of matter falling into black holes and shooting out of exploding stars. Not only will the telescope be 1,000 times more capable of finding new black holes than anything previously launched into space, but it will also give us an unprecedented look at the origins of the heavy elements we're all made of.

Named the Nuclear Spectroscopic Telescope Array--or NuSTAR, for short--the project has just been pegged by NASA for detailed study in the competitive Small Explorer Program (SMEX), which seeks out new technologies and new proposals for space missions that can be launched at low cost. NASA announced earlier this week that an unrelated mission called the Interstellar Boundary Explorer will be launched by 2008, and that NuSTAR will be given an up-or-down decision by next year for launch in 2009.

According to California Institute of Technology astrophysicist Fiona Harrison, the principal investigator of the NuSTAR project, an April high-altitude balloon flight in New Mexico should help to demonstrate whether the advanced sensors invented and built at Caltech are ready for space.

The balloon phase of the project sports the intuitive acronym HEFT (for High-Energy Focusing Telescope), and will mark the first time that focused pictures at "hard X-ray" wavelengths will have been returned from high altitudes. In fact, the HEFT data from the balloon is expected to be superior to any data returned so far from satellites at high X-ray energies.

NuSTAR will be much better than the balloon experiment, Harrison explains, because it's necessary to get above Earth's atmosphere for extended periods to get a good view of the X-ray sky. NuSTAR will orbit Earth at an altitude of about 300 miles or so for at least three years.

The reason that the new technology will be superior to that employed by existing X-ray satellites for certain observations is that high-energy, or hard, X rays, tend to penetrate the gas and dust of galaxies much better than the soft X rays observed by NuSTAR's forerunners. Thus, NuSTAR will get the first focused hard X-ray images for three basic science goals:

--The taking of a census of black holes at all scales. NuSTAR will not only count them, but will also measure the "accretion rate" at which material has fallen into them over time, and the rate supermassive black holes have grown.

--The detecting and measuring of radioactive stuff in recently exploded stars. These remnants of supernovae will provide a better idea of how elements are formed in supernova explosions and then mixed in the interstellar medium, which is the space between stars. NuSTAR will be especially good at observing the decay of titanium to calcium, which tends to be produced in the region of a supernova where material either is ejected forever from the explosion or falls back inward to form a compact remnant of some sort. NuSTAR will thus be an especially good probe of this region, and the data returned will contribute directly to NASA's "Cycles of Matter and Energy" program.

--The observing and imaging of the highly energetic jets that stream out of certain black holes at nearly the speed of light. Coupled with observations from the Gamma-Ray Large-Area Space Telescope (GLAST), NuSTAR will provide data to help scientists explain this still-enigmatic but powerful phenomenon.

The technical difficulties of obtaining hard X-ray images has been overcome with groundbreaking work in various Caltech labs, including that of famed inventor Carver Mead, who is the Moore Professor of Engineering and Applied Science, Emeritus, at Caltech. Both HEFT and NuSTAR will rely on an array of coaligned conical mirrors that will focus X rays from about 20 to 100 kilo-electron-volts on a pixel detector made of cadmium zinc telluride. The sensor is segmented into squares of about half a millimeter each, and these will take thousands of individual readings of X-ray photons and turn them into electronic signals.

"With this mission, we'll open the hard X-ray frontier and look at things never seen before," says Harrison, who is an associate professor of physics and astronomy at Caltech.

In addition to Caltech, the other participating organizations and universities are the Jet Propulsion Laboratory (managed by Caltech for NASA), Columbia University, the Stanford Linear Accelerator (SLAC), the Lawrence Livermore National Laboratory, Sonoma State University, the University of California at Santa Cruz, and the Danish Space Research Institute. NuSTAR's spacecraft will be built by General Dynamics Spectrum Astro.

JPL handles project management, the metrology system, and the extensible mast, and is involved in the mission's science. The mast is based on a previous JPL mission, the Shuttle Radar Topography Mission.

The selected proposals were among 29 SMEX and eight mission-of-opportunity proposals submitted to NASA in May 2003. They were in response to an Explorer Program Announcement of Opportunity issued in February 2003. NASA selected six proposals in November 2003 for detailed feasibility studies.

The Explorer Program is designed to provide frequent, low-cost access to space for physics and astronomy missions with small to mid-sized spacecraft. NASA has successfully launched six SMEX missions since 1992. The missions include the Reuven Ramaty High Energy Solar Spectroscopic Imager, launched in February 2002, and the Galaxy Evolution Explorer, launched in April 2003 and led by Caltech physics professor Chris Martin.

NASA's Goddard Space Flight Center, Greenbelt, Md., manages the Explorer Program for the Science Mission Directorate.


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