NuSTAR: Origins

 

The above composite shows images of the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. The field of view for each image is the same size, 6 arcseconds. Image Credit: CM Hubert Chen, Fiona A. Harrison, Principal Investigator, Caltech Charles J. Hailey, Columbia Principal, Columbia, Finn E. Christensen, DSRI Principal, DSRI, William W. Craig, Optics Scientist, LLNL, Stephen M. Schindler, Project Manager, Caltech

We now continue our review of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission, which launched June 13th. In this outing we will review NuSTAR's road from proposal to reality. And the road was indeed long and winding...

The Proposal

In February 2003, NASA formally requested new mission proposals for their Explorer mission program, described as an Explorer Program Announcement of Opportunity. The chosen proposals would fly as the tenth and eleventh missions of the Small Explorer (SMEX) program.  In response, NASA received 36 proposals. One of these was for an Earth-orbiting X-ray telescope to carry out a census of black holes with 1,000 times more sensitivity than previous experiments which have flown. The mission would be led by Fiona Anne Harrison of the California Institute of Technology, Pasadena, at a total mission cost to NASA of $132 million. The mission would be called the Nuclear Spectroscopic Telescope Array — NuSTAR.

The Studies

In November 2003, NuSTAR was one of five proposals selected by NASA for a five-month implementation feasibility study. Following the studies, NASA would select two of the five for future missions.

In January 2005, NASA announced its selection of the Interstellar Boundary Explorer (IBEX) to be the next SMEX mission. In addition, NASA announced that NuSTAR was also selected for mission development, pending a one-year feasibility study.

As part of the feasibility study, the NuSTAR proposal team had to prove that their instrumentation was ready for space. To do that, they planned a high-altitude balloon flight over the deserts of New Mexico. The balloon phase of the project was given the name High-Energy Focusing Telescope (HEFT).

The new technology would be superior to that employed by existing X-ray satellites for certain observations because 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 would get the first focused hard X-ray images for three basic science goals:

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

2. The detecting and measuring of radioactive stuff in recently exploded stars. These remnants of supernovae would 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 would 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 would 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.

3. 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 would provide data to help scientists explain this still-enigmatic but powerful phenomenon.

The technical difficulties of obtaining hard X-ray images was 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 would rely on an array of co-aligned conical mirrors that would 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 would take thousands of individual readings of X-ray photons and turn them into electronic signals.

In addition to Caltech, the other participating organizations and universities were 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 mast would be built by ABLE Engineering and the spacecraft would be built by General Dynamics Spectrum Astro.

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

Up, Up and Away!

In May 2005, the balloon-borne HEFT was first launched from Fort Sumner. The instrument carried one of the first focusing telescopes for the hard X-ray band (20–70 keV). It made use of tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT had an angular resolution of 1.5 arcminutes in half-power diameter, and an energy resolution of 1.0 keV full width at half maximum at 60 keV. The maiden flight of HEFT lasted 25 hours. The instrument performed within specification, and observed Cyg X-1, the Crab Nebula.

The flights of HEFT and the data it collected proved to NASA that NuSTAR would perform as advertised. Following the completion of the study, NASA green-lighted the NuSTAR program with an estimated mission launch in 2008.

To be continued.


And now, the mission particulars...


NuSTAR is a Small Explorer mission led by the California Institute of Technology in Pasadena and managed by NASA's Jet Propulsion Laboratory, also in Pasadena, for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation, Dulles, Virginia. Its instrument was built by a consortium including Caltech; JPL; the University of California, Berkeley (UC Berkley); Columbia University, New York; NASA's Goddard Space Flight Center, Greenbelt, Maryland; the Danish Technical University in Denmark; Lawrence Livermore National Laboratory, Livermore, California; and ATK Aerospace Systems, Goleta, California. NuSTAR will be operated by UC Berkeley, with the Italian Space Agency providing its equatorial ground station located at Malindi, Kenya. The mission's outreach program is based at Sonoma State University, Rohnert Park, California. NASA's Explorer Program is managed by Goddard. JPL is managed by Caltech for NASA. For more information on the NuSTAR mission, visit www.nasa.gov/nustar and http://www.nustar.caltech.edu/ .

To learn more about the High Energy Focusing Telescope (HEFT), visit www.srl.Caltech.edu/HEFT .

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