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”

–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 mast will be built
by ABLE Engineering and the 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

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.