HEFT’s mirror system has 72 shells. NuSTAR’s will be nearly identical, but bigger and with 130 shells.

NuSTAR ReNued

NASA has given the go-ahead to bring a mission back from the dead. Although they cancelled it in 2006, officials have revived the Nuclear Spectroscopic Telescope Array, or NuSTAR. The spacecraft will be the most capable instrument yet to explore the universe using high-energy X rays.

“It’s great that NASA was able to restart the mission,” says Fiona Harrison, professor of physics and astronomy and NuSTAR principal investigator. “I’m incredibly excited about our planned science program, as well as the unanticipated things we are bound to discover with a new telescope this sensitive.” NASA had scrapped the mission due to funding pressures within the Science Mission Directorate, but NuSTAR will now proceed to flight development, with an expected launch in 2011.

Researchers designed the mission to answer some fundamental questions about the universe: What powers the most extremely active galaxies? How were the heavy elements of the universe created? How are black holes distributed through the cosmos? NuSTAR will have more than 500 times the sensitivity of previous instruments that looked for black holes.

The members of Harrison’s team have been working on NuSTAR technology for more than 10 years, developing optics that can focus high-frequency X rays for the first time. X rays are at the high-energy end of the electromagnetic spectrum, and easily penetrate most materials—which is why doctors use them to see through skin and flesh. X rays can only be reflected and focused in a telescope if they hit the mirror at a shallow angle, like rocks skipping on a pond. But since they hit the mirror nearly end-on, it has a very small collection area. In order to catch as many as possible, X-ray telescopes have several nested mirrors called shells. NuSTAR will have two such multiple-mirror systems, each with 130 cylindrical shells of reflective material. The system was demonstrated on a balloon-borne experiment called HEFT, for High Energy Focusing Telescope, that Harrison’s group flew in 2005.

Each of the 130 shells is coated with an average of 300 thin layers of alternating high- and low-density materials of varying thicknesses, in order to reflect a whole spectrum of X rays. Other X-ray observatories, such as Chandra or the European Space Agency’s XMM-Newton, don’t have these multilayer coatings, which limits them to low-energy X rays.

NuSTAR’s X-ray detector is a special CCD, or charge-coupled device, analogous to the one in your video camera. But this one, developed by Harrison’s goup in Caltech’s Space Radiation Lab, is made of cadmium zinc telluride.

NuSTAR also incorporates an extendable structure developed by JPL and Alliant Techsystems Inc. for the Shuttle Radar Topography Mission that will fit the telescope into a small, inexpensive launch vehicle. Once in orbit, the arm will be extended to move the mirrors some 10 meters away from the detector, bringing the X-ray universe into focus.

In November 2003, NuSTAR was one of six proposals selected from 36 submitted to NASA’s Small Explorers Program, which funds lower-cost, highly focused, rapidly developed scientific spacecraft.
NASA anticipates that NuSTAR will bridge a gap in astrophysics missions between the 2009 launch of the Wide-Field Infrared Survey Explorer and the 2013 launch of the James Webb Space Telescope. Besides using high-energy X rays to map areas of the sky, the spacecraft will complement astrophysics missions that explore the cosmos in other regions of the electromagnetic spectrum. —EN