NuSTAR is a high-energy X-ray telescope designed to seek out black holes in the universe, even those hidden by space dust and debris.
The launch of NuSTAR, one of NASA's Small Explorer projects--a series of space exploration programs that cost no more than $120 million each--comes just days after the space agency announced the cancellation of the Gravity and Extreme Magnetism space observatory program. GEMS was another in the Small Explorer program; NASA spiked it after project reviews showed it would come in well over budget.
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GEMS also would have been an X-ray telescope, but one designed to study the polarization of the X-rays. "Imagine electromagnetic light as a rope that you can swing up and down, or side to side," Fiona Harrison, professor of physics and astronomy at California Institute of Technology and principal investigator for NuSTAR, said in an interview. "GEMS was trying to detect the angle, a very difficult thing to do."
By looking at the X-rays emitted by black holes, astronomers will be able to calculate how space-time is distorted and how fast they are spinning. To accomplish this, NuSTAR is equipped with two "grazing incidence" focusing optics. In order to gather the X-rays, the optics have to be almost-but-not-quite parallel to their trajectory, said Harrison.
To get the right kind of angles, the optics are comprised of 133 shells, nested inside one another. (NASA compares them to Russian nesting dolls.) Developing the material to coat each shell was one of the challenges. "X-rays need very smooth surfaces to reflect on--smooth at the level of a few atoms," Harrison said. "We had to find material that was affordable."
A special glass for laptop displays, called "Schott glass," turned out to be most suitable. It's smooth and comes in extremely thin sheets. Goddard Space Flight Center cut the sheets into segments and placed them over mandrils--molds shaped as the glass should be formed--then heated them in large, high-temperature ovens until the glass melted into shape.
But the shaped glass still wasn't smooth enough for the optics, so each segment was coated with about 200 layers of alternating thin films, one made of platinum and carbon, another of tungsten and silicon. Each film is only a couple of atoms thick. The films were developed in collaboration with the Danish Technical University. Researchers at Columbia University took the segments and glued them together into seamless tubes, nested the tubes inside each other, and mounted them into the telescope.
The optics are located at the end of a 10-meter mast, which will unfold after the telescope reaches orbit. At the other end of the mast are detectors that also had to be specially developed. Most detectors on space telescopes such as the Chandra X-Ray Observatory, launched in 1999, are digital and made of silicon, but high-energy X-rays of the type that NuSTAR is measuring will go through silicon. So the very large scale integrated circuits are made from a cadmium-zinc-telluride alloy.
Two lasers on the optics bench shine onto the focal plane bench. "We're detecting the position of the lasers all the time," Harrison said. "What we send to the ground is all the information on the lasers, their position and the energy of the X-rays." That data is then converted into images.
Once the satellite is launched, it will take roughly a month to run through an array of tests and instrument calibrations before it's ready to take pictures. The telescope will be controlled by an Italian Space Agency station in Kenya. ISA is partnering with NASA and Caltech on the project. Roughly four times a day, the station will lock on to the telescope and download the data that has been gathered.
"We save all the data on board in solid-state recorders; when they get half-full, they send the data to the ground," Harrison said. "Once they've received the data for the day, they send it to Caltech where they'll turn it into science products." The transmissions, including all the information on the spacecraft itself, will be about a gigabyte each.
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