After 20 years of planning, the NuSTAR X-ray telescope launched today from an island in the Pacific.
NuSTAR, the first telescope to focus very high-energy X-rays, launched successfully from Kwajalein Atoll at 9:00 a.m. Pacific time. NuSTAR will collect X-rays in the energy range of 6 to 79 keV, similar to the energy range of medical X-rays. Most X-ray astronomy has been done at lower energies, where X-rays are less difficult to focus. But rather than penetrate skin and muscle to look for broken bones, NuSTAR’s X-rays penetrate dust and gas to reveal supernova remnants and the flicker of gas around feeding black holes, where high-energy processes abound
To place NuSTAR in an equatorial, low-Earth orbit, one that will avoid interference from energetic charged particles trapped in Earth’s magnetic field, the launch had to take an unusual form. An L-1011 Stargazer aircraft climbed to 40,000 feet before dropping a Pegasus XL rocket strapped to its belly. The rocket fell for 5 seconds before igniting the first of three stages to carry NuSTAR into orbit. Watch a video of a Stargazer-Pegasus launch here:
NuSTAR was originally scheduled for launch in March, but a problem (now fixed) in the flight software delayed the launch by three months. For principal investigator Fiona Harrison (California Institute of Technology), three months was a drop in the bucket compared to the 20 years she has dedicated to this mission.
To build NuSTAR, Harrison and colleagues had to develop several new technologies. “Focusing [high-energy] X-rays calls for a new way of doing business,” says instrument manager William Craig (UC Berkeley). Unlike visible light, which comes to a focus when photon paths bend through a lens or bounce straight off a mirror, X-rays have too much energy, and too short a wavelength, to be directed in this way. If you send X-rays straight at a mirror, they’ll pass right through.
Instead, NuSTAR’s mirrors will deflect the X-rays by bouncing them at a very low “grazing incidence” angle barely skimming the mirrors. This method has long been used in other X-ray telescopes, but NuSTAR’s design is more extreme. With 133 mirrors nested inside one another in each of two optic units, NuSTAR can focus enough high-energy X-rays to make images ten times crisper and 100 times more sensitive than what is currently possible. Each image can be broken down into a spectrum with ten times the spectral resolution currently possible at these energies.
“It’s rare you get the chance of increasing a sensitivity factor by more than 100 times better than current methods,” says Bill Craig (Lawrence Livermore National Laboratory). “This is really a game changer.”
NuSTAR’s mirrors came surprisingly cheap — only $20 per sheet — because the mirrors are made of the same glass used in laptop displays. Each mirror has a special reflective coating only a few atoms thick.
Because the X-rays bend so slightly when they graze off the nested mirrors, they come to a focus at a great distance. So the camera imaging has to be placed far from the mirrors. A lightweight, but incredibly stiff mast holds the cameras 10 meters (33 feet) from the mirrors. This mast is a scaled-down version of a longer mast used successfully in a previous radar topography mission. The full-length mast wouldn’t fit in a Pegasus rocket, so it was folded up for launch. The whole telescope was no bigger than a refrigerator this morning. But roughly a week from today, scientists and engineers will be biting their nails as the mast deploys, each part unfolding and locking like a Tinkertoy set. See the simulation here:
Three weeks after the mast is deployed, the telescope should be ready for science operations. The primary mission is set for two years, but NuSTAR's mission could potentially be extended several years beyond that. Among its many targets are spinning black holes, radioactive titanium shining in supernova remnants, and the Sun’s atmosphere.
The launch of NuSTAR, a NASA Small Explorer mission, comes at an interesting time: even as X-ray astronomy celebrates its 50th anniversary, the next major X-ray observatory (the International X-ray Observatory) has been cancelled, and no major X-ray telescopes are planned for the near future. X-ray astronomers will depend on smaller satellites with shorter expected lifetimes, such as NuSTAR and the Japanese ASTRO-H (set to launch in 2014), as well as aging observatories, such as Chandra and XMM-Newton, to advance the field over the next decade or two.