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Friday, June 15, 2012

All about NuSTAR

A small X-ray telescope was boosted into orbit by an air-launched Pegasus XL rocket Wednesday, June 13, 2012, the first step in an ambitious low-cost mission to study supermassive black holes believed to be lurking at the cores of galaxies like Earth's Milky Way and to probe the creation of heavy elements in the cataclysmic death throes of massive stars.

Artist's concept of NuSTAR on orbit. NuSTAR has a 10-m (30') mast that deploys after launch to separate the optics modules (right) from the detectors in the focal plane (left). The spacecraft, which controls NuSTAR's pointings, and the solar panels are with the focal plane. NuSTAR has two identical optics modules in order to increase sensitivity. Credit: NASA/JPL-Caltech
While X-ray telescopes sensitive to lower energies have been operated with great success, the $180 million Nuclear Spectroscopic Telescope Array, or NuStar, is the first space telescope designed to focus higher-energy X-rays like those used for medical imaging and dental X-rays. 

The mission got underway with a dramatic pre-dawn launch from an L-1011 jet at an altitude of about 40,000 feet above the Pacific Ocean some 120 miles south of the Kwajalein Atoll in the Marshall Islands. Tucked into the nose cone of a three-stage solid-fuel Pegasus XL rocket, the NuSTAR spacecraft was dropped like a bomb at 12 p.m EDT (GMT-4; 4 a.m. Thursday local time). After a five-second fall, the first stage of the winged Pegasus booster ignited with a rush of flame to begin the steep climb to orbit. 

The Pacific Ocean launch zone was selected to enable the spacecraft to reach a scientifically favorable orbit tilted just six degrees to the equator. 

All three stages of the Pegasus booster operated normally, falling away as planned as their propellants were exhausted. Thirteen minutes after launch, NuSTAR was released into its operational 375-mile-high orbit. A few minutes after that, the telescope's transmitter was activated and telemetry confirmed the successful deployment of its five-segment solar array. 

NuSTAR's ability to detect high-energy X-rays is the result of improved mirror and detector technology. But its ability to be launched by a small, relatively low-cost rocket is the result of an innovative design incorporating an extendable mast, built by ATK Aerospace Systems, that was originally developed for a shuttle radar mapping mission.

Earlier X-ray telescopes, sensitive to lower energies, were built around fixed structures and required large launch vehicles. NASA's Chandra X-ray Observatory, for example, weighed more than six tons and was launched by the shuttle Columbia. NuSTAR weighs just 770 pounds. The mast providing the required separation between mirror and detectors was designed to fit inside a 3.3-foot-tall canister at launch. 

The mission is expected to last at least two years.

NuSTAR will be the first space telescope to create focused images of cosmic X-rays with the highest energies (6 - 79 keV). These are the same types of X-rays that doctors use to see your bones and airports use to scan your bags. Our view of the universe in this spectral window has been limited because previous orbiting telescopes have not employed true focusing optics, but rather have used coded apertures that have intrinsically high backgrounds and limited sensitivity.

During a two-year primary mission phase, NuSTAR will map selected regions of the sky in order to:
  • Take a census of collapsed stars and black holes of different sizes by surveying regions surrounding the center of own Milky Way Galaxy and performing deep observations of the extragalactic sky;
  • Map recently-synthesized material in young supernova remnants to understand how stars explode and how elements are created; and
  • Understand what powers relativistic jets of particles from the most extreme active galaxies hosting supermassive black holes.
The mission will work with other telescopes in space now, including NASA's Chandra X-ray Observatory, which observes lower-energy X-rays. Together, they will provide a more complete picture of the most energetic and exotic objects in space.

"Taking just over four years from receiving the project go-ahead to launch, this low-cost Explorer mission will use new mirror and detector technology that was developed in NASA's basic research program and tested in NASA's scientific ballooning program. The result of these modest investments is a small space telescope that will provide world-class science in an important but relatively unexplored band of the electromagnetic spectrum," said Paul Hertz, NASA's Astrophysics Division director.

The observatory is able to focus the high-energy X-ray light into sharp images because of a complex, innovative telescope design. High-energy light is difficult to focus because it only reflects off mirrors when hitting at nearly parallel angles. NuSTAR solves this problem with nested shells of mirrors. It has the most nested shells ever used in a space telescope: 133 in each of two optic units. The mirrors were molded from ultra-thin glass similar to that found in laptop screens and glazed with even thinner layers of reflective coating.

The telescope also consists of state-of-the-art detectors and a lengthy 33-foot (10-meter) mast, which connects the detectors to the nested mirrors, providing the long distance required to focus the X-rays. This mast is folded up into a canister small enough to fit atop the Pegasus launch vehicle. It will unfurl about seven days after launch, in 56 locking stages to a length of 10 meters, providing a precise focal separation between the mirrors and the detectors. About 23 days later, science operations will begin. 

NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation in Dulles, Va.  

The NuSTAR observatory, with key components labeled. Credit: NASA
NuSTAR spacecraft is in view following the removal of half of the Pegasus payload fairing in Orbital Sciences’ hangar on Vandenberg Air Force Base in California. Access to the spacecraft is needed for compatibility testing to verify communication with a tracking station in Hawaii. Credit: NASA/Randy Beaudoin, VAFB (April 10, 2012)
Technicians install the second half of the payload fairing over the NuSTAR spacecraft as they continue to process the spacecraft and its Pegasus rocket for launch. Image credit: NASA/Randy Beaudoin, VAFB (May 22, 2012)
Technicians roll the Orbital Sciences Pegasus XL rocket with NASA's NuSTAR spacecraft to the waiting L-1011 carrier aircraft known as "Stargazer." Credit: NASA/Randy Baudoin, VAFB (June 2, 2012)
Pegasus XL Launch Vehicle. NuSTAR was launched on June 13, 2012 into a low-Earth, near-equatorial orbit on a Pegasus XL rocket from Kwajalein Atoll in the Marshall Islands. The Pegasus launch vehicle, built by Orbital Space Corporation, relies on a unique air-launch system with the rocket released at approximately 40,000 feet from the "Stargazer" L-1011 aircraft. The rocket then free-falls in a horizontal position for five seconds before igniting its three-stage rocket motor. The Pegasus is one of the most flexible and reliable small launch vehicles, with 40 launches as of October 2008 and 26 consecutive successful launches since 1997. Credit: NASA / Caltech
Pegasus XL NuSTAR Mission Profile. Credit: NASA / Caltech
NuSTAR's Russian Doll-like Mirrors. (In reflection: Todd Decker.) NuSTAR has a complex set of mirrors, or optics, that will help it see high-energy X-ray light in greater detail than ever before. These images show one of two optic units onboard NuSTAR, each consisting of 133 nested cylindrical mirror shells as thin as a fingernail. The inner 66 layers are comprised of 6 segments with 12 pieces of glass per layer, while the outer 67 layers are comprised of 12 segments with 24 pieces of glass per layer. Each unit is 47.2 cm (18.6 inches) long, 19.1 (7.5 inches) cm in diameter and weighs 31 kg (69 pounds). The mirrors are arranged in this way in order to focus as much X-ray light as possible. X-rays don't behave like visible light. Instead of easily bouncing off surfaces, they tend to be absorbed. However, if an incoming X-ray grazes a surface at a very small, glancing angle, it will be reflected. By nesting mirrors of different sizes and angles, more X-rays can be reflected and focused onto the same spot. Credit: NASA/JPL-Caltech
NuSTAR's mirror structure is incredibly more complex than Chandra's. Credit: NASA
Nustar will fly two Cadmium Zinc Telluride (CdZnTe) pixel detectors. These high atomic number solid state devices provide good spectral resolution and high quantum efficiency without requiring cryogenic operation. The detectors are surrounded by an active CsI anticoincidence shield for low background. Credit: NASA
X-rays are in the most energetic portion of the electromagnetic spectrum. They are blocked by Earth atmosphere, so a space telescope is needed to collect enough photons in these very short wavelengths.
NuSTAR will provide more than an order of magnitude advance in spatial and spectral resolution and more than 2 order of magnitude advance in sensitivity with respect to previous X-ray space telescopes.
Credit: Daniel Stern (JPL / Caltech)
NuSTAR will locate massive black holes in other galaxies using the penetrating power of hard X-rays. Credit: NASA
Bringing Black Holes Into Focus. This image comparison demonstrates NuSTAR's improved ability to focus high-energy X-ray light into sharp images. The image on the left, taken by the European Space Agency's INTEGRAL satellite, shows high-energy X-rays from galaxies beyond our own. The light is "unresolved," meaning that individual objects creating the light – in particular, the active supermassive black holes – cannot be distinguished. The image on the right shows a simulated view of what NuSTAR will see at comparable wavelengths. NuSTAR will be able to identify individual black holes making up the diffuse X-ray glow, also called the X-ray background. The observatory will have 100 times better sensitivity than its predecessors, and 10 times sharper resolution. It will probe deeper into the mysterious regions surrounding black holes, and will discover never-before-seen black holes enshrouded in dust. Credit: ESA/NASA/JPL-Caltech
The inner few hundred parsecs around the Galactic center (2 by 0.8 degree) contains ~1% of the Galactic stellar mass, and up to 10% of its massive, young stars. Known sources in this luminosity range include accreting white dwarfs (specifically, intermediate polars), high- and low-mass X-ray binaries, rotation-powered pulsars, and magnetars. This region also contains unique high-energy features, such as mysterious magnetic radio filaments, light echoes from past outbursts of the supermassive black hole Sgr A*, and a TeV source coincident with Sgr A*. NuSTAR will survey the inner ~1 x 1 degree of the Galactic center.
One of the main goals of NuSTAR is to understand the physics of supernovae  and supernova remnants. Credit: Daniel Stern (JPL / Caltech)
NuSTAR will spend 6 months mapping historic supernova. The 44Ti lines at 68 and 78 keV provides important, new diagnostics. Credit: Daniel Stern (JPL / Caltech)
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