Guest blog: Benjamin Palmer on NuSTAR

Posted by David Eicher
on Monday, March 12, 2012

A new orbiting observatory is set to launch next week — NASA’s Nuclear Spectroscopic Telescope Array (NuStar). This pioneering mission will image the sky for the first time in orbit in high-energy X-rays, promising all kinds of cool findings regarding black holes, supernovae, the Milky Way, cosmic accelerators, and other good stuff. Here’s a guest blog by Benjamin Palmer of Queensbury, New York, last year’s Astronomy magazine Youth Essay Contest winner and now youth committee chair of the Astronomy Foundation. Enjoy!

By Benjamin Palmer

No environment is more conducive to revolutionary astrophysics than the compelling atmosphere of the space exploration community.  Here, you’ll discover astronomy’s best and brightest, striving for ubiquitous excellence in their quest to conquer the cosmos. Amidst this dynamic astrophysical backdrop, a cadre of avant-garde spacecraft reign supreme. These iconic voyagers shoulder the titles of the greats, immortalizing their human namesakes while scouring a transcendent tide of celestial paradoxes.

As vehicles like Cassini, Hubble, Herschel, and Kepler etch their profound observations into our collective memories, an imposing new space probe is rapidly taking form. Utilizing advanced technology and modern science, this elite spacecraft promises to be tomorrow’s “Top Gun,” its potent abilities rivaling the established legend of its predecessors. Get ready, science: the future of astronomical inquiry has arrived.  

NASA’s radiant superstar

Meet the NuSTAR. In the elegant atmosphere of modern astrophysics, this nimble X-Ray observer is the hottest spacecraft in town.

Scheduled for a March 22 launch, NuSTAR will sprint to the skies with a comprehensive, yet provocative science docket in mind. Encompassing a plethora of instrumentation and bursting with investigative potential, this remarkable craft has been assigned several arduous tasks of unparalleled significance. The research list: probe postulated solar axions, adumbrate theoretical microflares in an attempt to decode the solar corona; create a comprehensive census of black holes and collapsed stellar entities of varying size; fathom supernovae mechanics by charting recently synthesized remnant elements; and contemplate the workings of particle jets, ejected via supermassive black holes at the heart of active galaxies.

When NuSTAR and the heavens unite, a poignant place in history will be solidified. Even before her sensors enable, NuSTAR will be a scientific “first” in many regards. NuSTAR will become the first X- and gamma-ray oriented instrument to be launched by NASA since the Fermi Gamma-ray Space Telescope in 2008. As the first orbital satellite   to focus X-rays in high-energy states, NuSTAR offers astronomers a unique vantage point when examining X-ray enigmas. Dissecting energies greater than those observed by Chandra or XMM Newton, NuSTAR allows astrophysicists to delve deeper than ever before into X-ray and high-energy-related phenomena.

But every telescope has a behind-the-scenes narrative. Nearly a decade ago, NuSTAR’s odyssey began as most journeys do, with innately humble origins.

Her thrilling story began in February 2003 when NASA declared an Explorer Program Announcement of Opportunity. This initiative called for projects to fill the tenth and eleventh mission slots in NASA’s enterprising Small Explorers Program. Thirty-six high-caliber proposals, including NuSTAR, answered the call. Following thorough analysis, a handful of projects made the primary cut, progressing to a five-month-feasibility study beginning November 2003.

A full year elapsed. Finally, in January 2005, NuSTAR ousted the competition,  joining a prodigious coterie of Explorer spacecraft, including distinguished pioneers  WMAP and ACE.  After an additional yearlong flight-ability investigation, NASA gave the go ahead; NuSTAR could now prepare for operational status

Then came the budget cuts. Although a comparatively affordable spacecraft (NASA requires its small explorer missions to retain a budget under 120 million USD), slashes to NASA’s 2007 fiscal funds resulted in NuSTAR’s eventual cancellation in February 2006. Fortunately, the drama wasn’t to last. In September 2007, the NuSTAR project was formally reinstated, placing one of astronomy’s most sophisticated observational telescopes permanently on the table.

Now, headed by the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory (JPL), NuSTAR’s illustrious partners include members from both governmental and university settings. Led by principal investigator Fiona Anne Harrison from Caltech, this extensive group is comprised of representatives from NASA’s Goddard Space Flight Center, the Danish National Space Center, the Stanford Linear Accelerator Center, Columbia University, University of California, Berkley, University of California, Santa Cruz, and Sonoma State University. For launch and construction purposes, NuSTAR also plays host to numerous industrial collaborators, with the Orbital Science Corporation and ATK Space Systems-Goleta serving as primary contributors.

Having achieved stability in both financial and personnel sectors, NuSTAR can, quite literally, get down to the math. But to unlock the cosmic conundrums within its agenda, NuSTAR requires a premium standard in a spacecraft’s most critical element: observational apparatus. As every scientific detective knows, astrophysical scrutiny relies on capital investigative tools.  Given her captivating mission objectives, NuSTAR demands an intricate web of advanced instrumentation.  So what, in essence, makes this alluring spacecraft tick?

Armed and ready

Outfitting an astrophysical orbiter requires intricate coordination as the organizational pitfalls loom large. Any effective space probe must embody a lasting harmony, striking the perfect balance of affordability, dependability, and capability.

NuSTAR planners have done just that. Reliable technical systems have been employed and costs have been kept low. Yet thanks to her robust technologies, NuSTAR is armed to the teeth with cardinal scientific paraphernalia.

NuSTAR’s instrumentation closely mirrors that of its immediate predecessor, the High Energy Focusing Telescope (HEFT). Debuting in 2005, HEFT, a balloon-borne observatory, gleans information from hard X-rays (in this case, high energy rays in the 20-100keV band) through cadmium zinc telluride pixel detectors and depth-graded multilayer optics. Breaking new ground in both X- and gamma-ray observations, HEFT was an exemplary observational archetype, establishing the astronomical relevance of X-ray focusing telescopes.

NuSTAR runs along the same lines as HEFT, though with a myriad of significant advantages. Her technical composition breaks into three categorical elements: optics, detectors, and a deployable mast.

Optically speaking, NuSTAR manifests superlative celestial eyesight. This is achieved
using two distinct optical units.

X-rays aren’t observable in visual scopes, being transmitted and absorbed, rather than reflected. However, NuSTAR’s mirrors are carefully aligned to keep the angular bend from the plane of reflection very low. This technology, built free of Abbe sine condition (clear of spherical abrasion and coma) is known as grazing incidence, and has proved immensely beneficial in X-ray telescopes. Conceived by German physicist Hans Wolter in 1952, this arrangement consists of a paraboloid primary mirror accented by either a hyperboloid or ellipsoid secondary structure. With multiple design options possible, astronomers have classified a trio of mainstream configurations: the Wolter-I (paraboloid and confocal hyperboloid), Wolter-II (paraboloid and hyperboloid), and Wolter-III (paraboloid and ellipsoid).

NuSTAR’s dual optical units are both Wolter-I systems. Its optics display an overall length of 1.5 feet, with a peak radius of 7.5 inches. Focal length is an impressive 33 feet. Their conical shape is comprised of approximately 133 concentric mirrors shells, all with multi-layered coatings.

Jointly assembled by the Goddard Space Flight Center, DTU-Space at Danish Technical University, and Columbia University’s Nevis Laboratory, the mirrors are light, limber, and cost effective. They begin life as slim glass substrates, akin to the insert screen of a computer. After precision heating in a specialized, high-temp oven, the malleable sheets are draped over spotlessly polished cylindrical quartz mandrels, yielding a flawless baseline curvature.

Following initial shaping, NuSTAR’s mirrors acquire the next crucial ingredient: coatings. Like HEFT, NuSTAR’s optics exploit the benefits of depth-graded multilayers. These are constructed in compound fashion. Two alternate materials with converse densities, one high, the other low, are stacked in an overlaying manner, producing a strong density contrast. With several material combinations to choose from, NuSTAR will employ two distinct optical medleys. The first blends platinum (high density), and siliconcarbite (low density), while the second uses tungsten (high density) and silicon (low density). The by-product of this arrangement is maximum reflectivity of high energy X-rays, giving NuSTAR unprecedented observational power in the 6-79 keV range.

After coating, the final step is joint construction. With meticulous care, the optics are assembled from the inside out, the mirror shells fused together with epoxy, and separated by graphite spacers.

Superior mirrors alone cannot satisfy a research orbiter’s prerequisite. Enter scientific detectors, the literal brainpower of any astronomical satellite. Given NuSTAR’s research prowess, what technologies drive its paramount “mental abilities”?  

The answer lies in NuSTAR’s dual detector pods. Located at the focus of each Wolter-1 optical unit, the detectors and accompanying mirrors will be aimed at the same sliver of sky, amassing and relaying the data needed to create astrophysical X-ray images back on earth.

These detectors aren’t your typical backyard astrocams. Each pod contains a wealth of technical components, individually tailored to enhance the photographic process.

NuSTAR’s two focal planes engage 32 x 32 pixel Cadmium-Zinc-Tellurium (CdZnTe, or CZT) detectors. Manufactured by eV products, now EI Detection & Imaging Systems, these instruments are among the best in the business.

For this mission, four CZT detectors are tasked with converting high energy photons into electrons. As the detectors are contemporary room temperature semiconductors, the transfer process is quite efficacious. Once modified, the spacecraft’s elegant Application Specific Integrated Circuits (ASICs) take over. Fabricated by NuSTAR’s Caltech Focal Plane Team, the custom circuits utilize digital recording, thereby capturing the conglomerate electrons.

NuSTAR’s intricate focal planes also demand adequate shielding. That responsibility falls to cesium-iodine (CsI) crystals. Cultivated by the Saint-Gobain Corporation, this critical shield heavily augments NuSTAR’s astrophysical accuracy by chronicling extraneous cosmic rays and high energy photons, traversing the focal plane from directions unaligned with NuSTAR’s optical axis. This captious procedure permits astronomers to identify and subtract background elements, allowing NuSTAR to substantiate high energy photons of astronomical origin. As a result, the CZT detectors and CsI shield can be impacted simultaneously, without altering data the CZT encoders compile.

Without stable mounting, optical/detector packages immediately lose their value. This brings us to NuSTAR’s concluding feature: the deployable mast. What direct contributions does this salient structure render?

Enduring X-ray telescopes demand considerable focal lengths. This pertains to NuSTAR’s detectors and mirrors, which must be spread apart by a few respective meters.  The allocated space allows the photographic detectors to create quality X-ray images. In numerous missions, the spacecraft itself incorporates a sizable length, when assembled with large available finance and massive launch vehicles. However, NuSTAR’s cost and launch systems necessitate an effective, yet budget friendly alternative. Fortunately, NuSTAR planners have engineered an innovative solution.

NuSTAR has engaged the proven services of ATK-Goleta to construct a rugged, articulated mast. Meeting both launch weight and scientific specifications, this system is modern ingenuity at its finest. During takeoff, the mast will be stowed within a confine (seven feet) two meters long and (three feet) one meter in diameter. Once orbital flight has been achieved, the boom is free to fully extend, thereby providing the desired focal length separation.

The mast also manifests a few vital technological factors. To maintain ideal optical alignment, a mast-mounted mechanism will calibrate the optical units during the mission’s commencement. Further, the boom possesses a laser-based metrology system to aid the imaging detectors. This setup uses two optic-mounted lasers fired at a trio of light sensing indicators on the detector side. The resulting measurements will compensate for the mast’s motion blur as perceived by the detectors, with steady X-ray imagery as the outcome.

NuSTAR’s inspiring technology places this spacecraft on the cutting edge of astrophysics. Retaining the ideal amalgam of scientific instrumentation, NuSTAR will deliver more than two magnitudes of increased observational sensitivity, a vast improvement compared to former high energy observatories. NuSTAR’s revolutionary equipment will open new vistas for X-ray astronomy, and undoubtedly pave the way for the explorers of tomorrow.

Lights, camera, liftoff

“How are we going to get there?” Indeed, every comprehensive space mission must acknowledge this question with justifiable answers. On the NuSTAR front, cohesive launch framework has been expertly laid, with exquisite contributions from around the globe.

NuSTAR’s main launch contractor is the venerable Orbital Sciences Corporation (OCS). Currently celebrating three decades of astrophysical dedication, OSC has deployed a multitude of astronomical vehicles, ranging from GALEX to AIM.  From OSC’s extensive launch platform inventory, NuSTAR engineers have selected the ingenious Pegasus XL rocket as the means of transportation.

An Orbital Sciences mainstay since 1990, the Pegasus launch system has reshaped the landscape of low-Earth orbital insertion. With 35 successful deployments under its belt, this brilliant rocket produces auspicious orbital installments with the utmost ease. Supporting payloads up to 1,000 pounds, this durable craft satisfies myriad satellite configurations.  Capable of launching from a legion of sites worldwide, Pegasus caters to the direct needs of its users, offering incredible versatility with a heavily reduced price tag.

As NuSTAR’s regal chauffeur, the Pegasus XL integrates several technological elements. The vehicle’s independent stages embody a trifecta of Orion motors, one to each stage.  Manufactured by Alliant Techsystems, renowned for construction of the space shuttle’s boosters, these savvy propulsion devices will accelerate NuSTAR into space for approximately 219 seconds. Steering falls to tail fins (first stage only), main engine thrust vectoring nozzles (second and third stage), and nitrogen fueled thrusters (third stage).

For launch and flight accuracy, the Pegasus XL rocket contains an advanced internal guidance package. In addition to a GPS receiver, this assemblage includes a 32-bit computer accented by an inertial measurement unit (IMU).  During transit, each stage relies on algorithms for trajectory stabilization, the initial stage utilizing an exclusive model, with the second and third stages using space shuttle-based algorithms instead.

The remaining asset aboard the Pegasus XL is the efficiently designed wing. Molded to an immaculate 45° delta, this modernistic feature adds an extent of lift to the rocket, an essential action in early takeoff maneuvers.

Once NuSTAR is snugly fitted within the Pegasus XL cargo bay, the following launch
sequence will ensue. First, a modified Lockheed L-1011 TriStar will lift off from Kwajalein Atoll in the Pacific, smoothly ascending to 40,000 feet. Upon reaching the drop zone, the L-1011 releases the Pegasus System, which shall free fall in solitude
for five nail-biting seconds. Then the inaugural stage rocket, the Orion 50S ignites, driving NuSTAR toward the heavens with an earsplitting roar. Roughly 80 seconds after ignition, the Orion 50S sputters out, giving way to the second stage motor, the Orion 50. Also burning for around 80 seconds, this stage showcases vector control, distributed in pitch and yaw. At the halfway mark of second stage control, the launcher’s outer fairing breaks off, the discarded section revealing NuSTAR and the third stage. Next the second stage motor terminates. Pegasus and NuSTAR will cruise together for a brief time, until the pristine trajectory point is achieved. The second stage engine now drops away, handing the slack to the third motor, the Orion 38. Having ironed out roll, the nitrogen thrusters and third stage engine expire 64 seconds later. The final stage swiftly shears off, leaving NuSTAR in near-equatorial orbit.

NuSTAR will now officially be on her own. In the years to come, NuSTAR will circle our azure sphere on a quest that promises a capacious number of epic discoveries.   

Eye on the sky

When launched, NuSTAR will be the 31st X-ray telescope to roam the skies. Each of these orbiting observatories has brought something unique to astrophysics, decoding the extensive palette of X-ray anomalies.

Astronomy is a field of affluent perspectives, a varying potpourri of scientific interpretations. In decoding this lavish cosmic choreography, mankind labors to place the best minds, theories, and technology on the forefront of astrophysical exploration. With NuSTAR primed for launch, the future looks bright for progressive investigation of our breathtaking universe.

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