X-Ray Focus

JPL’s NuStar satellite is a low-weight, high-energy complement to Chandra

Astronomers have floated X-ray detectors on balloons, launched them on sounding rockets and put them on satellites since the late 1950s to sweep the sky. The goal is to unravel the story of a violent universe, where black holes swallow light, neutron stars are born and stars the size of the Sun puff up as red giants only to collapse in death as white dwarfs.

Since Earth’s atmosphere absorbs X-rays, astronomers wanting to use them need to rise above it. Their work so far has concentrated on detecting low-energy X-rays; the high-energy field has been investigated only in wide-angle views. Astronomers at the California Institute of Technology (Caltech) want to capitalize on the ability of high-energy X-rays to penetrate the cosmic dust that obscures the hearts of galaxies, where unseen black holes may be hiding out. Give us sharp, clearly focused images and we’ll give you exciting science, they promise.

NASA hopes the Nuclear Spectroscopic Telescope Array (NuStar) satellite will do just that. Championed for nearly a decade at Caltech by principal investigator Fiona Harrison, NuStar is set for launch on June 13 on an Orbital Sciences Corp. Pegasus XL rocket. Harrison expects the 780-lb. satellite to provide images that are 10 times crisper and 100 times more sensitive than those of previous X-ray observatories.

The 11th mission in NASA’s Small Explorer program, NuStar is funded at $165-170 million and will be capable of penetrating «supermassive» black holes that are dim at all wavelengths of light. One prime target is Sagittarius A at the heart of the Milky Way, which has a mass 4 million times greater than the Sun’s, says Project Scientist Daniel Stern of the Jet Propulsion Laboratory (JPL).

Astronomers are looking for «sleepers»—undetected black holes—and their more active cousins, which shine brightly as they swallow cosmic gas and dust. The NuStar task list includes locating the remnants of collapsed stars, mapping historic supernova remnants and observing high-energy gamma-ray sources. NuStar’s X-ray telescope should help astronomers understand how particles in some galaxies are accelerated to within a fraction of a percent of the speed of light.

The nominal mission length is two years but it could be extended to five. The spacecraft is distinguished by an instrument package that combines state-of-the-art high-energy detectors from Caltech, advanced mirrors and coatings, and the judicious employment of light-weight extendable mast technology from ATK Aerospace of Goleta, Calif. NuStar’s bus is based on Orbital Sciences’ LEOStar-2 design. Spacecraft and launcher integration were completed at Vandenberg AFB, Calif. The combined payload is to be flown by Orbital’s L-1011 launch aircraft to Kwajalein in the central Pacific Ocean on June 5.

The 56-ft.-long Pegasus XL gives small payloads a launch flexibility they cannot achieve if they ride as piggyback payloads on a larger mission. The three stage, solid-fueled rocket will be dropped from the L-1011’s belly at 41,000 ft. about 100 mi. from the Kwajalein atoll in the Marshall Islands. First-stage ignition will take place 5 sec. after the release.

NuStar is to be placed into a 375-mi.-high orbit inclined only 6 deg. from the equator. Hugging the equator minimizes the satellite’s exposure to the South Atlantic Anomaly, where charged particles in the inner Van Allen radiation belt make their closest approach to the planet’s surface.

The telescope will operate at 5-80 kiloelectron volts, well above the energy level of NASA’s Chandra X-ray Observatory, which detects at between 0.1-10 keV. Launched in 1999, the 45-ft.-long Chandra is one of NASA’s premier observatories, but its scale dwarfs NuStar. Chandra is the longest satellite ever orbited by a shuttle.

NuStar’s 100-member astronomy team will work with observatories operating in different wavelengths. Chandra leads the partner list, but the European Space Agency’s XMM-Newton mission—also launched in 1999—is expected to be tapped, as will NASA’s infrared Spitzer Space Telescope, the Japan Aerospace Exploration Agency’s Suzaku X-ray telescope and Hubble.

Data comparisons with NASA’s Fermi Gamma-Ray Space Telescope, which spans the sky 16 times a day in extreme high-energy ranges that are far beyond NuStar’s, will be useful in studying jets of light that stream from black holes with so much energy they affect an entire galaxy, says Harrison. Unlike visible light, if X-rays strike telescope mirrors at high angles, they penetrate them. To reflect X-rays, the mirrors must be positioned at such slight angles that the rays merely graze off them. The telescopes use multiple mirrors, or «shells,» of different sizes, which are nested inside each other, to increase the telescope’s collecting area. Each shell’s surface has a slightly different angle but they all focus on the same spot on the focal plane.

NuStar’s higher energy levels require smaller reflecting angles than previous telescopes, including Chandra. Technical advances have allowed the shells to become much thinner so they do not block light. Where Chandra uses four shells that are 0.8-1.2 in. thick, NuStar requires 133 that are 0.008 in. thick.

More than 9,000 individual mirrors were crafted at NASA’s Goddard Space Flight Center in Greenbelt, Md., for the NuStar mission. They were coated in a vacuum chamber at the Danish Technical University Space Center with alternating layers of platinum and carbon or tungsten and silicon that are just a few atoms thick. Assembly took place at Columbia University’s Nevis Laboratory in New York.

Crystals of cadmium-zinc-telluride allow NuStar’s focal-plane detectors to stop the high-energy X-rays. Caltech printed the detectors in a 32 X 32-pixel pattern, and each one was connected to a readout chip designed to detect X-ray energy levels at better than one part in 60. Ironically, the telescope’s sensitivity makes it vulnerable to stray high-energy photons and cosmic rays, so the system is programmed to detect and ignore background interference.

Because its X-rays graze off mirrors that are nearly parallel, the telescope needs a very long focal length between its twin mirror sets and its focal plane/ detector. Both NuStar and Chandra’s focal lengths are 33 ft. The difference is that the larger, heavier Chandra spacecraft’s focal length is integrated inside the telescope. NuStar’s low-cost solution is to rely on a collapsible composite mast that can fit inside a З.З-ft.-tall canister when it is stowed for launch. Harrison quips that this is the «Tinker Toy» approach. The mast unfolds piece-by-piece once NuStar is in orbit. The buttoned-up NuStar is 3.7 ft. in diameter and 6.3 ft. tall. Once the mast is deployed, the observatory becomes 37.3 ft. long.

An adjustment mechanism on the mast will allow the mission operations center at the University of California-Berkeley’s Space Sciences Lab to fine-tune NuStar’s focal plane using X-ray reference sources.

ATK’s first 197-ft.-long extendable mast was used to separate antennas on the Shuttle Radar Topography Mission in 2000; the company employed the same design to hold the International Space Station’s solar arrays. Should the mast jam or not fully open, NuStar will be lost. Ken Steele, ATK director of business development, says the design has a 100% success rate after some 50 applications.

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