Neutron stars have been called the zombies of the cosmos. They shine even though they're technically dead, occasionally feeding on neighboring stars if they venture too close. Interestingly, these unusual objects, born when a massive star extinguishes its fuel and collapses under its own gravity, also may help future space travelers navigate to Mars and other distant destinations.
NASA recently selected a new mission called the Neutron-star Interior Composition Explorer (NICER) to not only reveal the physics that make neutron stars the densest objects in nature, but also to demonstrate a groundbreaking navigation technology that could revolutionize the agency's ability to travel to the far reaches of the solar system and beyond.
The multi-purpose mission, also known as NICER/SEXTANT (Station Explorer for X-ray Timing and Navigation Technology), consists of 56 X-ray telescopes in a compact bundle, their associated silicon detectors, and a number of other advanced technologies. Both NASA's Science Mission Directorate's Explorers Program and the Space Technology Mission Directorate's Game Changing Program are contributing to the mission's development.
"It's rare that you have an opportunity to fly a cross-cutting experiment," said Principal Investigator Keith Gendreau, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., who is leading NICER/SEXTANT's development. "The time is right for this experiment. This is one that we can do now."
In addition to NASA Goddard scientists and engineers, the mission team includes the Massachusetts Institute of Technology and commercial partners, who are providing spaceflight hardware. The Naval Research Laboratory and universities across the United States, as well as in Canada and Mexico, are providing science expertise.
Space Station Bound
Slightly larger than a typical college dormitory refrigerator, NICER/SEXTANT will be deployed on the International Space Station (ISS) in 2017. It will fly as an external attached payload on one of the ISS ExPRESS Logistics Carriers, unpressurized platforms used for experiments and storage.
The X-ray instrument's primary objective is to learn more about the interior composition of neutron stars, the remnants of massive stars that, after exhausting their nuclear fuel, exploded and collapsed into super-dense spheres about the size of New York City. Their intense gravity crushes an astonishing amount of matter -- often more than 1.4 times the content of the sun or at least 460,000 Earths -- into these city-sized balls, creating the densest objects known in the universe. Just one teaspoonful of neutron star matter would weigh a billion tons on Earth.
"A neutron star is right at the threshold of matter as it can exist -- if it were compressed any further, it would collapse completely in on itself and become a black hole," said Zaven Arzoumanian, a NASA Goddard scientist serving as the deputy principal investigator on the mission. "We have no way of creating or studying this matter in any laboratory. There are many theories about what it is and how it behaves, but the only way to test our models and understand what happens to matter under such incredible pressures is to study neutron stars," he added. "The closest we come to simulating these conditions is in particle accelerators that smash atoms together at almost the speed of light. However, these collisions are not an exact substitute -- they only last a split second, and they generate temperatures that are much higher than what's inside neutron stars."
Although the nuclear-fusion fires that sustained their parent stars are extinguished, neutron stars still shine with heat left over from their explosive formation, and from radiation generated by their magnetic fields that became intensely concentrated as the core collapsed.
Although neutron stars emit radiation across the spectrum, observing in the X-ray band offers the greatest insights into their structure, the ultimate stability of their pulses as precise clock "ticks," and the high-energy, dynamic phenomena that they host, including starquakes, thermonuclear explosions, and the most powerful magnetic fields known in the universe.
NICER's 56 telescopes will collect X-rays generated from its tremendously strong magnetic field and from hotspots located at the stars' two magnetic poles. At these locations, the intense magnetic field emerges from the surface. Particles trapped in the magnetic field rain down and generate X-rays when they strike the surface. As the hotspots rotate into and out of our line of sight, we perceive a rise and fall in X-ray brightness.
This subgroup of pulsating neutron stars, called pulsars, rotate rapidly, emitting from their magnetic poles powerful beams of light that sweep around as the star spins, much like a lighthouse. At Earth, these beams are seen as flashes of light, blinking on and off at intervals from seconds down to milliseconds.
Because of their predictable pulsations -- especially millisecond pulsars, which are the target of the navigation demonstration -- "they are extremely reliable celestial clocks" and can provide high-precision timing just like the atomic clock signals supplied through the 26-satellite, military-operated Global Positioning System (GPS), an Earth-centric system that weakens the farther one travels out beyond Earth orbit and into the solar system, Arzoumanian said. "Pulsars, on the other hand, are accessible in virtually every conceivable flight regime, from low-Earth orbit to interplanetary to deepest space," Gendreau added.
As a result, NICER/SEXTANT also will demonstrate the viability of pulsar-based navigation. "The hardware needed for neutron star science is identical to that needed for pulsar-based navigation," Gendreau said. "In fact, the mission's two goals share many of the same targets and the same operational concept. The differences are on the back end in terms of how the data will be used."
To demonstrate the navigation technology's viability, the NICER/SEXTANT payload will use its telescopes to detect X-ray photons within these powerful beams of light to estimate the arrival times of the pulses. With these measurements, the system will use specially developed algorithms to stitch together an on-board navigation solution.
If an interplanetary mission were equipped with such a navigational device, it would be able to calculate its location autonomously, independent of NASA's Deep Space Network (DSN), Gendreau said. DSN, considered the most sensitive telecommunications system in the world, allows NASA to continuously observe and communicate with interplanetary spacecraft. However, like GPS, the system is Earth-centric. DSN-supplied navigational solutions also degrade the farther one travels out into the solar system. Furthermore, missions must share time on the network, Gendreau said.
"We're excited about NICER/SEXTANT's possibilities," Gendreau added. "The experiment meets critical science objectives and is a stepping-stone for technology applications that meet a variety of NASA needs. It's rare that you get an opportunity to do a cross-cutting experiment like this."
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