NICER and SEXTANT demonstrate XNAV pulsar navigation system that may be used on Artemis.
As long as explorers have traversed Earth's surface, getting an accurate fix on location has been essential. Early explorers used a sextant and compass to gauge the position of the stars at sea, and modern travelers use satellite-enabled GPS technology. Now, future deep space missions may use a more exotic source to get a fix on their location: the beating hearts of dying stars, known as pulsars.
Pulsars, also referred to as neutron stars, are the swiftly spinning remnant cores of core-collapse supernovae. A famous example is the pulsar at the center of the Crab Nebula, or Messier 1, in Taurus. Discovered in the late 1960s, the ultra-precise signals from pulsars earned them the informal acronym LGM (for "Little Green Men") for a short time after their discovery.
NASA's Neutron star Interior Composition Explorer (NICER) instrument, which has operated on the International Space Station (ISS) for the past few years, is devoted to studying pulsars to understand their structure and evolution. A technology demonstration known as SEXTANT utilizes those observations as a test-bed for future pulsar-based navigation.
NICER is a large, box-shaped detector, located on the exterior of the Integrated Truss Structure of the ISS. Part of NASA’s Explorers program, NICER records X-ray photons with energies between 200 and 12,000 electron volts. The detector was deployed as a proof of concept in 2017 and demonstrated X-ray detection and navigation capability in 2018.
Along the way, NICER has discovered the brightest X-ray burst from a pulsar 11,000 light-years away in Sagittarius, and found the fastest known orbiting pulsar orbiting its companion star every 38 minutes.
With SEXTANT, the mission turns the detector's pulsar timings — particularly timing measurements of rapidly rotating millisecond pulsars — into an autonomous next-generation navigation system for accurate positioning throughout the solar system.
NICER observes specific pulsars during each 90-minute orbit around Earth. By comparing time-stamped arrival times of the pulsars' photons, the SEXTANT program measures the station’s precise position in space. This X-ray pulsar-based navigation is sometimes referred to as XNAV.
XNAV takes GPS to the next level, turning the "G" from "global" into "galactic."
“GPS uses precisely synchronized signals,” says Luke Winternitz (NASA Goddard Space Flight Center) in a recent press release. “Pulsations from some neutron stars are extremely stable, some even as stable as terrestrial atomic clocks in the long term, which makes them potentially useful in a similar way.”
Robotic missions in the solar system typically use a combination of star tracking and radio signals from NASA’s worldwide Deep Space Network to fix position. XNAV would give a mission an autonomous and accurate navigational system that doesn't degrade as the mission travels farther from Earth.
One advantage to XNAV is that it would work throughout the solar system — and beyond — with no delay due to intervening interstellar matter. X-ray signals, however, are fainter, so the technique requires a more robust detector.
XNAV and Artemis
There are also plans to use the system on the upcoming Artemis initiative, which will return astronauts to the Moon. The planned Lunar Gateway will orbit the Moon in a looping 6 ½-day near-rectilinear halo orbit, allowing for longer and more accurate pulsar observations.
A longer collection time also means that a NICER/SEXTANT-type system on board the Lunar Gateway platform could be much smaller that the one currently used on the exterior of the International Space Station. “NICER is roughly the size of a washing-machine, but you could dramatically reduce its size and volume,” says Jason Mitchell (NASA Goddard Space Flight Center) in a recent press release. “For example, it would be interesting to fit an XNAV telescope into a small satellite that could independently navigate the asteroid belt and characterize primitive solar system bodies.”
Classic sextant navigation has actually been a mainstay in crewed U.S. spaceflight in the past, going all the way back to the Gemini and Apollo missions. On Apollo 8, Jim Lovell demonstrated that such a method could be used as a backup for astronauts to find their way back to Earth in the event of an emergency. In 2018, ISS astronauts tested a similar technique for possible future use by Orion deep space missions. And the U.S. Navy even brought the method of stellar navigation back for officer training in the event of a contingency. Although the pulsar method is more exotic, it would give deep space missions an autonomous means of navigation.
Pulsar navigation may one day come as standard feature of future space missions.