Navigating to the stars using stars
In past columns we have talked about using the stars to navigate upon the Earth and we have also discussed how some stars become pulsars which emit periodic bursts of radiation in the form of radio, optical, and X-ray emission. Surprisingly, a new NASA mission ties together these two seemingly unrelated topics. A new instrument aboard the International Space Station, dubbed “Station Explorer for X-ray Timing and Navigation Technology” and more succinctly denoted by the acronym, SEXTANT, will demonstrate how to use pulsars for a new method of navigation for spacecraft.
Pulsars arise from old stars which have exhausted most of the hydrogen that previously fueled their starlight. Without the heat generated from the nuclear fusion of hydrogen, gravity will cause old stars to collapse to a smaller and denser object known as a neutron star. Concentrating all the mass of a star into a ball only a few miles wide drives the density so high that a spoonful of the star’s dense matter will weigh billions of tons. When the star shrinks, not only is its mass concentrated into a smaller volume but so is its magnetism. If the star had a significant magnetic field to begin with, the shrunken star will now have a much stronger field because the magnetic field is concentrated into a much smaller volume. As the star shrinks to its smaller size, its rotation speeds up, just as an ice skater’s rotation speeds up as she pulls her arms inward. This combination of rapid rotation and strong magnetic field create the pulses of radiation that give pulsars their name. Pulsars were discovered by Jocelyn Bell Burnell in 1967 when she detected unexplained pulses of radio emission appearing every 1.33 seconds. Eventually, astronomers realized that the pulses arise from the fact that the strong magnetic field of a pulsar confines its emission to a narrow beam and its rapid rotation sweeps that beam past us once in every rotation, like the beam of a lighthouse.
There is an instrument aboard the International Space Station devoted to the study of neutron stars. This instrument, known as the Neutron-star Interior Composition Explorer, or NICER, consists of 52 small telescopes which detect X-rays from many neutron stars, including pulsars. In 2017, scientists at the U.S. Naval Research Laboratory and the NASA Goddard Spaceflight Center conducted the two-day experiment, called SEXTANT, in which they adapted the NICER telescope for use as a new type of navigational device. Readers of this column may recall previous articles which described how sextants are used to measure the angle of stars above the horizon for purposes of navigation. Unlike familiar terrestrial sextants, the SEXTANT experiment does not measure the directions to pulsars to determine its location.
Instead, it relies upon precise measurements of the timing of the X-ray pulses coming from different pulsars.
Using a sextant to observe the directions of specific stars seen from the surface of the Earth is useful for navigation because the Earth is constantly rotating, so identifying which star is in a particular direction at a known time tells us which location on Earth is facing toward that star; that is essentially what we need to know to find our location at that time. But as we have discussed before, this sort of traditional celestial navigation has largely been superseded in recent years by the use of the Global Positioning System, or GPS. GPS receivers work by detecting radio signals from the two dozen Earth-orbiting GPS satellites. But while traditional celestial navigation depends upon measuring the directions to different stars, GPS navigation doesn’t directly measure the directions to the GPS satellites. The GPS navigator relies instead upon measuring the precise timing of the signal from each satellite. Since every GPS satellite contains a precise atomic clock, synchronized with the clocks aboard the other GPS satellites, comparing the travel time of the radio signals from each of three or more satellites allows the navigator to determine his distance from each of the satellites, thus fixing his position since the orbits of the satellites can be predicted with great accuracy.
GPS is great for those of us who spend most of our time on Earth but it is useless for navigating in deep space far from Earth since the GPS satellites are all orbiting just a few hundred miles above the Earth. When the Apollo astronauts ventured to the Moon, the GPS had not yet been created. But even if it had existed at the time, it would have been useless to astronauts so far from Earth. Instead, the astronauts used a traditional sextant to orient themselves in space and know which way their spacecraft was pointing – a vital piece of information before firing the engine to change course! But since they were not on the surface of a rotating Earth, their observations of stars told them only their orientation; that is, which way they were facing, not how far they had travelled. They had to rely upon radar and radio signals from Earth to determine their position. Farther from Earth, those signals become weaker and more difficult to use. But the X-ray emission of pulsars can be used to determine both the orientation and the position of a spacecraft anywhere.
Because pulsars are so massive and spin so rapidly, their rotation periods can’t change very quickly. Their rotation periods can be precisely measured and the tiny changes in those periods can be predicted with great accuracy years in advance. This means the time between the pulses from each pulsar is like a unique “fingerprint” of that pulsar. Identifying the direction to several pulsars allows SEXTANT to determine its orientation relative to those pulsars and comparing the precise arrival times of the pulses allows SEXTANT to determine its position to an accuracy of a few miles. Within eight hours of beginning the recent two-day test of SEXTANT, the system had determined the location of the space station to within ten miles as it circled the earth at 17,500 mph. Later observations improved the accuracy to under three miles and project manager Jason Mitchell expects to improve the system soon to achieve an accuracy of a few hundred feet.
Now that this unexpected use of the NICER telescope has demonstrated that pulsars can be used for navigation, scientists hope to develop future instruments for deep space navigation which will be smaller, lighter, and use less power. When pulsars were first discovered, their precise timing seemed so mysterious that some astronomers half-jokingly suggested they might be navigational beacons built by space-faring civilizations. Indeed, the first pulsar detected by Jocelyn Bell Burnell was humorously given the designation “LGM-1” for “Little Green Man.” Although they were not built by aliens, it now appears that pulsars may eventually be used as navigational beacons by a space-faring civilization after all: ours.
Though we can’t dazzle you with views of pulsars, the observers of the Adirondack Public Observatory invite you to enjoy viewing other wonders of the cosmos at our Roll Off Roof Observatory (RORO) that is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Please come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.