Buzz
Although our GPS satellite network is impressive, it has its limits. Photo credit: U.S. Department of DefenseKey Insights
- Despite GPS technology being effective on Earth and in low-Earth orbit (LEO), space missions rely on terrestrial tracking stations and the Doppler effect to navigate.
- NASA's Goddard Space Flight Center developed pulsar navigation, a method using the consistent pulses from neutron stars as cosmic beacons, allowing for accurate position tracking far beyond our solar system.
- The NICER/SEXTANT mission aboard the International Space Station is testing the feasibility of pulsar navigation for autonomous interplanetary travel, aiming to reduce the dangers of becoming "lost in space."
THE DONNER PARTY REACHES CALIFORNIA, REPORTING PLEASANT WEATHER AND SAFE JOURNEY
If George and Jacob Donner had access to the Global Positioning System in the fall of 1846, this could have been a headline. GPS, a precise navigation technology reliant on signals from satellites orbiting about 12,500 miles (20,200 kilometers) above Earth, might have saved them from the tragic fate they suffered. Sadly, the technology wouldn't be developed for another century, and the Donner brothers, along with their ill-fated group of pioneers, had to rely on compasses, maps, and bad advice to navigate to California. Their journey ended in disaster as they were snowed in the Sierra Nevada Mountains, where many died before rescuers arrived in the spring.
Space explorers could face similar perilous situations if they can't find a reliable way to orient themselves on their journeys to distant planets or even stars. While GPS seems like an ideal solution, it is only effective for Earth-based travel. The 24 satellites in the GPS 'constellation' send signals only toward Earth. If you're on the planet's surface, or in low-Earth orbit (LEO), you can determine your location. However, above LEO, the GPS receiver will no longer be able to catch signals because the satellites are no longer directly transmitting in that direction. Simply put, GPS works only when you're beneath the satellites.
However, missions traveling beyond Earth don't have to rely on GPS alone. Existing navigation systems use a network of ground-based tracking stations that direct their attention into space. When a rocket leaves Earth for distant planets like Mars or Jupiter, the tracking stations send radio waves toward the spacecraft. These waves reflect off the rocket and return to Earth, where instruments measure how long it took the waves to complete the journey, as well as the frequency shift caused by the Doppler effect. With this data, ground control can calculate the rocket's position.
Now imagine a journey to the outer limits of the solar system. By the time your spacecraft reaches Pluto, it would be 3,673,500,000 miles (5.9 billion kilometers) from Earth. A radio signal sent from a tracking station would take 5.5 hours to reach you, and then another 5.5 hours to return. This distance makes it increasingly difficult to pinpoint the spacecraft's precise location. As you venture farther, the accuracy of earth-based tracking systems declines even further. The best solution in such cases would be to equip the spacecraft with an autonomous navigational system, and that's where pulsar navigation, developed by NASA's Goddard Space Flight Center, comes into play.
Exploring the Cosmos with Neutron Stars
GPS relies on precise time measurements to perform calculations. Each GPS satellite is equipped with an atomic clock, which is synchronized with the clock in a receiver. By calculating the time it takes for the satellite's signal to reach the receiver and multiplying it by the speed of light, the receiver can determine the distance to the satellite. For example, if it takes 0.07 seconds for a satellite's signal to reach the receiver, the satellite’s range would be 13,020 miles (186,000 miles per second × 0.07 seconds).
In a similar way, a spacecraft could perform calculations by receiving time signals from a source in space. Fortunately, the universe offers highly accurate timekeeping devices—pulsars. These are rapidly spinning neutron stars that emit regular pulses of electromagnetic radiation. Once a pulsar lived a vibrant life and burned brightly. After exhausting its nuclear fuel, it exploded in a massive event. The result is a spinning, magnetized object with poles that emit powerful energy beams. As the dead star spins, the beams sweep across space like a lighthouse beacon. Although the star itself is invisible to observers on Earth, the pulses of light that travel through space are detectable.
Pulsars can blink every few seconds or much more rapidly, but their pulsing frequency remains constant, making them precise timekeepers. In fact, pulsars rival atomic clocks in terms of accuracy. In 1974, scientist G.S. Downs from the Jet Propulsion Laboratory proposed the idea of using pulsars for spacecraft navigation. However, this concept remained theoretical due to a lack of knowledge about pulsars and the fact that the only instruments capable of detecting them—radio telescopes—were massive and cumbersome.
Over time, the field progressed. Astronomers continued to discover pulsars and investigate their behavior. By 1982, scientists had identified the first millisecond pulsars, which have rotational periods of less than 20 milliseconds. In 1983, they also found that certain millisecond pulsars emitted strong X-ray signals. These discoveries allowed pulsar navigation to move beyond theory and into practical application.
Galactic GPS
This illustration depicts the NICER/SEXTANT payload, featuring a 56-telescope array designed to be launched aboard the International Space Station. Image credit: NASA.While the GPS system we use on Earth isn't suitable for navigating through the solar system, its underlying principles can still be applied to interplanetary navigation. In fact, using pulsars to guide spacecraft is somewhat akin to using GPS on Earth.
- Just like GPS relies on signals from four or more satellites to triangulate your location, pulsars work in a similar way, requiring multiple pulsars to pinpoint an object's position in space. Over 2,000 pulsars have been cataloged, but the most promising ones for navigation are stable millisecond pulsars with strong X-ray emissions. Among the top contenders are pulsars like J0437−4715, J1824−2452A, J1939+2134, and J2124−3358.
- The next step involves using a device that can detect the pulsars' X-ray signals. While current X-ray telescopes are too large for spacecraft, the future of this technology lies in compact XNAV receivers that can be easily carried into space.
- Finally, complex algorithms are needed to process the data. Scientists have spent years developing mathematical models to account for various factors like pulsar irregularities and external influences (gravitational waves or plasma). Though the calculations are intricate, the concept mirrors GPS on Earth: The XNAV receiver detects signals from four or more pulsars, each carrying a time stamp, allowing for precise distance measurements as a spacecraft moves.
The final challenge is testing the theory to confirm its viability. This is the primary objective of NASA's NICER/SEXTANT mission. NICER/SEXTANT, which stands for Neutron-star Interior Composition Explorer/Station Explorer for X-ray Timing and Navigation Technology, consists of 56 X-ray telescopes arranged together in an array roughly the size of a mini-refrigerator. The mission, set to launch aboard the International Space Station in 2017, has two goals: studying neutron stars and proving the concept of pulsar-based navigation.
If NICER/SEXTANT proves successful, it could bring us closer to autonomous space navigation. This advancement might one day help us avoid the dangers of getting lost at the edge of the solar system, far from Earth—a scenario that seems far more terrifying than losing your way to California.
