＂Discerning Eye＂ Satellite Exploration ＂Wandering Earth＂ has reliable navigation
Eye of Sight, our country’s first large-scale X-ray astronomical satellite, launched in 2017. Photo provided by respondents
Humans couldn’t go into space without navigation. Aircraft could use GPS, Beidou and other artificial satellites to navigate, and flying out of the earth would use the "Deep Space Exploration Network". But as humans’ footsteps to explore the universe grew farther and farther, the Deep Space Exploration Network would also be "beyond reach".
"Pulsar navigation" may be one of the most reliable navigation technologies for future deep space exploration.
Recently, researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences have successfully carried out X-ray pulsar navigation experiments using our country’s first X-ray astronomical satellite, the "eye", with positioning accuracy within 10 kilometers (3 times standard deviation), reaching the international advanced level.
Zheng Shijie, the head of the "Insight" satellite pulsar navigation experiment and an associate researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, recently gave an interview to a reporter from Nanfang Daily to explain the technology that will help humans "go to deep space".
How high is the accuracy achieved?
When we leave the earth and enter space, satellite navigation systems such as GPS and Beidou can no longer be used. Therefore, the United States spent a lot of money to establish the "Deep Space Exploration Network", establishing three huge antenna arrays at equal intervals on the earth, using them to receive weak signals from distant spacecraft to achieve communication and positioning, making important contributions to human exploration of the universe.
Zheng Shi introduced that using the above technology for positioning, the error will increase with the increase of the distance from the earth, and the average distance between the sun and the earth (about 150 million kilometers) will increase by 10 kilometers.
"When reaching Jupiter (4-6 solar distances from Earth), the error will reach tens of kilometers; for Voyager 1 and 2, which are flying away from the solar system, the error will reach thousands of kilometers," Zheng said. "Another problem with this technology is that the communication time is too long: the one-way communication time between us and Jupiter is at least 30 minutes, and the one-way communication time with Voyager 1 is 17 hours! This also means that we cannot make timely adjustments to the flight trajectory of these vehicles."
Using pulsars to navigate can avoid these problems. Zheng Shi introduced that the "cosmic lighthouse" observed by the "discerning eye" experiment, the Crab Nebula pulsar, is 6,000 light-years away from us, and the navigation error obtained by using pulsars will not change with our position. "This way, even if one day we really leave the solar system with the’wandering earth ‘, we can navigate it accurately."
In addition, Zheng said that pulsar navigation technology can reduce the aircraft’s dependence on the earth, and the use of artificial intelligence technology on the star can make a lot of independent judgments and operations.
Zheng Shi introduced, "The pulsar navigation experiment we carried out on the’discerning eye ‘satellite shows that its positioning error is within 10 kilometers, and it is better than the deep space network for deep space outside Jupiter, which means that pulsar navigation can already be applied in deep space." What does the positioning accuracy of 10 kilometers (3 times the standard deviation) mean? Standard deviation is a commonly used method in scientific analysis to judge reliability. 3 times the standard deviation means that this value is credible in 99.7% of cases.
It is reported that in May 2019, NASA also announced that it would apply X-ray pulsar navigation technology to the "return to the moon program" and future Mars exploration programs. Zheng Shijie said, "In the future, with the accuracy of pulsar navigation reaching 1 kilometer or less, it will definitely be more widely used in Mars exploration, asteroid exploration and other scenarios."
What kind of pulsar is suitable for navigation?
As of the end of 2018, the Australian Observatory had sorted and announced more than 2,700 pulsars, not including nearly 100 pulsars discovered by China’s "Sky Eye" telescope and more than 100 pulsars to be certified.
Which of these pulsars are better suited for navigation?
Zheng Shi introduced, "Among the nearly 3,000 known pulsars, we have selected about 10’navigation stars’ that are most suitable for navigation."
One of the "selection criteria" is to have strong pulsed radiation in the X-ray band.
Zheng Shi introduced that pulsar radiation signals cover radio, infrared, visible light, ultraviolet, X-ray, gamma rays and even higher energy bands. Among them, the X-ray band has the advantages of strong radiation energy and easy miniaturization of detection equipment. Therefore, using pulsar signals in the X-ray band to achieve navigation is the most likely way.
"This requires selecting pulsars with strong radiation in the X-ray band." Zheng Shijie said that there are currently more than 200 pulsars that emit radiation in the X-ray band, but their pulsed photon flux varies widely.
According to Zheng Shijie, the Crab Nebula pulsar selected in this experiment is one of the brightest "cosmic beacons" in the starry sky. It was born in a supernova explosion in 1054 AD, which was recorded in the "History of Song". Its pulse photon flux is about 0.1/cm2/s, which means that when we use a 100cm2 detector, we can receive 10 pulsed photons per second. The pulse photon flux of most pulsars is only one ten thousandth of its. For the same detector, we need to wait 1000 seconds to receive a pulsed photon.
Another "inclusion criterion" requires the pulsar to have a stable period. Zheng Shijie said that the pulsar’s pulse period is very accurate, but it is also gradually evolving, so we need to be able to accurately predict its evolutionary behavior. Generally speaking, "young" pulsars are usually "changeable" and sometimes have a little temper, and there will be "starquakes" like earthquakes. After the starquake occurs, the period of the pulsar changes. If you happen to use this pulsar for navigation, it will cause a large deviation in the navigation accuracy; "old" pulsars are much more "stable".
For example, he said, the most stable pulsar known, PSR J0437-4715, is more than 1 billion old, and its stability is comparable to the best atomic clock on Earth. It is also a very good navigation pulsar. In addition, the shorter the period of the pulsar, the higher the accuracy of the pulse signal obtained by observation. "Therefore, we mainly choose the pulsar with the shortest period and the pulse signal period in the order of milliseconds as our navigation star."
Another factor is that different pulsars emit different pulse shapes (i.e. pulse contours); navigation pulsars should choose pulsars with sharp pulse contours, so that navigation accuracy will also be improved.
How far is it from being put into practical use?
X-ray pulsar navigation has received increasing attention in recent years.
In 2004, ESA published a technical report "Spacecraft navigation feasibility study based on pulsar time information". In January 2018, NASA announced that the NICER/SEXTANT project aboard the International Space Station had successfully conducted the first real-time autonomous navigation experiment on an orbiting pulsar.
China has also conducted a lot of theoretical and experimental research on pulsar navigation. The Tiangong-2 space laboratory, launched in September 2016, carried the "celestial pole" telescope, the Gamma Burst Polarization Explorer (POLAR), and completed the first domestic space experiment on pulsar navigation. The pulsar test star XPNAV-01, launched in November 2016, also carried out pulsar detection and related research.
The pulsar navigation experiment conducted by the "discerning eye" further validated the X-ray pulsar navigation algorithm proposed by High Energy – "Correlation Analysis of Pulse Profile Significance and Satellite Orbit". The feasibility of this algorithm has been preliminarily verified in the POLAR experiment.
In the "discerning eye" experiment, the researchers further improved the algorithm and applied the navigation algorithm to the observation data of three kinds of telescopes on the "discerning eye" satellite, and the results showed that the discerning eye can achieve autonomous positioning; if the observation data of all telescopes for 5 days is combined, the positioning accuracy can reach 10 kilometers (3 times the standard deviation), indicating that the accuracy of the "discerning eye" pulsar navigation experiment is comparable to the results of NICER/SEXTANT.
In order to further test the feasibility and reliability of the navigation algorithm, the research team also conducted a sufficient theoretical analysis and selected various types of pulsars for simulation verification. The results show that the method is also applicable to other navigation pulsars, laying a foundation for the practical application of the algorithm.
The reviewers of the experiment, published in the Astrophysical Journal (Supplement), believe that "the in-orbit demonstration carried out by the Insight satellite is an important contribution to the development of pulsar navigation."
Zheng believes that from the current experimental progress to practical application, there are still two key issues to be addressed:
The first is the miniaturization and lightweight of X-ray detectors. Zheng Shijie said that X-ray detectors must be carried on space detectors to achieve navigation services, and for space exploration missions, every additional load and every additional kilogram of weight require a large amount of space and energy for rockets or satellites. "Therefore, the development of miniaturized and lightweight X-ray detectors is the key to the large-scale application of pulsar navigation in the future."
In addition, in theory, further research and breakthroughs in the theory of pulsars are needed. Zheng Shijie introduced that the precision of the pulse signal provided by pulsars determines the final positioning error. At present, there are some noise signals in the time signals of all pulsars, including long-term "red noise" and short-term "white noise", resulting in the theoretical limit of the current positioning error of sub-kilometer level. "Only when we further study pulsars and understand the physical mechanism of these noises can we accurately predict these signals, thereby further reducing the positioning error and making pulsar navigation more in-depth applications."
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Could pulsar navigation be used in everyday life?
According to Zheng Shijie, pulsar navigation is mainly used to provide navigation services for spacecraft or probes exploring in deep space. In the vicinity of the earth, especially in low-Earth orbit, aircraft or satellites can use GPS, Beidou and other navigation means for positioning, and the accuracy is significantly higher than that of pulsar navigation. But in some special cases, such as when the navigation satellite or ground station fails or is destroyed and cannot provide navigation services, pulsar navigation can be used as an important backup means to provide services in emergency situations.
In addition, since X-rays cannot penetrate the earth’s atmosphere, the pulsar’s X-ray pulse signal cannot be received on the ground, making it impossible to directly use pulsars for navigation and positioning.
Pulsars will be "beacons" for exploring the universe
In 1967, Joselyn Bell, a graduate student in the Cavendish Laboratory at the University of Cambridge in the United Kingdom, noticed some strange and regular pulses while detecting signals from a radio telescope. Bell immediately reported the discovery to her mentor, Anthony Hewish.
The signal they found came from a distant star about 2,200 light-years away, and the discovery immediately aroused people’s interest: such a precise signal was so similar to the message sent to us by aliens that the newly discovered star was named "Little Green Man 1". In less than half a year, scientists found several such pulses one after another, thus ruling out the possibility of aliens.
In February 1968, Bell and Hewish reported in the British journal Nature the discovery of a new type of celestial object, a pulsar.
Pulsars are a type of neutron star, a dense object left behind by a "supernova explosion" of a massive star late in its life. Neutron stars that emit pulses at uniform time intervals are called pulsars.
The pulsar rotates at a high speed like a flywheel. During the rotation, its magnetic field forms a strong radio wave that radiates to the outside world, which is like a periodic pulse signal to the observer.
One of the characteristics of pulsars is that the long-term time stability of their pulse signal is very high, comparable to or even better than the atomic clock on Earth, which can be used as a time reference in the universe. Therefore, pulsars are also called "cosmic lighthouses" or "natural GPS satellites" in interstellar travel.
Just as satellite signals are used for navigation on the ground, spacecraft can also achieve autonomous navigation by observing pulsars, that is, pulsar navigation. The principle can be understood as follows: although the time interval (or pulse period) between two adjacent pulses emitted by the pulsar is constant, if the spacecraft moves towards the pulsar, the received pulse interval will be shortened, otherwise, it will become longer, and the observed pulse profile will change accordingly; the precise time for the pulse to reach the X-ray detector is determined by the distance of the detector relative to the pulsar, that is, the position of the spacecraft in space. Therefore, by analyzing the characteristics of the pulsar pulse signal received by the spacecraft (in different directions), the three-dimensional position and velocity (or motion orbit) of the spacecraft in space can be inferred. ( Reporter, Wang Shikun)