Prospects for interplanetary time synchronization and what will happen after GPS

In this illustrations from the website of the Jet Propulsion Laboratory (JPL) it is explained how the work of satellites is coordinated using atomic clocks for deep space (Deep Space Atomic Clock). Designed by NASA under the direction of Eric Burt, this watch is described in article from Nature in June 2021. In 2019, such watches were launched into space and showed absolutely stable operation for 23 days. According to Burt and his colleagues, such a clock should be installed on an orbital ship that would promptly coordinate the landing of descent vehicles on other planets. Burt’s watch is insensitive to cosmic radiation and temperature changes, and when landing robotic or manned expeditions, it is precisely the “local” GPS that would make it possible to cope with difficult atmospheric conditions and not miss the chosen landing site. An improved model of such an atomic clock is currently being developed (Deep Space Atomic Clock 2). This clock is planned to be installed on the ship VERITAS, which will go to Venus, make a complete topographic map of the planet and land. Burt believes that in the future it will be necessary to check how long and stably the atomic clocks will work after landing, since it is precisely this method of “seeding” new planets with atomic clocks in the future that would allow them to be used as beacons when landing on the Moon or planets and during the arrangement colonies.

Navigation and accurate time on a galactic scale

It is possible to equip GPS according to the above principle within the framework of a planetary system, but when traveling from one star system to another or when crossing the Galaxy, such signals will not be enough – the distances are too great. As sources of accurate time on a ship or in a new colony, natural sources will have to be used, moreover, those that could be (in advance) recorded from the Earth. As far as it seems today, it is most convenient to use pulsars in this capacity. A pulsar is a rapidly rotating neutron star that steadily emits radio pulses with very precise equal intervals between them. The first pulsar, now known as PSR B1919+21, opened in 1967 Jocelyn Bell (b. 1943), working under the scientific supervision of Anthony Hewish. Hewish was so amazed at the periodicity of the pulses that he seriously suspected that this object was a radio beacon of an alien civilization, and suggested that this source be called “LGM-1”, where “LGM” = Little Green Men, “little green men”. In fact, pulsars are completely natural in origin and are the remnants of supernovae – rotating highly magnetized objects that spread radio pulses around them just like a modern rotating beacon lantern spreads light.

There are pulsars that “blink” at intervals of a few seconds, but there are also much more “accurate” ones. In 1974, J.S. Downes of the Jet Propulsion Laboratory first suggested that pulsars could be used for timekeeping, but at that time the version remained on paper, since the nature of pulsars was still unclear, and radio telescopes were so huge that it was possible to install a similar device on spaceship seemed impossible.

Gradually, pulsars were discovered with ever shorter periods. So, in 1982, a pulsar with a period of 20 milliseconds was discovered, and in 1983 it was discovered that with the same exact frequency, pulsars emit not only in the radio range, but also in the X-ray spectrum (therefore, their signal can penetrate through dense gas and dust clouds) . Such pulsars surpass in accuracy even atomic clocks. Currently, as “beacons”, well tracked from the Earth, considered pulsars J0437-4715, J1824-2452A, J1939+2134 and J2124-3358. The clock for an interplanetary expedition could be calibrated by a pulsar directly from the Earth, and then such a clock could be installed on the ship, as well as a device for simultaneous orientation by radio and X-ray pulses. Such devices already exist today and are called XNAV receivers. To navigate using pulsar signals, one cannot do without corrections for distortions caused by stellar plasma ejections and gravitational waves, but technically and mathematically, pulsar navigation is no more complicated than GPS. The first device for navigating pulsars, NICER/SEXTANT, was designed by NASA and delivered to the ISS in 2017. The tests went well. It is assumed that after refinement, this technology will bring the accuracy of interstellar positioning to 5 km.

Conclusion

While working on this article, I abbreviated it with all my might and could not tell only about time without talking about space. The described technologies make it possible to make sure that even at not too large cosmic distances, time and space are inseparable. The article intentionally bypassed the topic of the Martian Starlink, as it is still too speculative, and also did not touch on the topic of a potentially abrupt and uncontrolled change in time when falling into a deep gravity well. I hope to return to the last topic one day.