NASA’s Jet Propulsion Laboratory and American Airlines have jointly developed a method for detecting exoplanets using the Solar Gravitational Lens (SGL). This approach has attracted a lot of interest, so is it really feasible? Let’s take a look at it – some time ago, the news of NASA’s plan to use the sun as a gravitational lens to image exoplanets was in fact already in place in 2017.
Some people think this is a good novel idea, and some colleagues who study orbital dynamics question the unrealistic nature of such observations. But the news reminds me of an equally novel report I heard many years ago.
General relativity points out that light is biased in the gravitational field. Thus, an observational test of general relativity is the deflection of starlight as it passes near the sun. Until now, astronomers have been measuring the sun’s deflection of electromagnetic waves, instead of observing the position of stars, but by interfering with very long baselines.
Light is a common phenomenon in astronomy, including the powerful gravitational lens created by well-known galaxies or clusters of galaxies, and microgravity lenses produced by planets (or some unknown object). The sun can naturally also act as a gravitational lens, and the news of the heat of the previous days was NASA’s plan to use the sun as a gravitational lens to image exoplanets.
Figure 1. Strong gravitational lens. (Photo: ALMA)
Imaging with the sun as a gravitational lens may seem like a new idea, but i learned about it on October 24, 2011, when I listened to Dr. Claudio Maccone of seti (Search for Alien Civilizations). Dr Maccone’s report at the time was entitled “FOCAL space mission to 550 AU and beyond”, which attracted me more than the value of 550 AU. Turyshev et al.’s literature is 548 AU. The two values are close, how does this come about?
It can be calculated simply that the angle of the deflection of light in the gravitational field with a mass of M is r object is proportional to the mass M of the celestial body, and the radius r of the celestial body is inversely proportional. On the other hand, this deflection angle can be approximated to the ratio of the radius r and focal length d of the celestial body. Therefore, for a given object, the focal length obtained by it is proportional to the square of the radius r of the celestial body, and the mass of the celestial body M is inversely proportional to the celestial mass M. Through careful reflection and calculation, you will find an interesting fact that if you use objects in the solar system as gravitational lensing, the sun’s focal length is the smallest, is about 550 AU!
Figure 2. The orbit of a probe satellite that acts the sun as a gravitational lens. That is, if you want to put a probe satellite and use the sun as a lens to detect distant objects, then the orbit radius of the satellite should be 550 AU, which is the location shown in the figure. (Photo: NASA)
Turyshev et al.’s idea was to place a satellite about 550 AU from the sun to image exoplanets. Dr Macccne’s earlier idea was to place an antenna about 550 AU from the sun to receive signals from celestial bodies, or to send signals to use the sun as an amplifier. If we’re going to launch a spaceship to the nearest star to the sun, this may be a rare way to keep the ship in touch with us. In an orbit with a radius of 550 AU, with a period of about 12,000 years, it is possible for a satellite or antenna in this orbit to move 10 angular seconds a year, aiming in one direction within a year.
Nearly a decade on, I’m still impressed by Dr. Maccone’s report. The report sounded like listening to science fiction, but Dr. Maccone told us that these ideas should be written down, submitted to certain institutions, approved, and taken care of him, and that sooner or later they would come true. If you don’t write them down, these ideas can’t be preserved.
Now it seems that the idea has been preserved and developed.
A. Einstein 1936, Science, 84, 506
S. S. Shapiro, et al. 2004, Physical Review Letters, 92, 121101
S. G. Turyshev, M. Shao, V. T. Toth 2019, IJMPD, 28, 1950125
About the author
Qian Lei: Associate Researcher of the National Observatory. He received his Ph.D. in astronomy from Peking University in 2009. From 2009 to the present, work has been carried out in the FAST project of the National Observatory. Currently responsible for FAST spectral line data processing. He has translated the monograph “Black Hole Accor” and several popular science articles.