In 1936, Albert Einstein published what he described as a “little calculation”, showing how the Sun could one day be used as a giant telescope. As incredible as it sounds, the concept is not that far out of our reach, and one idea to make it a practical reality is in Phase III of NASA’s Advanced Concepts Institute.
Some time ago RW Mandl visited me and asked me to publish the results of a small calculation I had made at his request, Einstein wrote in Science magazine. “This message is in accordance with his wish.”
As implied by Einstein’s general theory of relativity, giant objects in space bend space-time, altering the path of light. This is not some abstract idea, but something we can do quite regularly using telescopes like JWST, essentially extending the reach of the telescope by looking at light that has been bent by massive objects on its way to Earth.
Einstein realized – although he clearly calculated it only under pressure from Mandel – that this meant that there was a region of our solar system where the light behind the sun was focused, bent by the gravity of our star.
How gravitational lensing works.
Image credit: NASA, ESA and Goddard Space Flight Center/K. Jackson
The area where this effect takes place is about 550 astronomical units (AU) from the Sun, where one AU is the distance between the Earth and the Sun. Place a telescope in that region and we could use it to observe the surfaces of exoplanets, without having to design the mind-bogglingly huge space telescopes (or arrays of telescopes) that would otherwise be required.
“The gravitational field of the Sun acts as a spherical lens to increase the intensity of radiation from a distant source along a semi-infinite focal line,” Von Russel Eshleman, who first proposed a mission to build such a telescope, wrote in the article. . “A spacecraft anywhere along that line could in principle observe, eavesdrop, and communicate at interstellar distances, using equipment comparable in size and power to that now used for interplanetary distances. If coronal effects are ignored, the maximum magnification factor for coherent radiation it is inversely proportional to the wavelength, which is 100 million per 1 millimeter.”
Right now we can use gravitational lensing to see incredibly distant objects, but we’re limited by the location of those objects and the objects behind them. Using spacecraft, we could place our telescope on the opposite side of the Sun from the distant object we want to see, dramatically increasing our viewing distance. A Phase III project at NASA’s Advanced Concepts Institute has proposed that we can use this method to image the surface of exoplanets in our stellar neighborhood.
“Even in the presence of the solar corona, [signal-to-noise ratio] is high enough that in six months of integration time the image of an exoplanet can be reconstructed from ~25 km [15.5 mile]-surface resolution,” explains NASA, “enough to see surface features and signs of habitability.”
“Of course, there is no hope of directly observing this phenomenon,” Einstein added. “It is unlikely that we will ever get close enough to such a center line.”
While that’s still a huge distance—Voyager I has reached just over 160 AU since its launch in 1977—it seems a lot more achievable than it was when Einstein ruled out such a mission. The NASA project proposes using a “swarm architecture” of small satellites that use solar sails to bring them to the required position in less than 25 years.
Although there are still astronomical challenges ahead for such a mission (including significant distortions introduced by gravitational lensing and moving the spacecraft long distances to observe the object behind it of interest), it’s possible that we could make images of the actual surfaces of alien exoplanets in our lifetimes. Which is pretty cool, even if Einstein considered it a distracting chore to record and publish.