NASA wants to use the Sun as a giant telescope to observe the surface of alien planets

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 of ​​how to achieve it in practice is in Phase III at NASA’s Advanced Concepts Institute.

“Some time ago RW Mandl visited me and asked me to publish the results of a small calculation that I had done at his request,” Einstein wrote in the journal Science. “This note meets his wish.”

As implied by Einstein’s theory of general relativity, giant objects in the universe bend spacetime, altering the path of light. This is not some abstract idea, but something that we can do quite regularly using telescopes like JWST, essentially extending the reach of the telescope by observing light that has been bent by massive objects on its way to Earth.

Einstein realized – though he apparently only calculated it under pressure from Mandl – that this meant that there was a region of our solar system where light from behind the Sun was focused, having been bent by the gravity of our star.

How gravitational lensing works.

Image credit: NASA, ESA and Goddard Space Flight Center/K. Jackson

The region 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. Put a telescope in this region and we could use it to see the surfaces of exoplanets without the need to design the mind-bogglingly huge space telescopes (or telescope arrays) that would otherwise be required.

“The Sun’s gravitational field acts as a spherical lens to increase the intensity of radiation from a distant source along a semi-infinite focal line,” wrote von Russell Eschlemann, who first proposed a mission to create such a telescope, in an article. “A spacecraft anywhere along this 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 we ignore coronal effects, the maximum magnification factor for coherent radiation 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 that lie behind them. Using a 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. It was proposed in a Phase III project at NASA’s Advanced Concepts Institute that using this method we could image the surface of exoplanets in our stellar neighbor.

“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 with ~ 25 km can be reconstructed [15.5 mile]-scale 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 much more attainable than it was when Einstein ruled out such a mission. The NASA project proposes the use of a “swarm architecture” of small satellites using solar sails to propel them to the required position in less than 25 years.

Although there are still astronomical challenges for such a mission (including significant distortion introduced by gravitational lensing and moving a spacecraft huge distances to observe the object behind it of interest), it is possible to construct images of the actual surfaces of alien exoplanets in our lifetime. Which is pretty cool, even if Einstein thought it was a distraction to write down and post.

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