black hole with gravitational lens effect in front of bright stars. 3d illustration, Elements of this image are furnished by NASA
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Astronomers in the U.S. and Europe are actively working on a revolutionary new optical telescope concept that would harness our Sun’s own massive gravity to view distant celestial targets.
Perched on the very outer fringes of our solar system, a Solar Gravitational Lens (SGL) telescope, sometimes dubbed a Curved Space Telescope, would use the gravity of our own Sun to obtain extraordinary optical images at a focal point some 650 Earth-Sun distances (astronomical units) away. From this vantage point, such a telescope (or telescopes) could, in theory, obtain highly precise images of exoplanets, galaxies and supermassive black holes.
The crux of the technology rests on Albert Einstein’s theory of general relativity in which massive objects such as stars and galaxies gravitationally-bend space and time. For decades, astronomers have been using Einstein’s discovery to gravitationally lens galaxies and even planets circling other stars.
But now astronomers want to take gravitational lensing to the next level.
Longtime astrophysicist Slava Turyshev at NASA’s Jet Propulsion Laboratory and his team did a 2020 comprehensive study of what they term an SGL telescope as part of NASA’s Innovative Advanced Concepts program. Meanwhile, a European group led by the University of Pisa in Italy calls their project a Curved Space Telescope.
Our SGL concept exploits the natural curvature of spacetime around the Sun to yield a light amplification factor on the order of 100 billion, Turyshev tells me via email. This makes it the only known method capable of directly imaging surface features of an exoplanet, he says.
The Need To Scan
The most likely first use of this technology is to target an exoplanet that has already been deemed suitable for life.
Leading exoplanet researcher Sara Seager, a planetary scientist at MIT, and her team is very interested in using this technology.
Our main interest is using the solar gravitational lens to directly image earth-like exoplanets located tens of light years away, Seager tells me via email.
Astronomers using the technology would need to use at least one telescope per target exoplanet to ensure highly precise observational alignments.
The resolution would be so fine that individual pixels could correspond to areas only a few tens of km across the planet’s surface, allowing us to distinguish features such as continents, oceans, and even global cloud patterns, says Seager.
An Einstein Ring
Such telescopes would collect light from so-called Einstein Rings, which as the Harvard Smithsonian Center for Astrophysics notes can be created when light from a distant object can be so distorted that it creates a complete ring. The distortion maps the distribution of matter creating the warp and brightens the light source to make otherwise distant objects visible, the center notes.
And if we move the telescope to a greater distance, the separation between the Einstein Ring and the solar disk increases, says Turyshev.
When the spacecraft reaches 650 AU, this separation becomes large enough to make it possible to block light from the Sun using an optical coronagraph. That is, an optical device that would effectively block out the Sun’s light.
For an earth-like planet 100 light years away, the Solar Gravitational Lens (SGL) would project an image that is roughly 1.3km across, says Turyshev.
Pixel By Pixel
Moving the spacecraft within the 1.3km image that the SGL projects and measuring the changing brightness of the Einstein Ring amounts to “scanning” the image, one pixel at a time, says Turyshev. Our calculations show that even for an earth-like planet at this great distance, it is possible to scan as many as 100 x 100 pixels or more over the course of a year, he says.
But such telescopes will be difficult to point.
Once the spacecraft has arrived at the desired region it can move around the focal plane to point to specific locations of the “projected image,” but it would be too late to point to a totally different target, Mario Palos, a research fellow in space technology at the University of Tartu in Estonia, tells me via email.
If initially powered by an e-sail, the solar wind’s electrically charged particles are deflected by the tethers of the e-sail, producing a very small, but continuous and propellant-free, thrust.
At the beginning of the mission there will be plenty of solar power, both for electrical power generation and for the electric solar wind sail (e-sail) to function, Palos tells me. But with such a far-away target, the available solar power will inevitably go down to unusable levels, he says.
The Need For Radioisotope Generators
So, there will be a point in the mission in which the sails are jettisoned, perhaps even the solar panels as well, says Palos. From then on, the power needs of the spacecraft will need to be fulfilled with some variant of a radioisotope generator, he says.
An Incredibly Long Journey
With solar sails, it is possible to achieve speeds greater than 20 AU per year and thus get to the telescope’s desired focal region in some 25 years, says Turyshev.
The Pisa-led team, however, is much more conservative about the timeframe needed. They note that a spacecraft with a mass of up to 800kg could take up to 70 years to reach 650 AU. That’s a point roughly halfway between our solar system’s Kuiper Belt and the Oort Cloud of comets, which lies at something like 2000 AU.
Any such spacecraft must operate with a high degree of autonomy since data beamed back to Earth would take some 80 hours to arrive.
Yet such missions won’t be cheap.
Cost estimates range from a few billion dollars to as much as twenty times that amount. But we are now fully in an era of unbridled innovation in materials science and aerospace propulsion, largely enabled by sheer computer power and AI.
Sending robotic missions out to destinations four times more distant than NASA’s Voyager spacecraft might have seemed impossible only a decade ago. But today’s current innovation paradigm is rapidly enabling such wildly ambitious observational astronomy ideas to suddenly take root.
A pathfinder mission consisting of one spacecraft headed to the outer solar system could cost up to $1.2 billion while Turyshev says a full SGL imaging campaign to map an exoplanet could cost as much as $5 billion.
The Bottom Line?
From initial launch to “first light” for primary science observations, the timeframe extends to approximately mid-century, says Turyshev. If all goes as planned, we could see the launch of a such a telescope to 650 AU by the mid-2030s, he says.
