A space telescope placed far beyond the orbit of the dwarf planet Pluto could one day deliver high-resolution images of distant exoplanets, according to NASA physicist Slava Turyshev and colleagues. The secret to the telescope’s extraordinary power is the gravitational field of the Sun.
This artist’s concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Image credit: NASA / Ames / SETI Institute / JPL-Caltech.
In just the right spot, a space telescope would see the image of an exoplanet amplified by orders of magnitude.
The Sun would bend the light from the planet around it, focusing it at a point on the opposite side and, in effect, magnifying the planet into a gigantic image.
“It’s our next step, to go beyond the Solar System. And so that’s where we need to go,” said Dr. Turyshev, who recently took a big step of his own.
Dr. Turyshev was the first to publish the intricate quantum mechanical equations that capture the behavior of light at the gravitational lens focal point, beginning some 50 billion miles (80 billion km) away from Earth. His equations show that a close-up image of, say, an Earth twin 100 light-years away could be recovered by a suitable space telescope, at least in principle.
“That was a roadblock. The equations were previously unsolved. But now everything has checked out well. We have a good understanding, and confidence in the solution,” Dr. Turyshev said.
While he works out the optical details of the solar lens, other NASA specialists concentrate on the engineering.
NASA engineer Nitin Arora is part of a team developing a suite of conceptual plans for robotic interstellar voyages — including what it would take to get a space telescope to the gravitational lensing point at the icy outer reaches of the Solar System.
“You could see geological features on that planet. I’m pretty sure you could see lakes and oceans, mountain ranges,” Dr. Arora said.
Sending a telescope so far into deep space would require precision technology yet to be invented. To find such a tiny pinprick of a target, the telescope’s pointing accuracy will have to be at least 100 times that of today’s instruments.
And the target exoplanet won’t be sitting still, it will be orbiting its star. To cope with the motion of the planet at such extreme close-up range, smearing the planet into a blur, advanced image processing could be used. But another possibility involves tracking the exoplanet by moving the telescope itself in a kind of corkscrew pattern. This would call for delicate thruster control that would have to be invented, too.
“How to fly there, how long it will take, how to point your communications: it’s all challenging,” Dr. Turyshev said.
A space telescope that could image distant exoplanets would need to be placed outside our Solar System, a distance farther than NASA’s Voyager 1 has traveled in 40 years. Image credit: NASA.
The list of potential telescopic troubles is a long one.
The minimum distance for a solar lensing telescope is 547 astronomical units (AU). And in reality, proper positioning would likely require the telescope to be placed even farther out — perhaps as far as 2,000 AU or more.
The Kuiper Belt, which includes Pluto, extends to about 55 AU. The Oort cloud, the realm of dormant comets that are the most distant objects gravitationally bound to the Sun, forms a shell extending from 5,000 to 100,000 AU. The nearest star, Proxima Centauri, would require a journey of 271,000 AU.
“Just getting to the solar lensing position beyond the Kuiper Belt would likely take decades using today’s technology,” said NASA scientist Dr. Geoffrey Landis, who wrote a critique of solar gravitational lensing concepts.
“Think about NASA’s New Horizons spacecraft. It was the fastest probe ever sent into space [at launch]. It did not get to Pluto very quickly; it’s a very long way. The gravitational lens of the Sun [position] is more than 10 times as far away as Pluto,” he said.
The trip time could be cut substantially with exotic new propulsion systems, like ion thrusters, solar sails, or even laser-pushed sails.
“Reaching the solar gravitational lens within 25 years — instead of the 50-year time frame suggested by present-day technology — might even require a new form of nuclear propulsion,” Dr. Arora said.
“We might want to use a mix of two propulsion technologies, starting with a nuclear fission reactor or perhaps solar-thermal propulsion.”
“This could be combined with an ‘Oberth’ assist from the Sun — flying close to our star, dipping deeply into its gravitational field, then making a perihelion maneuver to greatly increase the spacecraft’s speed and fling it toward the outskirts of the Solar System.”
Once in place, the solar lensing telescope would not be confronted by an image of an entire exoplanet.
Instead, the planet’s light would be smeared into a ring around the Sun, an effect produced as the Sun’s gravity bends light from an object — in this case, the distant exoplanet — that is behind the Sun but is also in alignment with the Sun and the telescope. This smearing effect is known as an Einstein ring.
Within the Einstein ring, strips of the planet’s light could, at least in principle, be captured by the telescope.
And after the great investment in designing, building and launching such a telescope, and perhaps decades of transit time to the right spot in the dark, frozen reaches of the Solar System, it turns out that the telescope’s range would be extremely limited. It could only observe a single target.
The problem is that to turn to a different exoplanet, even one separated from the first target by a single degree, the telescope would have to move some 10 AU.
To justify such a complex but limited telescope, a worthwhile target would have to be identified in advance: an Earth-like exoplanet where signs of life already have been detected by other instruments.
“It is very fulfilling and exciting to be involved in something that can shape humanity’s future, and change life as we know it,” Dr. Arora said.
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Slava G. Turyshev. 2017. Wave-theoretical description of the solar gravitational lens. Phys. Rev. D 95 (8); doi: 10.1103/PhysRevD.95.084041
Slava G. Turyshev & Viktor T. Toth. 2017. Diffraction of electromagnetic waves in the gravitational field of the Sun. Phys. Rev. D 96 (2); doi: 10.1103/PhysRevD.96.024008
Geoffrey A. Landis. 2016. Mission to the Gravitational Focus of the Sun: A Critical Analysis. arXiv: 1604.06351
This article is based on text provided by the National Aeronautics and Space Administration.
