Using the sun as a gravitational lens would allow us to see exoplanets with incredible resolution

Using the sun as a gravitational lens would allow us to see exoplanets with incredible resolution

Have you ever seen dull arcs and rings in astronomical images taken by the Hubble Space Telescope and other observatories? These unusual properties are caused by a peculiarity in nature called gravitational lensing, which occurs when light from a distant object is distorted by a closer solid object along the same line of sight. This distortion effectively creates a giant lens that magnifies the backlight source, enabling astronomers to observe objects embedded in the lens-shaped arcs and rings that are otherwise too far away and too faint to see.

A group of scientists are working on plans to build a spacecraft that can apply this peculiarity by using our sun as a gravitational lens. Their goal is to see distant exoplanets orbiting other stars, and to image an Earth-like exoplanet, see it in exquisite detail, with a resolution even better than the well-known Apollo 8 Earthrise photo.

Slava Turyshev, a physicist at NASA’s Jet Propulsion Laboratory, has led research on this proposed concept, called the Solar Gravity Lens (SGL). The idea would be to send a spacecraft to the exact location where it can use the focus area of ​​our own sun to deflect light from an exoplanet and enlarge it into a giant image.

Hubble image of a bright red galaxy (LRG) that gravitationally distorts light from a much more distant blue galaxy, a technique called gravitational lensing.  Credit: ESA / Hubble & NASA
Hubble image of a bright red galaxy (LRG) that gravitationally distorts light from a much more distant blue galaxy, a technique called gravitational lensing. Credit: ESA / Hubble & NASA

“Using the solar gravitational lens is similar to using a conventional lens with a diameter equal to the sun, which is 1.4 million km,” Turyshev told me. “Physics is there, now it’s just a matter of finding out the technology.”

Turyshev and other scientists say they could potentially image an exoplanet of Earth size up to 100 light-years away, and detect surface properties as small as 10 kilometers in diameter.

This image taken by The Planetary Society’s spacecraft LightSail 2 on May 31, 2021 shows Afghanistan, Pakistan and the Arabian Sea. The Caspian Sea is down to the left. The shadows of the spacecraft’s solar panels can be seen on the sail. North is approximately at the top left. This image has been color-adjusted and some distortion from the camera’s 180-degree fisheye lens has been removed. Credit: The Planetary Society

Turyshev said that spaceship architecture is still being studied and discussed, but instead of using a single large spacecraft such as Voyager or Cassini, the SGL concept can rely on a fleet of smaller satellites that would use sunsets for propulsion. An exciting roadmap for SGL is to use a gravitational aid from the sun to reach speeds where it would take only 25 years to travel 60 billion miles to the destination’s destination in interstellar space.

“You do not need to fly systems like the size of Voyager or Galileo, etc, but can use smaller spacecraft,” said Turyshev. “There are alternative options such as mounting in space, so if you need to build a larger aperture. Or if we have smaller, more spacecraft, we can use different ways to take them to space, such as carpooling to get to low orbit around the earth. or geostationary orbits where we have our system self-assembled and then use solar sails for propulsion into space. ”

If the idea sounds great NASA Innovative Advanced Concepts (NIAC) Program – which funds research on visionary ideas that can change future assignments with groundbreaking concepts – believes that the SGL concept is healthy. In 2020, the Solar Gravity Lens project was the sole recipient of NIAC Phase III funding worth $ 2 million, and is one of the very few ideas funded for all three phases of NIAC support. Phases I and II enabled the team to demonstrate the feasibility of basic concepts and then invent a new mission architecture, and Phase III has led further research and development into how the telescope would work and perform.

In a newspaper published in April 2022, Turyshev and colleague Viktor Toth show that an exoplanet imaging mission is challenging but feasible, using technology that is either already available or in active development.

“That paper represents the realistic sensitivity that one can get with SGL,” Turyshev said via email. “It underscores the fact that under realistic conditions, megapixel imaging of Earth-like exoplanets in our galactic neighborhood requires only weeks or months of integration time, not years as previously thought.”

The concept of observing an exo-earth at 30 pc via SGL, using a small 40-cm telescope in the middle or thermal infrared part of the spectrum in conjunction with an external star screen / sunshade to block the sun. Left: the source image with a resolution of 128 × 128 pixels; Center: the blurred image projected by SGL in an image plane. Right: image after deconvolution. This concept could deliver high quality spatially or spectrally resolved images over a wide range of wavelengths. Credit: Turyshev et al.

The team’s latest paper was published in June 2022 discusses the possibility of reusing the sun sails once they have performed their propulsion tasks and using them as star screens.

“One of the more significant challenges of using the solar gravitational lens to image distant, faint targets is the fact that all light from such targets appears in the form of an Einstein ring on top of the bright solar corona,” Turyshev and Toth wrote in their paper. “For a target like an Earth-like exoplanet, despite the significant light amplification of SGL, the solar corona remains brighter than the target’s Einstein ring with several orders of magnitude.”

The star screen would work better than an internal corona graph to block this light. In addition, it would allow a telescope with a smaller aperture to be used. The paper says that quite remarkable is that even a 40-centimeter (15.7-inch) aperture telescope is enough to restore a resolved image of good quality of an exo-earth as far as 30 parsecs from the earth.

A list of some of the recently discovered potentially habitable exoplanets. Credit: High Performance Computing Facility at the University of Puerto Rico (hpcf.upr.edu).

“We see that in connection with the outer star screen, you can use quite small openings and increase the observation bandwidth to medium infrared,” Turyshev explained. “Medium-infrared observations are particularly interesting because planets are luminescent at these wavelengths and produce a strong signal, while there is significantly less noise from the solar corona. This part of the spectrum contains many functions of interest for exobiology and biosignature detection.”

Turyshev called this development “very significant”, and it provides strong motivation to study the use of navigable star screens – and especially solar sails that are reused as star screens – as essential components in future SGL missions.

Among space enthusiasts, there has always been much discussion about humanity’s first steps outside the solar system. The SGL concept can provide the point for several technology efforts to really come together to make it possible.

“I think we are at the beginning of an exciting period where several efforts are being gathered in the space industry,” he said, “such as more economical access to space, small resilient spacecraft and distributed opportunities where costs are now practical to bring with them. Exciting science. So in the lifetime of the current generation, we have the chance to really ask and perhaps answer the question we have been asking for at least 5,000 years, “are we alone.” We are at the point where technologies exist, and the challenge is how to use that technology, accelerate development, motivate the steps to test this technology and then really use it. “

Caption for lead: Artist concept of countless exoplanets. Credit: NASA

Further reading: NASA / NIAC Article on the Solar Gravitational Lens Project
Planetary Society article on SGL

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