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Smallsats + solar sails = Photos of exoplanets at 1970s digital camera resolution

Sundiver spacecraft could snap the first surface pics using solar lens

It's time to stop hemming and hawing about the monetary and temporal costs of exploring the outer solar system and beyond, say an international group of boffins. We've all the materials we need to do it faster and cheaper by combining modern smallsats with solar sails, and the end result could be actual photographs of exoplanets.

Authored by a group led by NASA Jet Propulsion Laboratory physicist Slava Turyshev, the team of 27 researchers say that current solar sail technology, combined with the increasingly popular smallsat form factor (classified by NASA as being under 180 kg) would be enough to build a craft able to reach speeds of up to 33 kilometers per second, or around seven astronomical units (the rough distance between the Sun and Earth, around 150m kilometers) per year.

At that speed, a solar-sailing smallsat could reach Jupiter in a year, and Saturn's moons in less than three. Seven AU/yr is fast - around double the speed of the Voyager probes.

"Velocities larger than seven AU/yr will require novel sail materials and technologies," the team wrote, but that doesn't mean we'll be waiting forever to get considerably faster. "With sail materials already being developed, smallsat velocities up to 20-25 AU/yr will be achievable in 5-7 years," the researchers added.

All of this, mind you, is without any propellant, greatly reducing weight and volume of potential spacecraft. Solar sails are highly reflective, and accelerate by reflecting photonic energy from the Sun. It's a small force but it adds up over time, although the further you are from the Sun the more acceleration drops off.

Solar sailing spacecraft may seem like science fiction, but a pair of experiments over the past few years have proved their efficacy. LightSail 2, a cubesat deployed in 2019, used a 32 square meter sail to increase its speed and gain altitude over the course of a couple of years in orbit, while the Japan Space Agency's IKAROS used a 14x14 meter sail to power its flight past Venus in 2010.

With those projects as reference, the researcher say they've developed potential craft that could be built for between $30-$75 million (£24-£60m), as opposed to the $2-$5 billion (£1.6-£4b) cost "of a typical flagship-type deep space mission." 

Because the craft themselves are quite small - NASA describes smallsats as being, at their largest, around the size of a refrigerator - they can piggyback on crafts like SpaceX's Falcon Heavy, as was the case with LightSail 2, further reducing costs.

In order to achieve their record-breaking speeds, solar-sailing smallsats would need to get as close to the sun as their materials allow in order to get on a high-energy hyperbolic trajectory that would allow them to capture as much energy as possible. 

All in the name of exoplanet photography

The team behind the paper proposes a new class of solar sail craft it calls "Sundiver" that's designed with the ultimate goal of fulfilling NASA's desire for a spacecraft that can reach a location in the deep recesses of the solar system known as the solar gravitational lens (SGL) focal region. Doing this in less than 25 years, would require speeds greater than 20 AU/year. 

The SGL focal region begins at a whopping 547 AUs from the Sun (dwarf planet Pluto, for comparison, is around 30-49 AUs from Sol), and is the point in space where Albert Einstein once predicted an observer could use our star's gravitational field to magnify distant objects. 

Exoplanets, which we've been able to confirm the existence of but not directly observe, could theoretically be photographed in relative detail, as the SGL offers intense brightness amplification and angular resolution. 

Even in the presence of the solar corona, NASA said that six months of integration time at the strong interference region of the SGL (beyond 547.6 AUs) would allow researchers to "reconstruct the exoplanet image with ~25 km-scale surface resolution, enough to see surface features and signs of habitability."

By contrast, a traditional optical instrument would need an aperture some 90.5 kilometers wide "to capture even a single-pixel image of an Earth-like exoplanet," according to the boffins.

Another frequently used space photography tool, the interferometer (which merges multiple light sources to create a single image), would also be unsuitable for the task: thousands of 30m telescopes would require around 105 years to get a signal-to-noise ratio greater than 1 when trying to snap an image of a planet orbiting another star, they predict. 

The proposed solar sail smallsat craft sitting at the SGL focal point, however, "can yield a 250 x 250-pixel image … in just 12 months," the researchers said. 


Concept art of the potential resolution of an exoplanet photographed at the SGL focal point - Click to enlarge. Credits: Slava Turyshev.

"Missions could also target the outer solar system quickly, including flights to potentially life-supporting bodies, such as Enceladus, and beyond, to the neglected ice giants, Uranus and Neptune," the team said. 

"None of these very small and special purpose missions would replace the desire and planning for the larger, more expensive flagship class missions for solar system science. But they would provide us with initial reconnaissance data while we wait for the orbiters and landers." 

Of course, that's for local science - no one is going to land a craft on an exoplanet in our lifetimes. Photographing one, on the other hand, might actually be possible. ®

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