I titled this topic thusly to avoid the connotation of "solar" as it is often used, speaking of the Sun's radiation as an asset.
This mission concept, which is almost shockingly radical, is on firm scientific grounds, but would deliver a tremendous capability at the cost of some considerable engineering resources.
The concept is: At a distance of >550 AU, the Sun could be used as a gravitational lens to magnify a given target located on the other side of the Sun. The magnification that could be achieved is in principle (here's a word you don't encounter often in the engineering realm) infinite. A recent study by Landis delves into the issues far better than I can here:
In a nutshell, a potential mission would fly a telescope out to >550 AU (four times Voyager 1's current distance) and would look back towards the Sun, which would be hidden behind an occulting screen, so that a target antipodal to the Sun would be magnified far beyond what any existing telescope can achieve. The image of, say, an exoplanet located tens of light years away would actually be too large (~10 km at the focal plane) for a telescope to collect the whole image at once. It would, in effect, see one "pixel" at a time, and to image an entire exoplanet, it would have to scan back and forth across the image, either actively or passively, either scanning one strip across the exoplanet or making some effort to gather a 2D image.
Because of the orbital dynamics, the craft would be essentially an interstellar craft rather than sun-orbiting as we normally think of it. It could not effectively observe multiple different targets because it would have to travel considerable distances to aim at a new target, although observing multiple planets in the same exoplanet system seems to be achievable. If we wanted to observe five different exoplanet systems, we would have to launch five different copies of this mission.
The Trappist-1 system introduces a case where a mission like this might have a respectable ROI. One telescope launched to Trappist-1's focal location from the Sun would perhaps be able to scan several or all of the planets in the system, either in 1D or 2D mode, perhaps repeatedly – the details depend upon the resources for propulsion to make the scanning work. The focal distance for Trappist-1, however, would be significantly farther than 550 AU. Landis' paper suggests that something more like 2500 AU would be required. That is very far, but not nearly as far as the stars are. It seems like the most painful requirement would be to get a telescope with a good amount of propulsive capability out to that distance before the mission planners die of old age.
The requirements are fantastic, but it seems like a good possibility that this would be cheaper than the seeming alternatives.
I'm not sure that I've ever read about a mission concept so radically different than anything I'd read before. It seems feasible – just hard.
The Landis article points out several problems with this idea (which goes back many decades). One is pointing: you have to know the position of the planet you're trying to image extremely accurately so you know where to send the telescope. For his example, the image of an Earth-sized planet would be only 12.5 km across. So if your estimate of its absolute position is out by only 0.1 AU, you'd miss the image by 15 000 km.
There will also be noise from the Sun's corona and extrasolar planet's star. And especially interesting is his claim that the FWHM blur of the image will be half the planet's diameter, so actually resolving the planet will be impossible without some kind of deconvolution. I don't know if anyone has proposed something like that...
Anyway, it is a lot of fun to think about...
I would imagine that one approach to this would be to send an array of small telescopes flying in formation at a fair distance from one another. That formation could scan the target area without a need to pinpoint it. Once a large bus underwent the massive maneuvers necessary to get to the right vicinity, small sub-cruises by small telescopes out there in deep space would seem fairly simple, but I'm not sure how large the telescopes would need to be to make a minimal observation.
The Sun is the only game in town for this strategy. Jupiter and other solar system bodies would require excessive distances to be achieved. Of course, non-solar system bodies create myriad other opportunities such as already exist. The problem there is that the alignments occur by chance and are unpredictable and unrepeatable. But perhaps a formation approach in solar orbit could try to make use of this meaningfully, scanning distant exoplanets using some other intermediate body as the gravitational lens. The opportunities surely exist, but the targets wouldn't be of our choosing.
Lensing by star- or planet-sized objects actually forms the basis of https://en.wikipedia.org/wiki/Gravitational_microlensing, which has been used to put strong constraints on models of dark matter, for example. In that case you're getting information about the distribution of the lenses, not about the sources.
In a cosmological context things are very different, since timescales are very long and we're seeing essentially a snapshot. There are many strong galaxy lenses known, including https://en.wikipedia.org/wiki/Einstein_ring, which are useful for various reasons, such as measuring the expansion rate of the Universe.
There isn't really a focal point as such. There is a distance at which the Einstein ring image just grazes the surface of the lensing object. Beyond that distance the ring appears larger and well clear of the lensing object. This makes very compact and dense objects like white dwarfs and neutron stars particularly effective because they present so little obstruction to viewing the Einstein ring.
How much energy can actually be focused on a single point by a gravitational lens is something I have not been able to discover or work out so far. I wonder, for example, if there is a (roving) point within the Sirius system where the light of A is concentrated strongly enough by the gravity of B to fry any passing object.
ADMIN NOTE: It's all been fun, but please review rule 1.9 re sci-fi engineering...let's keep our collective feet on the ground, so to speak.
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