Everyone kinda knows about gravitational lensing - one of relativity's greatest predictions and something which makes it very very hard to deny Einstein's geometric intuition of spacetime.

Turns out, Einstein figured if the sun was lensing light, why not use it as a telescope?

However, the sun is a very low mass star so it doesn't bend light *that* much. In fact, to get to the point of maximum light collection - the "focus" - we need to travel to 550 AU where the Earth Sun distance is just 1 AU.

Voyager I, the farthest sent man made object ever, is

only 25% of its way there after 40 years! It is so far away that light takes ~3.5 days to make the trip one-way.

But apart from exoplanet exploration, this is the greatest deep space exploration project we can currently conceive with a solid goal. This is truly deep space though

Our current best technological fantasy (as in we don't even have a working prototype) for fast space travel - solar sails - need much more advancement before the simple one way journey estimate can be brought to ~25 years.

But solar sails have a problem. They use the sun's light

to propel and since the sun is a constant source of energy, you don't need to carry fuel etc. However, to use mere photons for acceleration, you have to be terribly light. To achieve this journey in the ~25 year timeframe, you need ~1g/m2 density, which is incredibly light.

Our current tech is near 143g/m2. Yeah...

Secondly, you need to stop near 550 AU to actually image the planet / star you're observing. So how do you slow down? Turns out that part is simpler - just have a second sail that absorbs reflections from the first sail in the opposite

direction. I have no idea how that'd work, but I'll assume someone did the calculations.

Now the technological problems.

1) when you're 550 AU from the sun, the image of any given planet or star nearby is going to be moving at a good clip with its position relative to the

sun. So if proxima centauri happens to be on the other side of the sun from your focal point, it may move by fractions of a nanodegree if you were watching from earth, but at the focal point, that image is moving at like 50m/s.

To visualize, imagine you had a straight line going

from your focal point into the sun and through proxima centauri. Now the imaging device is stationary wrt the sun but needs to compensate for proxima centauri's movement to keep it in a straight line.

How would you achieve that? Well it is relatively easy to give an orbital

velocity to an object that simply doesn't decay. So if you launch from earth, you already have a 30m/s velocity around the sun. Take a trip around Saturn or something and add a tiny bit more and most likely nothing else in your trajectory can slow you down.

Of course, you see

the problem with this - what if you're done observing proxima and wanted to observe something else? No dice. The sun is directly in front of the sail so there is no way to add or subtract orbital (or even altitudal) velocity.

2) How do you communicate with the device over such

large distances? We don't know. The voyager has an antenna pointed at earth and we can still kinda communicate with it. But it is only 1/4th of the distance we want to achieve and it is not sending us images or anything high fidelity.

You can improve communications in two ways

a) increase power, size of antenna etc. this requires equipment on board our craft, a craft whose density we need to *really* minimize as much as possible.

b) increase the frequency range for higher throughput. But higher frequencies attenuate quicker so that is also problematic

Of course, modern receiver sensitivities and processing techniques are akin to magic compared to the 1970s, but it's still a big challenge to actually get the signal here even if we deploy multiple clusters to do the actual de-noising.

We can also use the chance to deploy the

first of many deep space communication infras - maybe a series of sun-stationary deep space antennas powered by nuclear in order to boost signals for future missions as well. But that's a separate project in itself.

3) Finally ofc, we have the problem of fitting everything we

need to do good imaging into the structure along with the sail to bring our density down. Same point as mentioned before except the density now includes the device itself.

But let's leap into the realm of fantasy and assume all this is possible. What are our results?

This is the exciting part. The resolutions according to this theoretical paper from NASA - https://www.nasa.gov/directorates/spacetech/niac/2020_Phase_I_Phase_II/Direct_Multipixel_Imaging_and_Spectroscopy_of_an_Exoplanet/ - are in the 25km range for planets at 30 parsecs.

So using advanced imaging techniques, we can reconstruct from the data, an image of an exoplanet such that every

pixel is 25km square. Which is truly *insane* if you think about it.

To create one of my own of those tired analogies from pop science articles - it'd be like imaging the moon from the earth at a sub-atomic level. Or to about the width of a couple of human hairs on Pluto.

Basically, it'd be more than adequate to notice even inhabited settlements, much less nightly lit cities etc. This assumes, ofc that our alien friends aren't ants and have large scale structures visible at that resolution.

This would possibly be our greatest engineering achievement in a few decades if it could even launch by 2040, much less reach its destination at any point before 2070 or so.