Today I’m going to tell you about the craziest proposal for an astrophysics mission that   has a good chance of actually happening.

A train of spacecraft sailing the sun’s   light to a magical point out there  in space where the Sun’s own gravity   turns it into a gigantic lens.

What could  such a solar-system-sized telescope see?

Pretty much anything.

But definitely it  could map the surfaces of alien worlds.

Thanks to the Kepler mission, we  now know that there are billions of   extrasolar planets - exoplanets - in our galaxy.

And we’re learning a ton about them - for example,   we’ve figured out that there are 40 billion or  so Earth-like planets, at least in terms of size   and mass.

Of course if we want to find life or  actually visit these planets it’d be nice to   know a bit more than that.

The James Webb Space  Telescope is helping - it can detect different   molecules in the atmospheres of exoplanets a bit  larger than the Earth.

JWST will even take images   of some exoplanets.

But those images will never  reveal anything more than a single dot in orbit   around a star.

If we find evidence for life, we’re  going to want to study it in detail - ideally with   images of the planetary surface.

And if we  ever want to visit one of these exoplanets,   it sure would be nice to know what lies  at the end of such a many-decade journey.

But resolving the surface features of an  exoplanet seems pretty impossible.

There’s   an absolute limit to the resolving power of  any telescope, and it depends on size.

Bigger   is always better.

When light passes into a  telescope, its wave nature interacts with   the edges of the aperture, causing diffraction,  and that blurs the focus of the mirror or lens.

The bigger the aperture, the smaller that blur.

A  telescope’s diffraction limit is the best possible   resolution it can achieve.

If you try to create  an image of anything smaller than this limit,   it will always get blurred to the size of  the diffraction limit.

That’s a problem,   because planets are pretty small when you’re  trying to see them from many light years away.

For example, to see a planet 100 light years  away as anything more than a dot you’d need   a telescope way bigger than New York City.

You might recall these pictures of the black   holes in the M31 galaxy and the center of  the Milky Way.

These were taken by bringing   together radio signals from telescopes all  across the planet, effectively giving us a   planet-sized telescope with a tiny diffraction  limit.

We are not yet able to repeat this trick   with visible light because this requires  exact measurement of the arrival time and   phase of the electromagnetic wave  - which  gets harder the shorter the wavelength.

But there is one way to take a direct image of an  exoplanet in visible light that could reveal it’s   detailed surface features.

And that’s by sending  a spaceship.

Not TO the planet - that would take   way too long.

In fact, it’s by sending a spaceship  in the opposite direction.

If you travel directly   along the line connecting your favorite exoplanet  and the Sun, but away from them both, you’ll reach   this spot where light rays from the exoplanet are  bent inwards by the Sun’s gravitational field to   all come together.

Forget about a New York sized  telescope - at this spot, we have a star-sized   telescope.

The result is an amplification of  the brightness of the exoplanet by a factor   of a trillion, and a magnification of the  surface details by a factor of 100 billion.

The technical name for this location  of incredible light-converging power   is the locus of focus hocus pocus.

LFHP.

OK, that’s just what it should be called.

For some reasons scientists went with  SGLF - solar gravitational lens focal   region - missed opportunity if you ask me.

But if we can get a telescope into the “SGLF”,   then we could start making detailed desk  globes of alien worlds.

Let’s talk about how   we might actually achieve this.

Because the  plan is further along than you might think.

Actually, first let’s review gravitational  lensing.

Einstein’s general theory of relativity   tells us that gravity is due to curvature  in the fabric of spacetime due to massive   objects.

But that curvature also bends the path  of light.

You know what also bends light?

Lenses.

So a gravitational field can also act like a  lens, although admittedly a kind of crappy one.

Regular lenses are designed to bring light  from the same point to a single focus point,   allowing an image to be formed.

Gravitational lenses produce highly   distorted images, like these stretched out  galaxies seen through the gravitational field   of a giant galaxy cluster.

If the alignment  is close enough, we can see an Einstein Ring,   like this galaxy being lensed by an intervening  galaxy.

The sun also has a gravitational field   that would create an Einstein ring of any distant  object - including an exoplanet - as long as you   were watching from the correct location.

Let’s  just imagine that we can find that location.

So   let's imagine we can find that  location.

As I mentioned, you'd catch something like a trillion times more light  from the exoplanet, making it possible to even see   the thing in the first place.

And the planet's  surface area would be expanded by a factor of   around 100 billion.

If only it were possible  to remove the distortion we could map that   surface in intricate detail.

Well, it’s not only  possible.

It’s kind of easy.

Gravitational lens   images pretty messy.

For example, here are some  simulations of distant galaxies that have been   lensed by a second galaxy much closer to us.

The detailed structure is scrambled.

But this   is what you get when you try to reconstruct the  original galaxy.

The results are remarkably close   to the originals.

We can go into the details  of this process another time.

And we should,   because it’s a big part of my own research.

But  not today.

Today we’re looking for aliens.

If it’s possible to reconstruct the  image from a messy galaxy lens, then   it’s completely straightforward to do it with the  very clean, well-understood gravitational field of   our Sun.

All we need to do is get our telescope  to the right spot.

Unfortunately that spot is   pretty far away.

Like,10 times the distance  of Pluto, or well over 500 times the Earth’s   orbital radius.

Around 550 “astronomical  units” or “AUs” in astronomer-speak.

For comparison, Voyager 1 is our most  distant probe.

It’s been traveling for   45 years and is now around 150  astronomical units from the Sun.

To get a telescope to the SGLF we’d want it to  travel a bit faster than that.

So the researchers   outline two possibilities in their report to NASA:  one is the “flagship” model, in which a single   craft with a 1-2 meter telescope is sent to do the  work.

The second is the string of pearls option,   in which many so-called small-sats are sent in  a long train, each riding on the light of the   sun with a solar sail.

That second seems the  preferred option, and it’s alway way cooler,   so let’s talk about that.

Just as regular sails  accelerate a ship by catching the momentum of   the wind, solar sails catch the momentum  of light - of photons from the Sun.

More   traditional propulsion methods that carry their  own fuel have hard constraints on payload size   and acceleration period because they’re  weighed down by their own fuel supply.

But a solar sailing vessel doesn’t carry fuel,  making them great options for long-range missions.

And this isn’t even some sci-fi far-future  tech.

In 2010 the Japanese space agency sent   the IKAROS probe to Venus using a 20 meter solar  sail, and plenty more solar sails missions are in   the design phase.

But getting to 550 astronomical  units would be the most ambitious among these.

In order to reach the destination during  the working lifetime of at least some of the   astronomers and engineers who witness the launch we need a travel time of 25-30 years.

So our   spacecraft need an average speed of more than 100  km/s - several times faster than Voyager.

We’d   want each craft to be very light - ideally under  100 kg.

But with advanced modern materials that   seems possible.

Even with that mass, the solar  sail would need to be enormous - with more surface   area than a football stadium.

Unfurling and then  controlling such a giant sail is very difficult,   so the scientists are proposing an advanced  solar sail design called the SunVane - multiple   controllable sail panels mounted along the narrow  structure of the craft.

These sails would need   to be made of some advanced, low-density  metal alloy that’s A) highly reflective,   B) has a high melting temperature because, as  we’ll see, it actually gets very close to the Sun,   and C) is only a few hundred atoms thick  so it doesn’t blow out our mass budget.

Solar sails experience more acceleration the  closer they are to the Sun.

To reach the speeds   we need, our spacecraft can’t start their  outward journey from the Earth - they needs   to get closer to the Sun first.

This is  how the proposed mission would play out.

Our spacecraft starts out by launching backwards  compared to Earth’s orbital direction, using sails   to slow down and sort of tack inwards.

They  speeds up rapidly plummeting towards the Sun,   ideally at around a quarter of Mercury’s  orbital radius, assuming we can make the things   sufficiently heat resistant.

Then the craft then  whip around the Sun and set their sails squarely   against that intense up-close solar radiation.

That propels the craft on a trajectory that   will take them out of the solar system and  towards our first image of an exoplanet.

Hitting the right spot is a feat of  incredible astro-navigation and maneuvering.

The solar gravitational lens focal range is  indeed a range.

While a regular lens creates a   focal point, the Sun’s gravitational field creates  a focal line, starting at 550 astronomical units,   and extending indefinitely, with the Einstein  ring getting wider and more diffuse the further   you go.

The column for an Earth-sized  exoplanet at 100 LY is only 1.3 km across,   so this really is like threading very, very  tiny a needle.

To hit the right spot the craft   will maneuver with tiny ion thrusters.

Once in  the zone, the craft deploys its telescope.

One   possibility is that multiple craft will assemble  into a larger scope.

It may even be possible to   repurpose the light sail as a mirror, if we want  to get really clever about this.

Once in place,   our telescope just needs to point back at the  Sun and take an image of the faint einstein ring   surrounding it.

You might note an issue here.

The  last thing you ever want to do with a telescope   is to point it at the Sun - that’s a great way to  fry your camera - and good luck seeing anything   next to the Sun’s intense glare.

To deal with  this our telescope will use a coronagraph - a   giant circular mask that’ll block the Sun’s light.

From any one location within the focal column, the   newly deployed telescope will see an Einstein ring  formed from a single tiny patch on the surface of   the planet, only 10km across.

In order to see the  entire planet it’ll have to move around the focal   column, mapping the surface one patch at a time.

The ion thrusters come into play here also.

On top   of this, the entire Einstein ring will be moving  due to the motion of the exoplanet but also due to   the wobble of the Sun as its tugged by the planets  of our solar system.

Our telescope is going to   execute this shifting pirouette as it races away  from the Sun, and that entire dance will have   to be performed without any guidance from Earth  due to the several day light travel time back to   Earth.

As you might have noticed, our spacecraft  have no way of actually stopping.

But that’s   OK.

The long focal line means that an Einstein  ring will be visible for years of travel time,   slowly expanding outwards as we get further from  the Sun.

Remember that the scientists called this   a string of pearls.

That first cluster of craft  was the first pearl.

Even if that wave doesn’t   get it quite right, its data will help the next  pearl learn, which will be following in about a   years time.

And improved positioning and observing  strategies will flow down the string of pearls.

Over time, the image that the train of  spacecraft sends back to Earth will get   clearer and clearer.

It should be possible to  achieve a resolution of around 25 km on the   surface of an exoplanet 100 LY away.

We could  map coastlines, see islands and mountain ranges   and lakes and ice caps and even vegetation - all  of which we may be able to distinguish from their   colours.

And if we spot bright points of light  on the planet’s night side - aka cities - that   would be pretty compelling evidence of a  technological civilization.

And over time   we’ll see changes in all of this.

That means we  can remove cloud cover, track the change from day   to night, see seasonal and tidal and changes,  and even changes due to the activity of life.

Every exoplanet that we want to image  requires a new fleet.

That sounds like a lot,   but remember that we build dedicated spacecraft  for each of our solar system’s planets - usually   more than one of them.

And these little  small-sats are designed to be cheap,   so hopefully we’ll eventually be able to do this  for many exoplanets.

And, actually, for distant   galaxies and black holes and literally anything  else for which we want extreme resolution imaging.

Maybe this all sounds a bit out there.

But all of  the technology involved is either existent,   or in the development phase.

Nothing seems like a  dealbreaker.

There is no funded mission yet,   but the scientists involved have been advanced  to the phase 3 stage by the NAIA program.

The step after that, hopefully, is for  the mission to be picked up by NASA.

We’ll let you know how that goes.

But it’s crazy to imagine that within   our lifetimes we may have mapped in  detail the surfaces of distant worlds,   brought into focus by our own Sun  and its lens of curved spacetime.