The next generation of space telescopes will
allow us to identify the first habitable exoplanets.
They’ll do this by dissecting the light
reflecting off these planets to determine
the composition of their atmospheres.
But resolving the surfaces of these planets
requires a telescope far more powerful than
anything we can hope to build in the foreseeable
future.
However, nature gives us a way to use the
Sun as a gravitational lens to magnify a distant
exoplanet by a factor of 100 billion.
That’s enough to let us map its continents
and oceans and even search for signs of life.
It sounds like science fiction, but scientists
and engineers are working right now to create
such an image within our lifetimes.
Welcome back to Launch Pad, I’m Christian
Ready, your friendly neighborhood astronomer.
In our previous video on future space telescopes,
we talked about how the proposed HabEx and
LUVOIR observatories will have the ability
to directly image Earth-sized exoplanets.
These telescopes use coronagraphs - and in
at least one case, a starshade - to block
out the light from the star and reveal the
planet, which is 10 billion times fainter.
For the first time we’ll be able to map
their orbits, monitor seasonal changes in
their brightness, and identify the chemical
make-up of their atmospheres.
But none of these telescopes can actually
resolve an exoplanet’s surface.
Such a feat requires a telescope much larger
than anything envisioned.
How large?
Well, let’s consider an exo-Earth 100 light-years
away.
In order to make a one-pixel image of the
planet’s surface, we'd need a telescope
90 km (56 mi) in diameter.
That’s large enough to extend from Philly
to Atlantic City!
It’s not impossible, but it will take a
while.
Light from the planet’s host star will contaminate
the image, either directly or by scattering
off the dust within its planetary system.
The only way to increase the signal to noise
ratio is to increase the integration time…to
100,000 years.
That’s how long it would take to make one-pixel
image of the planet.
What we really want to see is something more
like this, with a resolution of 1000×1000
pixels, and to create it within my lifetime.
Yours too.
This image was composed from 4 months of data
from NASA’s Terra Satellite.
At up to 1 kilometer per pixel, we can easily
make out continents and oceans.
We can see color variations and distinguish
desserts from vegetation.
Not only is vegetation proof of life on this
planet, but if we look closely, we can even
see lights on the night side.
In other words, proof of advanced life!
However, a similar-sized image of our example
exoplanet requires a telescope 90,000 km in
diameter.
That’s 7 times larger than Earth!
Even if the mirror were just a single micron
thick, it would have a mass of about 1 trillion
kg.
You can’t put it in orbit around Earth because
tidal forces would just tear it apart.
You’d have to put it into its own orbit
around the Sun.
But then it will become an instant light sail
and leave our solar system within a year.
Plus, no image.
However, there is a way to image an exoplanet
in months to years, instead of centuries to
millennia.
In fact, it’s really the only way to make
such an image.
General Relativity tells us that matter curves
spacetime, creating gravity.
As light passes through this curvature, its
path is deflected.
Eventually light rays passing around a massive
object converge at a focus, creating a Gravitational
Lens.
If we position ourselves exactly at the focal
point and look back toward the lens, we see
a ring of light surrounding the lensing object
called an Einstein ring.
We use Einstein rings to probe distant galaxies
that are being lensed by foreground galaxies.
However, Einstein rings are rare because seeing
them requires a chance alignment between the
source, the lensing object, and Earth.
But if we use the Sun as a gravitational lens,
we could, in principle position a telescope
at the focus for any target of our choosing.
As gravitational lenses go, the Sun isn’t
as handy as say, a black hole, but it is massive
enough to amplify a background source by a
factor of 100 billion!
That’s why a team of scientists and engineers
are developing a Solar Gravitational Lens
mission.
The team is led by Dr. Slava Turyshev at NASA's
Jet Propulsion Laboratory.
Turyshev and his teammates have been working
on the mission for a few years now and they’ve
already worked out most of the solutions to
some pretty staggering problems.
For starters, the SGL’s focus is far.
It starts at about 550 astronomical units.
Remember, Earth is one astronomical unit from
the Sun, Neptune is 30 AU, the Kuiper belt
extends to about 55 AU, and Voyager 1, the
most distant human-made object ever created,
is currently just shy of 150 AU.
By the way, it was launched in 1977!
At 550 AU, the SGL focus is well into interstellar
space.
But there’s a plan to get there in just
20-30 years after launch.
We’ll talk more about that in a little bit.
But once we do get there, we’ll have plenty
of time to make observations.
That's because parallel light rays that pass
farther away from the Sun aren’t bent as
strongly as rays passing close to the Sun,
so these rays come to a focus at greater and
greater distances.
Instead of a focal point, we get a focal line.
That simplifies things because we won’t
have to bring the spacecraft to a stop once
we reach the focus.
Instead, the spacecraft continues to image
the exoplanet as it flies along the focal
line.
That’s just as well, because pointing the
telescope to a new target isn’t going to
be very practical.
In order to change the telescope’s pointing
by just one degree, a spacecraft at 550 AU
would have to move 10 astronomical units in
the lateral direction.
That's the distance from Earth to Saturn!
It’s doable, but in practice such a telescope
wouldn’t be re-pointed.
So, you’d need to select the target planet
before setting out for its SGL focal line,
but that’s what the next generation of space
telescopes are for.
But the problem that’s blindingly obvious
is the fact that the Sun is…blinding.
Even at 550 AU, the Sun is far too bright
to image the Einstein ring.
It would need to be blocked with either an
internal coronagraph or an external starshade.
But even then, the Sun’s corona still puts
out a great deal of light.
That means we have to travel beyond 550 AU
before we can start making images.
But how much further?
Ideally, we’d wait until we’re far enough
away from the Sun that we’re only imaging
the light rays that avoid the corona altogether.
But that distance starts around 2200 AU from
the Sun.
A spacecraft traveling 25 AU per year would
take 88 years to reach that distance.
So, Slava Turyshev and Viktor Toth studied
the problem in greater detail.
They found that a specialized coronagraph
designed by Michael Shao at JPL could block
everything from the Sun all the way out to
the inner part of the ring.
Then, by turning off the pixels outside the
ring, they could image the Einstein ring with
an integration time of just a few seconds.
This approach allows the ring to be imaged
starting at 650 AU.
Yes, that's still far, but it’s a lot better
than 2200 AU.
But at that distance, the planet’s image
is going to be very large.
To understand why, let’s consider a pinhole
camera.
Light from a source passes through the pinhole
and makes an image on a screen.
The size of the image depends on the distance
from the pinhole to the screen.
The farther the light has to go to form an
image, the larger the image gets.
At 650 AU, the planet’s image is 1.3 km
across.
Instead of an image forming on the detector,
the detector would be inside the image!
This means an ordinary-sized telescope at
the SGL can only image a single “pixel”
of the planet.
In this case, a 1-meter sensor images a pixel
corresponding to a 10-kilometer patch of the
planet’s surface.
The telescope moves to the next pixel location
and makes another image.
This is a technique called rastering.
Meanwhile, the planet isn’t sitting still
for its close-up.
It rotates on its axis and orbits its star.
But not only is the planet moving, so is the
telescope!
Believe it or not, the Sun is not fixed at
the center of the solar system.
Instead, it moves around the solar system's
barycenter as it’s tugged back and forth
by the planets, particularly Jupiter and Saturn.
There are many other motions to consider,
but they all add up to a predictable “wobble”
of the telescope.
The wobble is slow enough that the spacecraft
can use ion microthrusters to generate the
necessary sideways velocities to cancel out
these motions.
With these issues addressed, let’s consider
what a telescope actually sees from the SGL's
focus.
For starters, the Einstein ring is as thick
as its image is wide.
In our example, the ring is just 1.3 km thick.
Therefore, this illustration is way out of
scale.
A 1-meter telescope at the SGL would see a
ring of light, but not be able to distinguish
the ring's thickness.
However, as we saw before, parallel light
rays passing farther away from the Sun aren’t
bent as strongly as rays passing close to
the Sun.
This means that if we were looking at a point
source through a gravitational lens, we will
see a bright ring from the rays that are in
focus, surrounded by concentric rings from
the parallel rays that are out of focus.
This results in a blurring effect called spherical
aberration.
A distant exoplanet is very tiny on the sky,
but it's not a point source.
Light from different points on the planet
form their own Einstein rings which correspond
to different pixels on the image.
Thanks to spherical aberration, all of these
rings are blurred together.
However, we can take advantage of this as
we construct an image.
As the telescope rasters across the image
plane, it views the planet’s Einstein ring
from a slightly different angle, resulting
in a unique mix of light coming from the region
of the planet being imaged, plus the blurred
light from the rest of the planet.
This causes the ring to dim or brighten as
the telescope moves from one pixel to the
next.
As the telescope scans across the focal plane,
it builds up a brightness map of the image,
one pixel at a time.
The result is a blurred image of the planet!
However, we don’t have to settle for a blurred
image, either.
After all, a blurred image is ultimately caused
by light from a particular point scattering
into multiple pixels.
If the source of the blur is understood well
enough, it's possible to reassign that light
to their intended pixels and reconstruct the
original image.
This is a technique called image deconvolution,
and it’s a pretty standard trick of the
astronomical trade.
However, deconvolution increases noise at
the expense of the signal.
That's why the signal to noise ratio is so
important.
There are a number of ways to increase SNR,
such as longer integration times, larger telescope
apertures, and increased instrument sensitivity.
But the real key to image deconvolution lies
in understanding exactly how the telescope
blurs the light from a point source.
This can be modeled with something called
a Point Spread Function.
The better the PSF, the better the deconvolution.
Slava Turyshev and Viktor Toth developed a
PSF of the Sun’s gravitational lens and
used it to simulate how Earth would appear
if it were 100 light-years away and lensed
by the Sun.
Their simulation described the brightness
of each location on the image as measured
by a telescope at each pixel.
Then they applied deconvolution algorithms
over a range of realistic signal to noise
ratios.
They showed that with a high enough SNR, an
image can be reconstructed with about 25-km
resolution!
That's 25 kilometers of exo-real estate 100
light-years away per pixel.
Planets that are closer will yield even better
resolution!
With repeated observations over time - preferably
with multiple telescopes - regular changes
in brightness due to the planet’s rotation
will be distinguished from irregular changes
caused by, say, cloud cover.
That means over time it will become possible
to “remove the clouds” and map the planet’s
surface.
None of this will be easy, but it is possible.
In fact, it’s very possible.
But it's also a gigantic undertaking, with
a fair amount of risk.
That’s why the team decided to go small.
Rather than a single “flagship” spacecraft,
one or two-hundred small spacecraft would
be used in a novel “string of pearls”
approach.
Each “pearl” is an array of 10-20 telescopes,
each 1 or 2 meters in diameter.
Pearl groups are launched annually over a
10-year period, adding to the string.
So how do they actually get to the SGL?
Nuclear propulsion would be ideal for this
sort of mission.
It was first developed in the 1960’s under
the Nuclear Engine for Rocket Vehicle Application
program, or NERVA.
Had it continued, it surely would be the fastest
propulsion system available and would have
provided plenty of power for the SGL mission.
But it was canceled by the Nixon administration
in 1973 as a cost-cutting measure, and this
is why we can’t have nice things.
Perhaps one day NERVA will be reinstated and
made ready for an SGL mission, but we’re
trying to get there within my/our lifetime.
This is why the spacecraft in each pearl group
will consist of smallsats equipped with solar
sails.
The smallsats can either launch together or
separately in a series of ride shares aboard
commercial launchers.
They’d rendezvous in cislunar space, outside
the immediate gravity wells of the Earth and
Moon.
There, they’ll deploy advanced solar sails
called SunVanes.
Unlike a traditional solar sail, the Vanes
can be individually articulated to control
the spacecraft’s direction.
This makes the SunVane highly maneuverable.
The spacecraft tack into the Sun, accelerating
as they spiral in.
At the moment of closest approach, the Vanes
are turned face-on to the Sun to achieve the
maximum solar radiation pressure.
The closer the sails can get to the Sun, the
faster they’ll accelerate.
But the sails have to withstand the intense
radiation as well.
Current technology can handle a 10 solar radii
approach.
That's only one solar radius further than
the Parker Solar probe, which uses a large
heat shield as it passes through the Sun’s
corona.
However, advanced sails may be able to handle
approaches as close as 5 solar radii.
Because of the SunVane’s maneuverability,
the spacecraft can make small, precise adjustments
as needed, setting it on course to the SGL.
By the time the spacecraft are 5 AU from the
Sun, they cross Jupiter’s orbit just a couple
of months after their slingshot around the
Sun.
At this distance, the solar radiation pressure
will have diminished to the point that the
Vanes are no longer needed.
Most of the Vanes can be jettisoned to reduce
mass while others could be repurposed as antennae,
telescope mirrors, or navigation aids.
The smallsats race on at 22 AU/year.
They cross Pluto’s orbit less than two years
after launch.
Within two and a half years, they exit the
Kuiper belt.
Within seven years, they smash through Voyager
1's distance record at 150 AU.
The smallsats are now in interstellar space.
There’s nothing out here, except for the
photons of the Einstein ring, 500 AU ahead.
They’ll navigate using laser beacons from
Earth, intra-cluster ranging between satellites,
inter-pearl ranging between clusters, and,
very likely, timing signals from pulsars.
The smallsats will spend most of the next
20-years of their cruise hibernating.
They'll occasionally wake up to transmit health
and status information.
But they’ll have one very important task
to complete before they reach the SGL.
Like a swarm of space Legos, the smallsats
use their ion microthrusters to assemble themselves
into 10 or 20 telescopes.
Each telescope is equipped with a 1- or 2-meter
mirror and an internal coronagraph.
The mirrors could be made from segments carried
aboard some of the smallsats.
Alternatively, remaining SunVanes could be
repurposed as crude mirrors and a smallsat
carrying an adaptive optics package would
correct for the distorted light.
I know that all of this sounds like science
fiction, but all of these technologies are
real.
Self-assembling spacecraft are currently being
studied by the U.S. Space Force...yes, that’s
a thing now..., DARPA, and are under development
at private companies such as NovaWurks and
Arkysis.
The SunVane concept was originally developed
at L’Garde and a technology demonstration
mission is now under development by Xplore,
Inc.
The goal of this first mission is to reach
speeds of 5-8 AU per year.
That’s two to three times faster than Voyager
1.
The Autonomous Assembly of a Reconfigurable
Space Telescope, or AAReST, is a cubesat mission
currently under development at Caltech.
The Deformable Mirror Demonstration Mission,
or DeMi, is another cubesat mission being
developed at MIT.
Meanwhile, advances in artificial intelligence
and machine learning will allow the SGL spacecraft
to operate autonomously through all stages
of flight.
The Parker Solar Probe is already doing this
as it navigates around the Sun, which is 8
light-minutes away.
By the time the spacecraft reach the SGL,
the round-trip light travel time will be greater
than a week.
So, the spacecraft will need to collectively
handle everything including navigation, fault
management, data transfer, and observation
strategy, all autonomously.
Upon arriving at the SGL, at least one of
the telescopes in a pearl group uses the Einstein
ring of the exoplanet’s star as a beacon.
With the beacon acquired, the other telescopes
move laterally to the predicted location of
the planet’s image, some 5- or 10,000 km
away.
The first pearl group to arrive at the SGL
will undoubtedly run into unexpected problems.
Perhaps there was a little bit more light
contamination than expected.
Maybe the planet was just a little bit off
from its predicted position, and the telescopes
to reposition themselves to another location.
This first pearl group will learn from the
mistakes of their creators and serve as the
pathfinder for the pearls to follow.
The pearls communicate with each other, sharing
health and status information, and passing
down the lessons learned to the next group.
Each pearl group improves upon the work of
its predecessor.
The string of pearls architecture allows for
redundancy, efficient power management, and
communications.
Multiple telescopes in each pearl mean more
data can be collected in shorter periods of
time.
As more data is acquired, the picture literally
gets clearer.
The pearls can follow the SGL focal line for
years at a time, monitoring the exoplanet.
It would be like having a virtual orbiter,
allowing us to watch the planet go through
seasonal changes.
Another advantage of this approach is that
it’s completely agnostic of its target.
The Solar Gravitational Lens always starts
at 550 AU from the Sun, no matter the distance
to the object being lensed or its location
in space.
With so many smallsats being mass produced,
missions to the SGLs of multiple targets could
be launched.
SGL missions could image the surfaces of individual
stars in other galaxies, create high-resolution
images of the first galaxies to form in the
early Universe, and map out the event horizons
of black holes.
Closer to home, it could create atlases of
habitable planets and possibly their civilizations.
Over time, we could use SGL to write the first
edition of Encyclopedia Galactica.
Most of the technologies needed have either
already been created or are under active development.
NASA’s Innovative Advanced Concepts program
awarded small grants to the SGL team to develop
their initial Phase 1 and 2 studies.
In April 2020, NIAC awarded the SGL team a
$2 million grant for a two-year Phase 3 study.
This is only the third time a Phase 3 grant
has been awarded.
It will allow the team to finalize the mission
concept, design the technology demonstration
mission, and transition from a study to an
actual mission proposal.
It’s quite possible that the first mission
to the SGL could be launched by the end of
this decade.
What if, 35 years from now, the first tentative
blurry images of an exoplanet began to trickle
back to Earth?
And a few years later, enough data had been
collected to resolve that image into a clear
picture?
Challenging?
Yes.
But it’s also a feasible mission.
One that is worthy of a civilization.
Just thinking about it and knowing there is
real work being done to make this mission
a reality, gives me hope for the future.
But in the meantime, we need to discover candidate
habitable exoplanets.
That’s why NASA is also working with the
astronomical community to design a new generation
of advanced space telescopes to find them.
I made a video about these new telescopes,
so I’ll see you over there when we’re
done here.
A huge thank-you goes out to Dr. Slava Turyshev
at JPL for his assistance - and his patience
– in helping me to understand all of this
well enough to make this video.
There’s a LOT of details I didn’t get
to cover, but I’ve linked to his papers
below so make sure you check them out because
they’re extremely well detailed.
And thanks, as always to my Patreon supporters
for helping to keep Launch Pad Astronomy going,
and I’d like to welcome my newest patrons
Vincent L. Cleaver,
Brandon Davis,
Carolos Gross Jones,
Ryan Suder,
and Wouter Westnbrink,
And a special thanks to Anna for sponsoring
at the Intergalactic level and to Michael
Dowling and Steven J Morgan at the Cosmological
levels.
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Until next time, stay home, stay healthy,
and stay curious, my friends.
