The mysteries of the universe seem limitless
However to unlock them we're going to need some incredible technologies to peer deeper and more sharply than is currently possible.
Fortunately, the imagination and ingenuity of telescope engineers also seems to be without limit.
We recently talked about some of the most exciting new
observatories currently being built. One of those, the James Webb space telescope will succeed the Hubble space telescope
with more than a factor of five increase in collecting area.
Getting such a large telescope into space is a major challenge and, in fact, it may be difficult to go much larger
using traditional mirrors. Those things are really hard to get into space in one piece.
Fortunately NASA isn't constrained by traditional methods.
The NASA Innovative Advanced Concepts N.I.A.C.
program has some ingenious ideas for overcoming this limitation. The first big idea may
revolutionize our study of terrestrial
exoplanets. The Kepler Mission has
determined that terrestrial planets; that is, rocky planets like our Earth, are extremely common and may orbit most stars in the Milky Way.
But these planets are extremely difficult to directly image because they're dense and small and, in addition, because
rock solidifies at a much higher temperature than volatiles like ammonia and water,
terrestrial planets tend to form close to their parent star. This brings a special challenge:
Glare. Our sun is about 10 billion times brighter than Earth.
Train a distant telescope on us and it would be overwhelmed by the sun's rays. So how can we find a terrestrial planet
around a star light-years away?
Maybe it's simple: Blot out the star.
We've done this for decades with the coronagraph, a disc inside a telescope that
occults a star, blocking its light so that any planets can be seen more clearly.
However, there's no such thing as a perfect shadow. The wave nature of light causes it to bend, or
diffract, around the edges of a coronagraph, back towards the central optical axis.
This means it's never possible to completely block the star's light.
Good coronagraphs can allow detection of objects from a hundred thousand to a million times fainter than the central star, but nowhere
near the factor of ten billion difference between the Earth and the Sun.
In 2005, Dr.Webster Cash proposed a successor to the coronagraph:
The Star Shade. It's actually a Spacecraft outfitted with thrusters to align Itself between a space telescope
and a star. A Star Shade will be up to 50 meters in diameter and hover
80,000 kilometers in front of the telescope,
assuming a four-meter diameter telescope mirror. At that distance, the Star Shade acts like an artificial eclipse.
The effect of diffraction would be easier to isolate than for a typical internal coronagraph, but the Star Shade goes much further.
It will not be a simple opaque disc like a standard coronagraph.
Instead it'll have a flower-like shape whose cleverly calculated petal geometry is
designed to diffract light away from the central axis, not towards it. The number and length of petals
optimizes each star shade for a particular wavelength of light.
Each of those petals is articulated to fold into a very compact
bud for launch and then open up like an actual flower once in space. A bit like the James Webb Space Telescope.
The main motivation for building Star Shades is to suppress the glare of stars enough to see the planets that orbit them.
Configured right, glare is suppressed by a factor of 10 billion
at 50 milliarcseconds from the star, so one of these things would allow us to see Earth in orbit around the Sun
from 60 light-years away.
There are a couple of thousand stars within that range, and hundreds of Sun-like stars,
many of which certainly have Earth-like planets.
With the Star Shade we may soon directly observe
terrestrial exoplanets with cameras and spectrographs; we'll be able to detect continents, oceans, ice caps and cloud banks of far-away worlds.
Besides Earth-like exoplanets,
the Star Shade would also be an enormous help in studying quasars and other high-contrast phenomena.
The first Star Shade may launch with NASA's 'WFIRST' mission in the 2020s, for a budget of around
750 million dollars, and a run time of five years. It's pricey but may ultimately save money as its
beneficiary telescope will require no coronagraphs or wavefront correctors, or other high contrast compensators.
Oh, and one Star Shade could theoretically serve multiple telescopes.
Diffraction is a challenge for coronagraphs, but the phenomenon is a challenge for any telescope.
The edges of a telescope's primary mirror or lens also cause diffraction. This introduces an unavoidable blur.
The Diffraction Limit
defines the best possible resolution of a telescope and it gets smaller or sharper
proportional to the size of your telescope's aperture but that's a tough trade-off.
To improve resolution by a factor of two you need to double the diameter of the scope, which means the volume and mass,
roughly speaking,
increase by a factor of eight and that's a problem when you're trying to launch your telescope into space.
Diffraction is expensive to deal with, but what if we could use the phenomenon to our advantage?
Enter, the Aragoscope, another one of Webster Cash's strokes of genius.
It's a revolutionary idea to use diffraction optics to focus light, instead of what we call geometric optics
so: reflection or refraction, like in traditional telescopes.
Imagine an opaque disc of a hundred meters to a kilometer in diameter,
suspended in front of a satellite detector. Light diffracts around the disk, coming to a focus on the optical axis
Where the light's wavefronts line up in constructive interference.
Place some minimal traditional optics and a camera at that focus and you have an image of the distant source
that's a hundred or a thousand times better in resolution than the Hubble Space Telescope.
The resolution of The Aragoscope is still proportional to its size,
but because we're talking about a foldable plastic disc rather than a chunky solid mirror, it's possible to scale up the Aragoscope in size
much more easily than a regular scope.
Now, there is a big downside to the Aragoscope: it's also a giant coronagraph,
blocking most of the light from the object of interest. All you get is the thin ring of light diffracted around the edge.
Although that light comes to an incredible focus, the actual amount of light you get is the same as if you didn't
have a telescope at all. Ways around this are to add a largeish lens or mirror to the satellite
but that adds a lot of mass.
It's also possible to break the disc into a set of concentric rings so that you get many
diffraction edges. There are challenges to bringing the light from each ring to the same focus
but fortunately humans are pretty smart and there are ways to do this.
An Aragoscope in geosynchronous orbit could resolve a hamster on the surface of the Earth.
Pointed outwards, it could spot a terrestrial planet at tens of light years distance and even map the cloud structure of a gas giant,
especially if you add a Star Shade to the Aragoscope, because why not?
One of the most powerful uses of the Aragoscope is in X-Ray Astronomy
X-Rays have such a short wavelength that telescope mirrors have to be astoundingly smooth to reflect them cleanly.
A mirrorless Aragoscope avoids this problem. In this case the disc would have to sit
thousands of kilometers in front of the detector so It would be an independent spacecraft, just like the Star Shade.
However you will be able to see X-rays right down to the event horizons of
supermassive black holes in distant galaxies. And now for something completely different.
This one from
Dr. Marco Quadrille at JPL. We've Just seen ways to forgo heavy glass mirrors and lenses,
but what if we could ditch the giant discs and support structures too? The future could lie in
orbiting rainbows, an idea as creative as it sounds.
Inspired by how water droplets focus light into colorful arcs across the sky,
scientists have proposed we use photon pressure to suspend a cloud of tiny reflective particles in Earth's orbit;
a laser-confined glitter cloud, if you will. The particles would be fractions of a millimeter in size,
Small enough that commercial lasers could corral and shape the particles into an effective mirror or lens.
Scientists could expand the aperture to tens of meter in diameter,
but on launch it fits into a small box and there's literally nothing to break during that launch.
The pile of shiny grit is already as broken as it can get. The immediate question is how can such a
disordered granular material create a clear image?
Tests of glitter coated lenses show they're inherently noisy,
but scientists can counter this by taking multiple exposures of the same target and then use advanced
algorithms to combine the images and remove speckle.
Results probably won't top those of the magnificent Star Shade and Aragoscope,
but the orbiting rainbow is cheap. It may be possible to launch
multiple such telescopes that have several times the light-collecting power of the Hubble Space Telescope.
NASA has some brilliant plans to overcome the limitations of traditional
space-based telescopes.
What once seemed like
fundamental limits to our ability to observe the universe are now being overcome by some incredible human ingenuity.
As we launch our new observatories our vision will become keener,
allowing us to peer more sharply, and to ever greater depths, into space-time.
I want to take this opportunity to thank all of our Patreon supporters. Your contributions, no matter how small,
really help keep the show running, and a very special thanks to Dean Fuqua for supporting us at the Quasar level.
Dean, when the first Orbiting Rainbow is launched and that sparkling glitter cloud is thrown into the universe,
we will consider it to be a celebration of your great generosity.
Huzzah Dean!
Last week we talked about the tantalizing rumor that the LIGO
Observatory had detected gravitational waves from the merger of a pair of neutron stars.
Let's see what you guys had to say:
Kai Widman would like to know what would happen if a black hole merged with a neutron star.
Well, the black hole would win, for one thing. The neutron star would be tidally disrupted when it got very close to the black hole,
producing a blast of observable radiation. After that, everything that wasn't ejected in the blast
would be eaten by the black hole.
Nicolas Martino asks whether gravitational waves are redshifted by the expansion of the universe.
Yeah, they sure are; they have to travel
along the same space-time fabric as light waves after all; I mean, they're waves in that fabric so,
stretch out the fabric and you stretch out its waves.
Feynstein 100 asks whether a black hole forming in the death of a massive star first goes through a neutron star-like phase.
Well, in a way yes, but it's never actually a neutron star.
The final phase of the core of such a star is a giant ball of nickel and iron,
held up briefly by electron degeneracy pressure.
Basically, the electrons are crammed as close together as quantum mechanics allows.
That support gives way when pressure rams electrons into protons in the nuclei, to turn them into neutrons.
As this happens, the stellar core succumbs to the gravitational crush and collapses incredibly quickly.
During that collapse the core looks more and more like a neutron star,
basically a giant ball of neutrons with the density of an atomic nucleus.
While neutron stars halt this collapse when they hit neutron degeneracy pressure,
The most massive stars don't manage to stop before the core is smaller than its own event horizon,
forming a black hole. So it's reasonable to imagine that neutron star-like conditions exist extremely briefly
before the event horizon forms,
but after that the neutrons themselves will disintegrate into quarks and then who knows what as the core continues to collapse
towards the singularity.
Galicki Band wants to confirm that the gold in their ring may have been created in the collision of two neutron stars.
Well, we can't say for certain yet, but it is looking very possible and
if not merging neutron stars then it was likely a supernova explosion.
So, yeah, your ring was forged in the death of a star or the birth of a black hole.
Who needs Mount Doom?