
The “Terrascope”: On the Possibility of Using the Earth as an Atmospheric Lens - sohkamyung
https://arxiv.org/abs/1908.00490
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blackhaz
This is pretty exciting but doesn't look very efficient and the Strehl ratio
is probably going to be as high as looking through a bottle of water. The cost
of a 100-meter Overwhelmingly Large Telescope (OWL) proposed in the 90s is
around $1.7B (the bid was lost to the ELT design.) Assuming adaptive optics is
used, this instrument might achieve comparable level of resolution. To launch
a mission like Terrascope, which with a 1-meter sized detector is equivalent
to a 150-meter telescope, we are probably looking at expenses on the order of
$10B - I'm looking at James Webb and Hubble. Imagine the spacecraft technology
of exposing a 1-meter mosaic of CCDs in orbit and keeping it thermally
controlled.

Although I am only an amateur astronomer, the biggest disadvantage I see is
that the Terrascope won't have the collecting area of a 150-meter telescope,
as Earth is not transparent. We get the resolution of it, but we're basically
looking through a very large atmospheric ring with what - >90% central
obstruction? I cannot imagine the exposure times required to get anything
meaningful and, also, the complexity of post-processing required to correct
the ringing artifacts. Then, you're limited to just 1 square degree of sky to
observe - unless you maneuver the Terrascope detector in orbit, which is
prohibitively expensive due to the limited fuel. Imagine the slew times
measured in months... Finally, it's going to have shorter mission design
lifetime, if compared to a ground-based telescope. It can possibly be extended
with a refueling mission, which would add to the cost, however. I think the
overall value of this instrument is rather low, a curious proposition, but
why?

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SiempreViernes
> a curious proposition, but why?

I think the initial part of this snippet answers the question: this is mostly
an effort by someone to type up a fun idea. Said someone is most likely a
theorist, who doesn't let small details like slewing speed or exposure time
prevent them from writing up a fun idea.

Honestly it seems like David Kipping could be a pretty fun guy to sit down and
dream up weird stuff with, witness arxiv:1903.03423,

"The Halo Drive: Fuel-Free Relativistic Propulsion of Large Masses via
Recycled Boomerang Photons"

"Abstract: Gravitational slingshots around a neutron star in a compact binary
have been proposed as a means of accelerating large masses to potentially
relativistic speeds. Such a slingshot is attractive since fuel is not expended
for the acceleration, however it does entail a spacecraft diving into close
proximity of the binary, which could be hazardous. It is proposed here that
such a slingshot can be performed remotely using a beam of light which follows
a boomerang null geodesic. Using a moving black hole as a gravitational
mirror, kinetic energy from the black hole is transferred to the beam of light
as a blueshift and upon return the recycled photons not only accelerate, but
also add energy to, the spacecraft. It is shown here that this gained energy
can be later expended to reach a terminal velocity of approximately 133% the
velocity of the black hole. A civilization could exploit black holes as
galactic way points but would be difficult to detect remotely, except for an
elevated binary merger rate and excess binary eccentricity"

~~~
pavel_lishin
"We didn't want to spend too much energy, so we just moved a black hole."

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jcims
Moonset from the ISS demonstrating this property:
[https://youtu.be/bc_jSezwruc](https://youtu.be/bc_jSezwruc)

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andyljones
The author seems to be pretty receptive to questions on Twitter:
[https://twitter.com/david_kipping/status/1157088879817764864](https://twitter.com/david_kipping/status/1157088879817764864)

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adito
The video from the author, "Turning Earth Into a Telescope | The Terrascope"
[https://www.youtube.com/watch?v=jgOTZe07eHA](https://www.youtube.com/watch?v=jgOTZe07eHA)

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thedevindevops
What would it be for one of the gas giants?

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rjmunro
Sounds like a smaller version of
[https://en.wikipedia.org/wiki/FOCAL_(spacecraft)](https://en.wikipedia.org/wiki/FOCAL_\(spacecraft\)).

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cellular
This is BIG news.

