Author Topic: Mission to the Gravitational Focus  (Read 20764 times)

Offline DLR

  • Full Member
  • ****
  • Posts: 494
  • Angus, Scotland
  • Liked: 14
  • Likes Given: 0
Mission to the Gravitational Focus
« on: 12/09/2011 11:04 AM »
http://www.centauri-dreams.org/?p=785

This would make for one awesome telescope. We could directly image exoplanets in very high resolution. I read that with a Hubble-sized telescope, you would theoretically  be able to make out features only a few hundred meters in size on a planet orbiting Alpha Centauri, if you exploit the gravitational lens of the sun.

So NASA, the next time you plan flagship missions, think about getting a telescope out to 550 - 1000 AU!

How to do it? I believe nuclear electric propulsion would be up to the task. It could get a probe to cover that distance in twenty years of flight or so. Or a close flyby of the sun and then sailing on the solar wind. That may work as well.
"Bei der Eroberung des Weltraums sind zwei Probleme zu lösen: die Schwerkraft und der Papierkrieg." - Wernher von Braun

Offline Tass

  • Full Member
  • ***
  • Posts: 349
  • Liked: 51
  • Likes Given: 137
Re: Mission to the Gravitational Focus
« Reply #1 on: 12/09/2011 11:44 AM »
Trouble is that it looks in one specific direction, and steers very slowly. You have to move the probe around the sun at 500AU to change view.

Maybe it we, in a decade or so, have a really good candidate for an earth-like exoplanet, then it may be worth considering sending a probe to the specific point were it is in focus.

A big expensive mission, but much easier (and faster) than sending a probe to the actual planet.

Offline Warren Platts

Re: Mission to the Gravitational Focus
« Reply #2 on: 12/09/2011 01:32 PM »
Yeah, but Alpha Centauri would be a good candidate anyways because it will always (on human time scales) be the closest planetary system.
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Online Lee Jay

  • Elite Veteran
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6570
  • Liked: 853
  • Likes Given: 129
Re: Mission to the Gravitational Focus
« Reply #3 on: 12/09/2011 02:15 PM »
So NASA, the next time you plan flagship missions, think about getting a telescope out to 550 - 1000 AU!

Just for scale...the Voyagers are currently at about 100 and 120 AU from the sun.

Offline STS-200

  • Member
  • Posts: 90
  • UK
  • Liked: 9
  • Likes Given: 0
Re: Mission to the Gravitational Focus
« Reply #4 on: 12/09/2011 02:29 PM »
http://www.centauri-dreams.org/?p=785
How to do it? I believe nuclear electric propulsion would be up to the task. It could get a probe to cover that distance in twenty years of flight or so. Or a close flyby of the sun and then sailing on the solar wind. That may work as well.

You would be better off with one of these:

Fission fragment drive - a hypothetical 10t probe on a 10 year flight, using 180kg fuel.

Definitely at home in the "Advanced Concepts" section!
"Nothing will ever be attempted if all possible objections must first be overcome."

Offline sanman

  • Senior Member
  • *****
  • Posts: 3319
  • Liked: 359
  • Likes Given: 1
Re: Mission to the Gravitational Focus
« Reply #5 on: 12/09/2011 11:59 PM »
Pardon my ignorance, but what does FOCAL stand for again? Is it an acronym?

Online Lee Jay

  • Elite Veteran
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6570
  • Liked: 853
  • Likes Given: 129
Re: Mission to the Gravitational Focus
« Reply #6 on: 12/10/2011 12:04 AM »
Pardon my ignorance, but what does FOCAL stand for again? Is it an acronym?

The point of focus is called the focal point.

Offline sanman

  • Senior Member
  • *****
  • Posts: 3319
  • Liked: 359
  • Likes Given: 1
Re: Mission to the Gravitational Focus
« Reply #7 on: 12/10/2011 12:14 AM »
Fine, I know that - but why is FOCAL capitalized here? It just makes it look like it's an acronym which stands for something.

Offline sanman

  • Senior Member
  • *****
  • Posts: 3319
  • Liked: 359
  • Likes Given: 1
Re: Mission to the Gravitational Focus
« Reply #8 on: 12/10/2011 12:34 AM »
So using the example of Kepler 22b, which is said to be 600 light years from Earth, could it be imaged using the FOCAL mission approach?

Would there have to be a prolonged exposure time in order to image that planet?

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3209
  • Liked: 370
  • Likes Given: 81
Re: Mission to the Gravitational Focus
« Reply #9 on: 12/10/2011 10:39 AM »
So using the example of Kepler 22b, which is said to be 600 light years from Earth, could it be imaged using the FOCAL mission approach?

Would there have to be a prolonged exposure time in order to image that planet?
The article seems to claim it could make pretty much any object appear to be a ‘mere‘ 550 au away. sounds like a huge gain but surely you still need an impressive telescope.

I find this heartening just because it is one more interesting target we have yet to visit. We can just keep pushing the range of our robotic probes with each generation and there is always some new thing to make the extra range worthwhile.

Offline Tass

  • Full Member
  • ***
  • Posts: 349
  • Liked: 51
  • Likes Given: 137
Re: Mission to the Gravitational Focus
« Reply #10 on: 12/10/2011 12:38 PM »
So using the example of Kepler 22b, which is said to be 600 light years from Earth, could it be imaged using the FOCAL mission approach?

Would there have to be a prolonged exposure time in order to image that planet?
The article seems to claim it could make pretty much any object appear to be a ‘mere‘ 550 au away. sounds like a huge gain but surely you still need an impressive telescope.

I find this heartening just because it is one more interesting target we have yet to visit. We can just keep pushing the range of our robotic probes with each generation and there is always some new thing to make the extra range worthwhile.

The article seems to be written by someone who does not know terribly much about optics. I'll try to do some calculations when I have time.

Offline DLR

  • Full Member
  • ****
  • Posts: 494
  • Angus, Scotland
  • Liked: 14
  • Likes Given: 0
Re: Mission to the Gravitational Focus
« Reply #11 on: 12/10/2011 01:30 PM »
I read that with a sufficiently large light collector (a rotating tether which is reeled out), you could make out features the size of cars on planets orbiting Alpha Centauri ...

... so I suppose that even with a Hubble-sized telescope, you could directly image Kepler-22b and perhaps make out continents, clouds and oceans, if they existed.
« Last Edit: 12/10/2011 01:31 PM by DLR »
"Bei der Eroberung des Weltraums sind zwei Probleme zu lösen: die Schwerkraft und der Papierkrieg." - Wernher von Braun

Offline Tass

  • Full Member
  • ***
  • Posts: 349
  • Liked: 51
  • Likes Given: 137
Re: Mission to the Gravitational Focus
« Reply #12 on: 12/10/2011 02:25 PM »
By a quick calculation it seems that a diffraction limited pixel of the picture at 600 AU will extend over about 30 millimeter, corresponding to 2 kilometers of the object if it is 600 light years away.

But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc. To get a useful signal to noise ratio in a realistic time one will probably have to drop resolution quite a bit.

Still seeing continents, and maybe finer detail would not seem unrealistic.

Offline sanman

  • Senior Member
  • *****
  • Posts: 3319
  • Liked: 359
  • Likes Given: 1
Re: Mission to the Gravitational Focus
« Reply #13 on: 12/11/2011 12:03 AM »
Perhaps this sounds ignorant, but could one perhaps try to arrange for syzygy or colinearity with the Sun and Jupiter to improve the gravitational focus even further? Or is Jupiter's mass just not significant enough? At least it could help in blocking the Sun's disc, I suppose.


Online Lee Jay

  • Elite Veteran
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6570
  • Liked: 853
  • Likes Given: 129
Re: Mission to the Gravitational Focus
« Reply #14 on: 12/11/2011 12:26 AM »
But with an aperture to focal length ratio of about 65000 the intensity will be terrible,...

The math for this works out reasonable.  If the surface is as bright as Earth, a day-time shot would require about 41 seconds of exposure time at ISO 100 (unless I did that wrong).  Assuming the scope is tracking the planetary motion, a push-broom could be used to track planetary rotation making that exposure time work for that level of resolving power.

Offline sanman

  • Senior Member
  • *****
  • Posts: 3319
  • Liked: 359
  • Likes Given: 1
Re: Mission to the Gravitational Focus
« Reply #15 on: 12/11/2011 01:06 AM »
So you mean that the space telescope could be very finely adjusting its angle, to keep its gaze fixed on an exo-planet like Kepler 22b for some necessary exposure duration, like say one minute?

If the space telescope is out there at a distance of 550 AU from the Sun, then is it traveling in an orbital trajectory around the Sun? Or is it sort of motionless and stationary while it aims at its exo-planet target?

Online Lee Jay

  • Elite Veteran
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6570
  • Liked: 853
  • Likes Given: 129
Re: Mission to the Gravitational Focus
« Reply #16 on: 12/11/2011 01:32 AM »
I would suppose that the Sun's gravitational acceleration at such distance is so small that you could, if you'd like to, hold position there if you had an engine strong enough to get you there in the first place.

Offline hop

  • Senior Member
  • *****
  • Posts: 3029
  • Liked: 270
  • Likes Given: 589
Re: Mission to the Gravitational Focus
« Reply #17 on: 12/11/2011 02:03 AM »
If the space telescope is out there at a distance of 550 AU from the Sun, then is it traveling in an orbital trajectory around the Sun?
To get to 550+ AU in a reasonable time, it will be on an escape trajectory. Fortunately it doesn't have to stop there, the gravitational lens works at greater distances, and IIRC being farther away helps with some of the difficulties caused by having a star in the middle of your telescope.

However, pointing will be a challenge. If you actually manage to get km scale resolution on something light years away, you are going to need really precise and stable pointing to make use of it. Your target planet will also be moving at tens of km/s in it's orbit, and the star it orbits will probably be moving tens or hundreds of km/s relative to the sun.

We should keep in mind that the theoretical capabilities being discussed here are from spherical cow land. Achieving them in the real world would present tremendous challenges. However, you could fall short by many orders of magnitude and still do a lot better than conventional telescopes.

Offline Robotbeat

  • Senior Member
  • *****
  • Posts: 25683
  • Minnesota
  • Liked: 5805
  • Likes Given: 4315
Re: Mission to the Gravitational Focus
« Reply #18 on: 12/11/2011 02:23 AM »
Kilometer-scale resolution... If we can get resolution to even 100km or 1000km/s, we could study continents and possibly (if we get good spectral data) lifeforms if there are any pervasive on the surface in detail significantly greater than the single-point-of-light we have now and are likely to have for quite a long time (before a mission of this scale is done). The nice thing about a project like this is that it can be used to study planetary systems 10lightyears-100lightyears at reasonably good resolution that would be out of reach for even first-generation interstellar missions. There are at least 10,000 stars within 100 light years, compared to only ~10 within about 10 light years. Much better odds of finding good planets to study in detail. It would be interesting to read the details of someone doing serious analysis of this idea. This hand-waving is likely to be not too useful.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Sparky

  • Full Member
  • ***
  • Posts: 378
  • Connecticut
  • Liked: 0
  • Likes Given: 0
Re: Mission to the Gravitational Focus
« Reply #19 on: 12/11/2011 07:15 AM »
Getting out there quickly could be done with a solar sail. Once beyond the 550au point, the sail can double as your reflector.

Tags: