The Asteroid and the Telescope

by Paul Gilster on May 3, 2012

One of the topics receiving fairly little coverage in the excitement of the Planetary Resources announcement is asteroid deflection. It seems clear that learning how to reach an asteroid and extract everything from water to platinum-group metals from it will also teach us strategies for changing an asteroid’s trajectory, in the event we find one likely to hit the Earth. The recent report from the Keck Institute of Space Studies makes this point clearly in the context of its own mission study, a plan to retrieve a small (7 m) asteroid and park it in lunar orbit.

What Asteroid Operations Can Teach Us

Although Planetary Resources estimates there are more than 1500 asteroids that are as easy to get to as the Moon, we still have a long way to go in understanding basic facts about these objects and their composition. Take dust, which will probably vary from object to object, but which could cause problems for ‘gravity tractor’ concepts where a spacecraft is used to deflect an asteroid without physically contacting it. If the rendezvous with the asteroid can be managed far enough from Earth, the gravitational field of a nearby orbiting body as tiny as a spacecraft can, over a period of years or even decades, bring about the needed course change.

But assuming your vehicle works with the kind of solar electric propulsion envisioned by the Keck study, dust could be a factor if the engine exhaust reaches the asteroid as part of needed station-keeping (this is perhaps an argument for solar sail technologies in these scenarios). What seems to be a small issue becomes a big unknown when you think about the multi-year presence of a gravity tractor spacecraft around such an asteroid. Direct study, as via Planetary Resources robotic technologies or manned crews examining a captured asteroid in lunar orbit, should help us learn more about how dust is moved and settles on an asteroid surface.

Other factors listed by the Keck report:

Anchoring: We need to acquire the ability to land a robotic spacecraft on an asteroid and anchor it there, a challenge any mining venture will have to resolve.

Structural characterization: This is a big one. We need to understand an asteroid from the inside out, since a prime deflection method is to hit the asteroid with enough of a blow to change its course. But we know little about what happens to an asteroid when this occurs because ejecta from the impact could multiply the momentum given to the NEA by the impactor.

Proximity operations: How do we dock with the asteroid and navigate near it? We’ll learn many of these things through actual robotic asteroid operations, and as we saw last time, having a small asteroid available for examination in lunar orbit would far surpass the 60 grams of surface material we’re going to have returned from the upcoming OSIRIS-REx mission.

These are all technical matters, but it goes without saying that a successful asteroid retrieval of the kind Keck envisions would also draw public attention to the asteroid defense element of all our studies of near-Earth objects. And in addition to its uses in providing unique, space-based resources for radiation shielding and propellant extraction, an asteroid retrieval would offer up some of the options we may someday want to use in space elevators. Says the report:

One day, in the more distant future, it is possible that a small NEA (~10 m) returned to E-M L2/L1 could act as an orbiting platform/counter weight for a lunar space elevator to allow routine access to and from the lunar surface and also function as a space resource processing facility for mining significant quantities of materials for future human space exploration and settlement and possible return and inclusion in terrestrial markets.

Eye on an Exoplanet

The asteroid mining and retrieval idea seems so loaded with possibilities that the Keck Institute’s 51 page report can barely contain them all, but I want to close with the idea NextBigFuture has been discussing recently. Planetary Resources makes a point about the Arkyd Series 100 space telescopes it intends to begin launching as soon as 24 months from now. These are intended to begin with studies in low Earth orbit but the Arkyd Series 200 that follows would contain a propulsion system so that missions directly to new asteroid targets will become possible.

We get the same kind of look at an asteroid, says Planetary Resources, as we got when exploring the Moon with the Ranger missions (1961-65) or the Deep Impact mission at Comet 9P/Tempel in 2005. The name of the game is data acquisition as we try to decide which near-Earth asteroids are the best candidates for future operations. NextBigFuture took a look at all those telescopes — Planetary Resources describes them as “the first private space telescope… simple enough to be designed, manufactured, tested and integrated by a small team, yet robust enough to get the job done.” Could they be massed for deep space studies?

The principle is interferometry, which would allow the creation of huge telescopes, mixing signals from a cluster of small instruments to achieve high-resolutions unavailable from a single, monolithic lens. The idea has been thoroughly vetted, and with great success, with Earth-bound instruments, but French astronomer Antoine Émile Henry Labeyrie (Collège de France) has been studying what he calls a ‘hypertelescope,’ which would involve huge numbers of free-flying spacecraft combining their data to produce images that could show surface detail on exoplanets.

Labeyrie’s presentation on the topic at a European Space Agency meeting in 2009 describes a “laser-driven hypertelescope flotilla at L2” that could image continents and oceans on a world 10 light years away. These would be telescopes whose mirrors were placed kilometers apart, each of them small instruments but forming what he has called a ‘sparse giant mirror.’ Here’s the image from Labeyrie’s talk that NextBigFuture also ran. Note the resolution shown for Earth at the 10 light year distance, and the swarm of spacecraft that have been used to produce it.

In a 1996 paper, Labeyrie had this to say about interferometry and exoplanets:

As the technical difficulties will become mastered, a continuous evolution towards larger sizes is to be expected. Jupiter-like planets at 5 pc can be imaged from Earth with 10 km arrays, while Earth-like planets at 5 pc require 100 km arrays, preferably installed in space. Because such images can also yield spectra for each of their resolved elements, they should provide a better diagnostic for the presence of life, and possibly civilisation, than would spectra of unresolved planets. Other objects such as pulsars, galactic nuclei and QSOs [quasi-stellar objects] are also candidates for high resolution imaging.

Labeyrie went on to develop the concept he calls Exo-Earth Imager, one that made an appearance in New Scientist in 2006 in an article by Govert Schilling:

Labeyrie’s design for a hypertelescope takes dilute optics to the extreme. Ultimately his Exo-Earth Imager will consist of at least 150 mirror elements, each measuring 3 metres across, and spread out over an area of about 8000 square kilometres. Together, they would fly in formation around the sun to make a hypertelescope with a diameter of 100 kilometres – large enough to pick out clouds and continents on a distant relative of our home planet.

Whether or not Planetary Resources would eventually wind up creating a hypertelescope flotilla anything like this as an offshoot of its asteroid mining effort remains to be seen, but what is exciting here is the prospect of lower-cost space telescopes whose very presence may spur refinements in interferometric techniques. The same network could boost the effort to exploit sunshade concepts, in which the light of the central star is effectively nulled and the faint light of exoplanets made visible. All in all, an effort to reach and take advantage of asteroid resources could have large ramifications indeed, not all of them confined to our own Solar System.

Two papers by Antoine Labeyrie are relevant here. They are “Resolved imaging of extra-solar planets with future 10-100km optical interferometric arrays,” Astronomy and Astrophysics Supplement, v.118 (1996) p.517-524 (abstract) and “Snapshots of Alien Worlds: The Future of Interferometry,” Science 285 (1999), pp. 1864-65 (abstract). The Schilling article is “The hypertelescope: a zoom with a view,” New Scientist 23 February 2006.

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{ 20 comments }

bill May 3, 2012 at 14:16

With regards to yesterday’s article “bringing asteroid to lunar orbit” I’d like to address some of the comments that were made yesterday. The idea of directing an asteroid to hit a deserted place on earth so that you could pick up the metals sounds like a recipe for disaster. One small error and you could end up taking out a city. In response to the person that wrote about a cargo ship which became infected by a alien entity, it sounds like either a storyteller ripped off the movie “Aliens” on the movie ripped off the printed story. Speaking of the movie Aliens you recall that in that movie they were transporting perhaps millions of tons of ore to be processed, it makes you wonder why they didn’t process the ore where they minded and sent it to earth. A similar situation we find ourselves in with regards to these asteroids.

Wojciech May 3, 2012 at 15:07

The hypertelescope swarm is exciting indeed.
I can imagine them being controlled by same progamming as those experimental drone swarms:

http://www.youtube.com/watch?v=FubP0KzeS4w

I wonder would it be possible for those telescopes to be much smaller like so-called nano-satellites weighting couple of ten of kilos but indeed placed like swarms in hundreds?

philw1776 May 3, 2012 at 16:36

Interferometry requires precise sub wavelength of light spacing control. Difficult to do even here on “stationary” Earth between the two Keck telescopes whose complex interferometer was recently de-funded by NASA which apparently prefers funding climate hysteria research. The fleet of space telescopes is the trivial part of the problem.

Enzo May 3, 2012 at 17:49

ESA launched Herschel at 1.1 euro billions. Guessing that each of the 150 telescopes would have a similar complexity, the cost is such that it’ll never happen. Even considering the economy of scale, there is still the launching cost for each telescope.

Photonic sieves are cheap and interesting, but not good at collecting the tiny amount of light coming from an exoplanet :

Enzo May 3, 2012 at 18:11
Eniac May 3, 2012 at 22:15

philw1776:

Interferometry requires precise sub wavelength of light spacing control. Difficult to do even here on “stationary” Earth between the two Keck telescopes whose complex interferometer was recently de-funded by NASA which apparently prefers funding climate hysteria research. The fleet of space telescopes is the trivial part of the problem.

This is not strictly true. What Wang and (I think) Paul are referring to is intensity interferometry, which requires only an information link between individual devices. There is a rudimentary stub here (http://en.wikipedia.org/wiki/Intensity_interferometer), and a fairly detailed explanation here (http://mysite.du.edu/~jcalvert/astro/starsiz.htm , towards the bottom). It has limitations, compared with true interferometry, but apparently they can be overcome by clever computational strategies. See also the Next Big Future article Paul has linked to for further interesting references.

It looks like this is going to be astronomy’s Next Big Thing, and I am not kidding.

Interstellar Bill May 3, 2012 at 22:55

On Hyperscopes:

Let’s remember that the number of informationally distinct pixels in the final image roughly equals the number of mirrors, which basically act as point sources when seen from the focal plane.

Let’s also remember that each mirror needs a co-orbiting coronograph-style occulting disc to block the light of the primary star of the planet.

Given a particular number of mirrors, their size determines the hyperscope’s light-collection power. A total-weight ceiling for the fleet gives a tradeoff of luminosity vs. resolution.

Regarding gravity tugs, the tractor has two rockets slanted laterally so their exhaust misses the asteroid though their net thrust vector will be through the asteroid CM. What they need to be looking at is the tractor having a big cargo net, to be filled with gravel brought from the asteroid.

Also, regarding asteroid deflection, don’t forget tethers. Put some mass from the asteroid on the tether and let it slide out and get released. This could be used to alter the asteroid’s rotation as well as its momentum vector.

Finally, don’t forget tunneling robots, if you really want to know what’s inside an asteroid. With the low gravity you don’t have to worry much about tunnel collapses. We need to know about tunnelling anyway, since people would ultimately need to be inside the asteroid to shelter from cosmic rays on a long expedition.

Stan May 3, 2012 at 23:40

@Bill …Asteroids will have to be parked in lunar orbit until robots become a lot more autonomous than they are. The distances in solar orbit would bring operations to a halt. (Speaking of Sci-fi, the need for autonomous mining robots in space is the most likely breeding ground for those dreaded superior breeds of robot.)
The idea you rebutted of dropping a meteor-sized chunk of ore into the Sahara has another problem as well as the danger to cities: The problem of who can claim space material visavis the Space Act is no longer a problem once the ore is in the Sahara sand. It would simply be the property of Mali, or whatever country it landed in. Goodbye profits.

Joy May 4, 2012 at 4:25

@philw1776
Agree on optical interferometry being one of the few major disappointments in astronomy in the last two decades, it is looking much less practical than we hoped in the 1980s when the Keck twins were being designed. The VLT is also a major disappointment as an interferometer.

The Giant Magellan telescope design, separate primary mirror fragments (all part of a single parabolic surface) aimed at a common secondary mirror is expected to work as all 7 mirror segments will be mounted on the same rigid frame. Going from that to a fleet of mirror fragments flying in formation to the required accuracy is a big leap.

Worst of all, we do have a sort of assembly line for 3 metre class space telescopes, although it produces only about 3 scopes a decade. The last launch, USA-224 in 2011, cost $4.5 billion to LEO. To place a fleet of 150 in solar orbit … possible in theory at a cost of over $1 trillion? The control systems to maintain the formation of such a fleet are decades away in any case. Not to mention that gyroscopes do not last forever, see the history of Hubble servicing. The entire fleet would have to be launched in a very short time. Bradbury’s “Rocket Summer” at last, LOL!

Securis May 4, 2012 at 10:20

@bill: There are similarities between Dead Space and Alien but i thought it would be worth mentioning because it is a modern/nowadays story of space mining (from 2008 to be precise). Also i found the idea of “planet cracking” as a means of space mining quite interesting. Watch this intro when you want to get a glimpse of what the game is like:

http://youtu.be/Pv67FStr0jk (contains a little gore)

ljk May 4, 2012 at 11:12

Some day I hope Hollywood and the public will realize that space exploration can be exciting all by itself without invoking horrible looking and vicious alien mutants.

And I am also beyond tired of the gritty-looking spaceships. Unclean spacecraft can spell disaster to a crew and mission. That’s why NASA flipped out when John Young brought an unauthorized corn beef sandwich aboard Gemini 3 in 1965. They feared that any floating crumbs could get into delicate equipment and cause problems.

http://news.discovery.com/space/the-case-of-the-contraband-corned-beef-sandwich.html

http://www.strangehistory.net/2010/09/20/the-cornbeef-sandwich-that-almost-destroyed-a-spacecraft/

Eric May 4, 2012 at 13:03

Please see my comments regarding Planetary Resources’ first generation of telescopes, TESS, and the TESS Project’s Sara Seager here:
http://scienceblogs.com/catdynamics/2012/04/linkedy_links_ii_1.php#comments

The future looks bright for transit searches around nearby stars!

Paul Gilster May 4, 2012 at 13:46

Eniac writes:

This is not strictly true. What Wang and (I think) Paul are referring to is intensity interferometry, which requires only an information link between individual devices.

You’re right, Eniac, with this qualification: I wish I had someone who is really conversant with correlation or intensity interferometry who could write the method up for a layman like myself. If anyone wants to have a go at it, please let me know — the method seems to have major implications, as Eniac says. For that matter, Eniac, want to have a go at it yourself?

Wojciech May 4, 2012 at 14:50

“ESA launched Herschel at 1.1 euro billions. Guessing that each of the 150 telescopes would have a similar complexity, the cost is such that it’ll never happen”
I would to know just as you what will happen in 10.000 AD ;) In any case-a lot of costs are connected R&D, reproducing the same design on more mass scale lowers the costs significantly.

“Even considering the economy of scale, there is still the launching cost for each telescope.”
Launching costs with such large number would go down, and besides-who says you have to launch them ;)

Eniac May 4, 2012 at 18:05

bill:

The idea of directing an asteroid to hit a deserted place on earth so that you could pick up the metals sounds like a recipe for disaster. One small error and you could end up taking out a city.

Stan:

The problem of who can claim space material visavis the Space Act is no longer a problem once the ore is in the Sahara sand. It would simply be the property of Mali, or whatever country it landed in. Goodbye profits.

Such considerations are moot in the face of reality. A finished platinum ingot need be no bigger or more dangerous than any old returning space capsule. Reentry, landing, and retrieval of these has lots of precedent with a high rate of success.

Eniac May 4, 2012 at 18:09

Interstellar Bill:

Also, regarding asteroid deflection, don’t forget tethers. Put some mass from the asteroid on the tether and let it slide out and get released. This could be used to alter the asteroid’s rotation as well as its momentum vector.

Excellent suggestion. Rather than simply releasing the mass, it is better to spin it up on a rotating tether and then let go once it has reaches an “exhaust velocity” of 2 km/s or so, which is about the maximum that a good tether can sustain. This way, you can use solar power and no propellant other than the asteroid material itself.

Eniac May 4, 2012 at 18:44

For that matter, Eniac, want to have a go at it yourself?

I can try to give my own limited understanding of it. Conventional interferometry brings light from two different paths together so they can interfere constructively or destructively based on their respective phases. In 1956, Hanbury Brown and Twiss discovered that some of the phase information can be recovered from the correlation of temporal fluctuations in the intensity of two detectors. The effect was first used to determine the diameter of many nearby stars using a 10-200 m baseline at the Narrabi interferometer. More recently, with the advent of high-powered computation, the concept has been extended to multiple detectors, which allows the computation of an image, not just the diameter. Interstellar Bill is probably correct that the number of pixels cannot exceed the number of mirrors, although I am not completely sure of that. The most accessible presentation of the imaging work (to me) has been this paper: http://planetimager.com/docs/Orbiting-Constellations.pdf.

Your milage may vary…

Eniac May 4, 2012 at 19:00

Worst of all, we do have a sort of assembly line for 3 metre class space telescopes, although it produces only about 3 scopes a decade. The last launch, USA-224 in 2011, cost $4.5 billion to LEO. To place a fleet of 150 in solar orbit … possible in theory at a cost of over $1 trillion?

I believe the Planetary Resources telescopes are designed to be light, simple and low-cost (at $5- $10 million, as I recall?). For intensity interferometry you do not need high quality optics, so it would seem that a reasonable flotilla of a few hundred detectors could be flown for ~$ 1 billion.

Unrealistic? Perhaps. I wish them luck….

Eniac May 4, 2012 at 19:32

Let’s also remember that each mirror needs a co-orbiting coronograph-style occulting disc to block the light of the primary star of the planet.

I believe the idea here is that the starlight will be removed computationally, in a similar manner as the image is resolved. In a way, the setup is a coronagraph, one with spectacularly high resolution at that.

It all sounds really great, but exoplanet light is extremely sparse. I am not sure if feasibility under this aspect has been completely established, even though some of the recent literature does give that impression.

ljk May 7, 2012 at 10:14

As I recall growing up, in my neighborhood it was the rich families who were usually the stingiest with their money.

My presumption is that these rich guys would not be investing the many millions of dollars that they are on this planetoid mining plan without knowing that they are going to be making back a lot and them some on their investment. And so publicy at that.

In summation, they will do it and it will work. And we will finally gain a permanent foothold into the rest of the Sol system. And has G. Harry Stine once said, once you are in orbit you are halfway to everywhere else.

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