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Tuesday July 10th 2012

The Asteroid and the Telescope

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.


This article was originally posted on Centauri Dreams