Weekend! Which means I finally have the time to put up another entry or two.
What a month for near-Earth asteroid studies! First there was the Caltech Space Challenge, where undergraduate and graduates from all over the world came together to study a manned sample return mission from a near-Earth asteroid/object (NEA/NEO). Then this week was the start of a professional workshop to study moving a small asteroid to a the L1 Lagrangian point (Lagrangian points are locations in a two-body system (say Sun-Earth) such that any body at that location will remain stationary in the Earth's rotating frame), where it can then act as a staging area/refuelling station for missions into deeper space. Asteroids at Lagrangian points are not new - we have known of the Jovian Trojans, and just recently the Wide-Field Infrared Survey Explorer (WISE) spacecraft found an asteroid at Earth-Sun L4. (http://blogs.discovermagazine.com/badastronomy/2011/07/27/wise-finds-the-very-first-earth-trojan-asteroid/
So the workshop began with a public panel discussion organized by the Keck Institute of Space Studies (KISS) and the Planetary Society (TPS) right here in Caltech Cahill Hameetman Auditorium. The discussion was moderated by Lou Friedman, the co-leader of the Keck Institute Asteroid Retrieval Mission Study (ie the workshop I was referring to) and the former Executive Director of TPS, and the panel included Rusty Schweickart, former Apollo astronaut and Chairperson of the B612 Foundation (a private organization to protect Earth from asteroid strikes), Tom Jones, former shuttle astronaut, John Lewis, planetary scientist, and Bill Nye, the current Executive Director of TPS.
The discussion started with Rusty explaining the hazard posed by asteroids. Asteroids impact events at the Tunguska scale have a period of several hundred years, more than the typical lifetime of a human and thus it may feel "remote" to some people. Nonetheless, asteroids present a natural hazard that can be predicted many years in advance, and mitigation measures can be taken. These measures can be classified into two kinds: civil defense (ie to run xD) or to deflect the asteroid by some means, perhaps by kinetic impact (send an impactor hurtling into the asteroid) or by gravity tractor (send a spacecraft near the asteroid to pull it off course by mutual gravity). He pointed out that both technologies are in fact available now, and should there be an asteroid coming towards Earth, what is stopping us from doing anything would mostly be politics. The uncertainty in the orbit of the asteroid, the deflection method and possible unintended consequences would lead to long discussions that may delay any action before it is too late. He gave an example. Say an asteroid is believed to impact the Earth with its landing eclipse centered in the Atlantic Ocean but spanning from the US to Russia. Should we slow the asteroid (ie move it to the West) or speed up the asteroid (move it to the East)? What if the deflection attempt is aborted halfway, and the asteroid is insufficiently slowed down, and thus its landing eclipse is squarely over the US and not off the planet as intended? Deflection of the asteroid off its orbital plane would be ineffective, resulting in mostly a cyclic motion about its original orbit.
Rusty then gave the example of the asteroid 99942 Apophis, which will pass close to the Earth on 13 April 2029 2100 UTC. At this close pass, the Earth's gravity will change the orbit of the asteroid. This change may in fact result in the asteroid orbit intersecting with Earth's orbit, and lead to a future impact. Such narrow windows have been termed "keyholes". Fortunately, Apophis is not expected to go through any keyhole in its 2029 pass, but it highlights the threats we have due to the NEA around us.
Source: Rusty's Presentation
Tom Jones proceeded to talk about mission to study asteroids. The complexity of any manned mission and the uncertainty with the topology and spin characteristics of the asteroid requires robotic scout missions. Studies of the radiation environment shows that radiation effects of staying 180 days in deep space do not have too adverse effects on the astronauts, and Tom also outlined various technologies that can be used (e.g. jetpacks (Manned Maneuvering Units), ropes and exploration vehicles). Such technologies can be tested on the International Space Station (ISS), and perhaps since not all components of the ISS expire by 2025, some components can be reused instead of being sent to burn up in the atmosphere.
John Lewis took a different approach, describing asteroids as places of opportunity for science and resources. Not only are asteroids made up of primitive material from the formation of the solar system, they also have abundant resources. Carbonaceous (C-type) asteroids have 6% organic matter and 10% water, useful for sustaining astronauts and also as a source of fuel. He pointed out that the smallest known asteroids contain more metals than what has been mined on Earth so far, and that NEAs, with their orbits going out to the asteroid belt, can serve as traveling hotels for astronauts to hitch a ride to further destinations.
Bill Nye then suggested other interesting ideas for asteroid deflection, including the use of a swarm of small small satellite equipped with mirrors, cutely named "Mirror Bees", to deflect the asteroid using radiation pressure.
Thursday also saw the release of new results from the WISE mission (NASA news conference here
). While originally designed to look at cool stars, WISE has turned out to be very useful in searching for asteroids in our solar system. The low temperatures of the asteroids make it easier to observe them in IR (Wien/Planck's law), but apparently IR observations also give a more accurate measurement of the size of the asteroid.
I am not so sure how this works though. True I can understand why chalk will appear much brighter than charcoal in optical and perhaps of similar brightness in the IR, but certainly IR is not any more special from optical and minerals/molecules can appear very different in IR too. A direct conclusion that a brighter objects in IR means a larger object seems as naive as a similar conclusion in optical. Perhaps someone can enlighten me about this.
Nonetheless, WISE completed a one-year survey of the sky, cataloging more than 90% of NEAs with diameters greater than 1 km. While the estimated number of asteroids agree with the number detected at this size, there seems to be about 40% fewer asteroids at smaller diameters than previously expected as estimated from WISE data.
While the smaller number of estimated asteroids may mean that Earth may be safer than we originally thought, we have to remember that it does not take that large an asteroid to devastate large areas on the Earth. Meteor crater was due to an asteroid ~50m across, and the line of "?" for that size category in the chart above is certainly not very comforting.
Asteroid studies have just started. There are new discoveries every day, but they merely highlight how much we do not know about them. As Rusty pointed out, the understanding of these NEAs and an ability to manipulate them would be an "entrance exam" into the league of space-faring civilizations. It is only in this way that we can take the first step in protecting our survival as a species against threats from space.