Laser asteroid defense

Could lasers protect the Earth from wayward asteroids? A number of schemes have been proposed for pushing asteroids gradually to move their orbits away from the planet. Now, two California professors are proposing a bold scheme to build solar-powered space lasers powerful enough to evaporate a 500 m asteroid in about a year--or to make short work of a 17 m asteroid like the one that exploded near Chelyabinsk, Russia, on February 15.

Philip Lubin of the University of California (Santa Barbara, CA) and Gary Hughes of California Polytechnic State University (San Luis Obispo, CA) began planning the project they call DE-STAR--for Directed Energy Solar Targeting of Asteroids and exploRation--a year ago. On February 14, they issued a press release timed to the close approach by asteroid 2012 DA14. They were as stunned by the Russian explosion as everyone else.

Their bold proposal seeks to take advantage of the dramatic improvements in high-power diode lasers and solid-state lighting to build giant orbital phased arrays of lasers powered by electricity from huge solar panels. They envision starting with a desktop 1 m array called DE-STAR 0, then scaling up to a 10 m array called DE-STAR 1. They have proposed that NASA support a conceptual study of scaling up to a 10 km DE-STAR 4 array, powerful enough to vaporize a half-kilometer asteroid 150 million kilometers away. Even bigger versions could be used for laser propulsion; they estimate that a 1000 km DE-STAR 6 array could accelerate a 10 ton spacecraft close to the speed of light.

Future DE-STAR array samples composition of an asteroid as it propels an interplanetary spacecraft. (Courtesy of Philip Lubin)

The scheme may sound fantastic, but Lubin says it violates no laws of physics and requires no "technological miracles." It merely envisions continuing technological progress at the rate of the past 50 years, which took us from the feeble LEDs and diode lasers of 1963 to today's powerful emitters. They assume photovoltaic cells that can convert 70% of incident solar energy into electricity, and diodes which can convert 70% of the input electrical power into light.

Lubin doesn't think it will be easy. He worries about issues including the mass needed to build the giant array, and controlling output phase across the array with the precision needed to tightly focus the emission. But he predicts his assumptions will be considered "extraordinarily conservative and modest" in 30 to 50 years.

That remains to be seen, but space-based solar-powered diode arrays are worth investigating. They could go beyond asteroid defense to could help move asteroids, collect valuable materials from them, or provide power resources in space--as well as inspiring some fun science-fiction stories.

Seeing Pluto

In July, the Hubble Space Telescope spotted the fifth moon of Pluto, an icy ball 10 to 25 km across that was just a pinprick of light in the image. Much of the press coverage focused on whether that discovery should make Pluto a full-scale planet. But I was far more interested in Pluto, its moons, and the amazing optical feat of finding something so small and so far away.

My interest in optics grew from a fascination with astronomy. I'm old enough to remember the 1978 discovery of Pluto's largest moon Charon. The discovery images show a small bump on the fuzzy ball of Pluto, recorded on a photographic plate by a ground telescope. Comparison of a series of images showed that the bump moved as the unresolved moon orbited Pluto. In the days before adaptive optics, seeing even that much seemed amazing.

Hubble resolved Pluto and Charon soon after its launch in 1990. It was a badly needed success for Hubble in its troubled early years, but scattered light in the background of the photo clearly shows the spherical aberration of the telescope's primary mirror. Pluto and Charon are both blurry and diffuse, but the photo clearly shows them as separate worlds, with Pluto clearly the larger and Charon roughly half its size. Once NASA added corrective optics to fix the spherical aberration, the Faint Object Camera produced much sharper photos in 1994.

Further upgrades have made Hubble even better. In 2005, it spotted two roughly 100 km moons, later named Nix and Hydra. Last year, astronomer Mark Showalter of the SETI Institute (Mountain View, CA) began a series of Hubble observations to check for other little moons which might scatter dust into the path of the New Horizons spacecraft when it visits Pluto in July 2015. Earlier this month, Showalter downloaded a new batch of Hubble data, and in an hour was on the phone reporting the discovery. A few days later, he told me "I'm still struck by just what an amazing instrument Hubble is. This little object, [called] P5, is fainter than Pluto by a factor of 100,000 and separated by one arc second."

It's amazing and wonderful. And so far Hubble's images show New Horizons is on a good path to avoid any dangerous dust, so we can see close-ups of Pluto three years from now.