Asteroid Deflection

April 19, 2012

One characteristic of modern laboratories is the plethora of warning signs. We see warnings about high voltage, X-rays, toxic chemicals, toxic gases, and so forth. As they say, "A word to the wise is sufficient" (Verbum sapienti satis est). The Earth has a proximate warning sign, also. The cratered surface of the Moon warns us that we live in a dynamic solar system in which celestial bodies frequently pummel each other.

There's an interesting episode of Star Trek (Season 3, Episode 3), entitled The Paradise Syndrome (October 4, 1968, Jud Taylor, Director) that deals with a potential asteroid impact on a primitive civilization.[1] Not even Spock, with all the power of the Enterprise, could divert an asteroid on a collision course with this populated planet.

The primitive people were saved by a device put there in a distant past by an alien race. The device successfully deflected the asteroid. The Earth, unfortunately, does not have any alien asteroid deflectors, so we need to handle things ourselves.

There goes the neighborhood! This is an artist's illustration of a supposed asteroid impact on Earth about 250 million years ago. This impact, off the northwestern coast of Australia, left a 125 mile diameter crater and caused massive extinctions. (NASA image, Continental Dynamics Workshop/NSF).

Humans, the putative caretakers of this planet, have existed as a species for about 200,000 years, but we've only had the capability to front a planetary defense in the last few decades. Meteors were considered to be omens from the gods until just a few hundred years ago.[2] They were collected, given poetic names, such as as the Thunderstone of Ensisheim, and eventually their origin as asteroids was deduced. Presently, we have the capability to track these in space and launch spacecraft to their vicinity.

Interest in a possible collision of an asteroid with the Earth was piqued early this month when the 150 foot (45 meter) asteroid 2012 EG5 passed between Earth and the Moon's orbit, a mere 143,000 miles (230,000 km) from Earth's surface, on April first.[3] This asteroid was discovered just two weeks prior to that. This isn't an extinction-sized asteroid, but it could cause the equivalent destruction of a nuclear weapon.

Another near-Earth asteroid, 2012 DA14, is about the same size (45 meters), and it has a mass of about 120,000 metric tons. This asteroid is expected to pass within 48,000 miles (77,250 km) of Earth on February 15, 2013. This is just 20% of the distance of the Earth to the Moon.[4] Geosynchronous satellites orbit at an altitude of 22,236 miles (35,786 km).

Asteroid 2011 AG5, with a diameter of about 140 meters and a mass of about four million metric tons, is predicted to pass within a million miles of Earth in 2023, and ten million miles in 2028. In each such pass it will be affected by Earth's gravity, so there's a very small chance that its trajectory might change enough for it to impact Earth in 2040.[8]

Quite worrisome is the asteroid known as 99942 Apophis, which has a diameter of about 270 meters, and a mass of about 27 metric tons. Its orbit, as presently known, has it coming closer to Earth than a geosynchronous orbit on April 13, 2029. If the orbit of 99942 Apophis is greatly affected by Earth's gravity during the 2029 encounter, it might impact the Pacific Ocean in 2036, causing widespread tsunami devastation.[8]

So, what is our possible defense strategy? The first important part is identifying all asteroids with a potential to impact Earth, the so-called PHOs, potentially hazardous objects. NASA has been surveying asteroids within a radius of 120 million miles from Earth in its Spaceguard program. Spaceguard has found no extinction-sized asteroids of the type that caused the demise of the dinosaurs about 65 million years ago, but it has found 20,500 objects with the destructive potential of a nuclear-weapon.[3]

NASA has studied quite a few possible methods for diverting an asteroid from an impact orbit with the Earth.[5-6] Some of these use techniques that would slowly nudge the asteroid's orbit while it's still at a distance from the Earth. Others rely on a single, high energy event. These are tabulated in a recent NASA report,[5] and they appear below.

Impulsive Alternatives (Table 14 of Ref. 5)[5]

Impulsive Technique	Description
Conventional Explosive (surface)	Detonate on impact
Conventional Explosive (subsurface)	Drive explosive device into PHO, and then detonate
Nuclear Explosive (standoff)	Detonate on flyby using a proximity fuse
Nuclear Explosive (surface)	Impact, detonate using a contact fuse
Nuclear Explosive (delayed)	Land on surface, detonate at an optimum time
Nuclear Explosive (subsurface)	Drive explosive device into PHO, and then detonate
Kinetic Impact	High velocity impact

Slow Push Alternatives (Table 15 of Ref. 5)[5]

Slow Push Technique	Description
Focused Solar	Use a large mirror to focus solar energy on a spot to vaporize material
Pulsed Laser	Rendezvous, position spacecraft near PHO and focus a laser on surface to vaporize material
Mass Driver	Rendezvous, land, attach, mine material and eject material from the PHO at high velocity
Gravity Tractor	Rendezvous with PHO and fly in close proximity for extended period, Gravitational attraction provides a small force
Asteroid Tug	Rendezvous with the PHO, attach to the PHO and push
Yarkovsky Effect	Change the albedo of a rotating PHO. Radiation from sun-heated material will provide a small force as the body rotates

The Yarkovsky effect is a little known effect that's based on selective heating of a rotating body. Since photons carry a small momentum, an anisotropic emission of thermal photons, caused by selective heating of an asteroid, will reshape its orbit, albeit slightly.

A particular laser pushing method is being investigated at the University of Strathclyde, Glasgow, Scotland. It's based on a swarm of small satellites flying in formation and firing solar-powered lasers.[7]

NASA has been working hard to dispel the notion that an asteroid is somehow involved in the apocalypse prediction of the Mayan calendar.[3] Noted popularizer of astronomy, Neil deGrasse Tyson, recently wrote in Wired that "The chances that your tombstone will read 'Killed by Asteroid' are about the same as they'd be for 'Killed in Airplane Crash.'[8] Tyson had a cameo appearance on The Big Bang Theory.[9]

The possibility of using asteroids as weapons was depicted in the 1955 film, This Island Earth (Joseph M. Newman, Director). In this film, Zagon spacecraft launch an asteroid bombardment of the planet, Metaluna.

References:

1. The Paradise Syndrome, Star Trek (Original Series), Season 3, Episode 3, October 4, 1968, Jud Taylor, Director, on the Internet Movie Database.
2. Massimo D'Orazio, "Meteorite records in the ancient Greek and Latin literature: between history and myth," Geological Society, London, Special Publications, January 1, 2007, vol. 273, pp. 215-225.
3. Rob Waugh, "Bus-sized asteroid hurtles past Earth - so close it flew UNDER the moon," Daily Mail (UK), April 2, 2012
4. Asteroid 2012 DA14 Page on Wikipedia.
5. Report to Congress: Near-Earth Object Survey and Deflection Analysis of Alternatives, National Aeronautics And Space Administration, March 2007.
6. Final Report of the Ad Hoc Task Force on Planetary Defense, NASA Advisory Council, Dr. Tom Jones and Mr. Rusty Schweickart, Co-Chairs, October 6-7, 2010.
7. Engineers set their sights on asteroid deflection, University of Strathclyde Press Release, March 25, 2012.
8. Neil deGrasse Tyson, "We Can Survive Killer Asteroids — But It Won't Be Easy," Wired, April 2, 2012.
9. Neil deGrasse Tyson Cameo appearance on The Big Bang Theory (YouTube video clip).