When a 60-foot meteor exploded 15 miles above the Russian city of Chelyabinsk almost exactly six years ago, the world was reminded of a sobering reality: thousands of rocky or metallic objects big enough to damage Earth are speeding through space not far from our planet.

To understand and reduce the risks that these can cause, a team from Lawrence Livermore National Laboratory is taking part in a federal program that aims to identify the most threatening of these Near Earth Objects and determine how to respond if they seem likely to hit Earth.

The partial government shutdown has temporarily grounded some parts of the federal effort, but LLNL research continues. Its team has some 20 members with diverse skills ranging from materials science to astrophysics, shockwave physics to computer modeling.

All these skills and more may be required, since possible governmental responses to an impending collision range widely.

As if to dramatize the dangers lurking in space, the dramatic 2013 Chelyabinsk meteor explosion was followed a few hours later by the passage of another big rock in the opposite direction at a distance of some 17,000 miles from Earth.

While the Chelyabinsk event injured more than 1,200 and damaged some 7,000 buildings, the other asteroid caused no damage. It was noticed mainly by astronomers and space buffs as news reports focused on the dramatic events in Russia.

Still, the day’s scary events nudged the U.S. further in the direction of a serious program to defend against future impacts.

Although impact craters on the moon had been visible through telescopes for centuries, the idea that the Earth could suffer similar collisions was somehow not taken seriously until relatively recently.

It wasn’t until 1960 that Eugene Shoemaker proved that Meteor Crater in Arizona was an impact crater.

In the years that followed, scientific expeditions and improved modeling demonstrated that a huge, forest-flattening explosion over Tunguska in Siberia in 1908 was caused by the arrival of some kind of object from space, although the exact nature of the object is still debated.

For geography buffs, Tunguska is some 1,200 miles east of Chelyabinsk, which is less than 100 miles from Snezhinsk, Livermore’s sister city, home of Russia’s scientific equivalent of LLNL.

Near Earth Objects

For several decades, NASA has been tracking Near Earth Objects (NEOs). Congress got involved in the 1990s by requiring NASA to identify at least 90 percent of the NEOs of a kilometer or more in size. (A kilometer is nearly two- thirds of a mile.)

NASA believes it has found nearly all of those, with none in an orbit that endangers Earth. Still, if an object that large were to hit Earth, the collision could threaten life around the globe.

NASA has also been creating an inventory of smaller but still dangerous NEOs ranging in size from a few meters to hundreds of meters. Some 20,000 have been identified to date, with thousands more thought to be orbiting the sun not far from near Earth’s orbit, difficult to spot because of their size.

The Chelyabinsk meteor is thought to have been about 20 meters across. It exploded with the energy equivalent of perhaps 20 Hiroshima bombs, doing limited damage because it entered Earth’s atmosphere at a shallow angle and exploded 15 miles high.

The rock that passed in the other direction, labeled 2012 DA14, was estimated to be 50 percent larger.

The 1908 Tunguska explosion flattened an area of Siberian forest roughly the size of Alameda County.

Scientists debate whether it was caused by a comet moving in at very high speed from a more distant part of the solar system or an asteroid circling the sun more slowly, but in either case, the explosion is estimated to have generated the energy equivalent of a multi-megaton thermonuclear bomb.

Planetary Defense Team

Not surprisingly, given LLNL’s decades of experience in developing nuclear and conventional explosions and modeling their effects on various materials and the atmosphere, the Laboratory brings special expertise to the problem.

According to team leader Megan Bruck Syal, who has a background in both geophysics and astrophysics, the national effort falls under the overall responsibility of the Department of Homeland Security and involves a range of federal agencies and scientific institutions like Sandia and Los Alamos National Laboratories in addition to LLNL.

Speaking to the Livermore Laboratory’s Retirees Association in November, she said that LLNL has worked particularly closely with colleagues at NASA’s Goddard Space Flight Center in Maryland.

There is international participation as well. Syal described a “tabletop” exercise in Tokyo that simulated a 250 meter asteroid crashing into the Sea of Japan, generating large waves that caused damage and required emergency rescues along the coasts of Japan, South Korea, North Korea and Russia.

Another exercise considered the impact of a 400 meter asteroid smashing into the Gulf of Mexico, generating huge, destructive waves. but also releasing so much chlorine from salty water that atmospheric concentrations of chlorine gas were doubled.

This hypothetical but plausible scenario was described last week to the Valley Study Group by another of the LLNL researchers, materials scientist Tané Remington.

Beyond the near-term destruction caused by the asteroid strike, Remington said that the climate was affected. The chlorine combined with hydrogen to create hydrogen chloride, which circled the globe and lowered worldwide temperatures by 1-3 degrees C., enough to threaten the onset of an Ice Age.

How to Respond?

The effect of an asteroid strike depends not only on the size and mass of the object coming in from space, but also where it hits, how fast it is moving and its path through the atmosphere.

Space debris is hitting the Earth’s atmosphere all the time: dust-like particles that drift in unobserved, bits of sand that streak across the night sky as shooting stars and occasionally larger pieces that reach the surface of the earth.

What happens if NASA identifies a large asteroid that is seriously dangerous because its orbit makes it look as if it will be hitting Earth someday, and it is too big to ignore?

The possibilities, Syal said, range from accepting the hit – evacuating the strike area, trying to push the asteroid into a different and safer orbit, or trying to pulverize it into harmless space dust.

Any of these approaches could be an option depending on the threat. Decisions would not be made by the scientists, of course, but by governmental authorities after input from technical and emergency response experts.

They would consider the answers to questions like these:

Is the approaching asteroid too close to deflect? How big is it, how fast is it moving and does it look as if is hitting the middle of the ocean or a heavily populated area?

Is it small enough to be pulverized? Is it solid or loosely aggregated? How fast is it spinning? Does it have an irregular surface or is it smooth and symmetric?

The most practical course might turn out to be taking no preventive action beyond evacuating and protecting people in the impact zone.

A second option is kinetic impact, sending up one or more rockets to smash into an asteroid and nudge it off its destructive path, Syal said. How to do that successfully will depend on a detailed understanding of the asteroid.

With that in mind, LLNL modelers like Remington are simulating kinetic impacts on asteroids of various sizes, shapes, composition and dynamics.

A very heavy asteroid might have to be hit by more heavy rockets than are available, suggesting another option: nuclear deflection.

The idea would be to explode a powerful nuclear device at a distance so that x-rays boil off the asteroid’s surface like a jet. The jetting action would push the asteroid into a new and safer path.

A third approach would be to blast the asteroid into small pieces that would burn up in the Earth’s atmosphere – to “shred” it, in Remington’s words.

This approach, like the others, would have to be carefully modeled in advance to make sure the result was improved safety for Earth. It wouldn’t do to break one large asteroid “into four large pieces” that continue traveling to Earth, Remington said.