Clips, Space

Defending Earth’s Cosmic Borders

Red Fez, 13 February, 2016.


The odds of getting killed by a meteorite are 1 in 75 million, according to the U.S. National Safety Council. The numbers suggest, therefore, that we face no threat from beyond the blue.


On a cold morning, on February 15, 2013, around 9:20 a.m., as people were going about their business in the Russian town of Chelyabinsk, just east of the Urals, an object weighing 28 million pounds, hurtled to Earth at 42,000 m.p.h., and exploded in the skies above them in a dazzling white flash that made the Sun look like a lantern in a Marrakesh souk. The spectacular hot blaze could be seen from miles and miles away.

Window panes blew out as the shockwave (created when a wave beats the speed of sound) hit the ground. Shards of broken glass struck people. 1,500 were hurt. The roof of a zinc factory caved in. In the neighboring town of Yemazhelinsk, the statue of Russia’s treasured national bard, Alexander Pushkin, was shattered. Over 7,000 buildings, in six cities, suffered cracks. This was the biggest thing to hit Earth in the last 108 years.

Turn the clock back to another morning—on June 30, 1908, a little after 7:00 a.m. The location is a conifer-rich boreal forest, near the top of the world. An asteroid, 37 meters across, plunges Earthward and as it does so, it heats the surrounding air to a broiling 44,500 Fahrenheit and blows up over Siberia’s Tunguska River.

The blast released the energy equivalent of about 185 nuclear warheads detonated over Hiroshima, Japan. Dense clouds formed over the region. As a side-effect, the night skies were aglow for days after the event. Trees across an area of 825 square-miles were flattened, and herds of reindeer perished.

Much earlier, when Earth was in its infancy, it was smacked by a chunk, half the size of Manhattan. The collision wiped out nearly all the large vertebrates, including the non-avian dinosaurs. Unlike those that appeared over Russia, this space rock made a violent touchdown. The thump created a giant crater, 110 miles wide, near what’s present-day Chicxulub, Mexico.


Earth resides in a swarm of “near-Earth objects” (N.E.O.s, for short)—comets and asteroids that have been deviated by the gravitational willpower of nearby planets into orbits that bring them into Earth’s neighborhood. Primordial denizens, they’re planetary flotsam left over from the formation of the solar system, 4.6 billion years ago. While comets are the debris from the outer planets (Jupiter, Saturn, Uranus, and Neptune), asteroids are detritus from the inner worlds (Mercury, Venus, Earth, and Mars.)

Asteroids are irregular-shaped bodies, anywhere in size between 600 miles and a pea, which float in the belt between Mars and Jupiter. Like the planets, they too, spin around the Sun. The smaller ones—anything below one meter—are called “meteoroids.”

Once a near-Earth object breaches Earth’s protective airy shell, it’s termed a “meteor.” Typically, they’re so small that they burn, and ablate in the mesosphere due to friction. On a cloudless night, far from urban pollution, one may see them as a glowing streak, popularly called a “shooting star.”

Sometimes though, they’re substantial, and come apart with sound and fury, turning into a “bolide” or a “fireball.” Rarely, they survive their long, fiery passage through the Earth’s atmosphere, and slam into a field, a farm, a tarn, a knoll, a garden, a patio—anywhere. At that stage, they’re called a “meteorite.” Additionally, every day, Earth is bombarded by more than 200,000 pounds of stardust and sand grain-size motes, raining down continually, without our knowledge.


In 1992, the U.S. Congress requested NASA to do a study of the near-Earth object population, and assess the risks they posed. The report, “Spaceguard Survey,” also led to a mandate that the agency locate most of the asteroids that were, at least, half a mile or more, within a decade.

It determined that an object, 0.6 miles or more, would have repercussions that’d be felt worldwide. Over and above leaving a vast trail of devastation, locally, it’d also spew so much ejecta into the air as to disrupt the weather pattern a whole continent away. A global pall of dust would block out the Sun. Crops would fail, and people would die of starvation.

A boulder, sill larger—six miles or greater—would trigger mass extinction. Fortunately, the chances of a projectile of this magnitude falling to Earth are very, very, slim. The last time it happened was 66 million years. The bigger the object, the less likely it is to stray into Earth.

Between 1980 and now, NASA’s near-Earth object program has discovered about 13,600 near-Earth objects. Of these, about 900 are gargantuan, making the inventory of the Brobdingnagian objects nearly complete. Some 1,600 of those have been judged “potentially hazardous”—objects that pass within 4.6 million miles of Earth and are robust enough to do harm and have the potential (repeat: potential) to strike.

In 2003, the agency appointed a group of researchers to take a fresh look. They recommended that the near-Earth object census should focus next, on locating midsize objects—those that are about the size of a football field. It’s this class of the near-Earth object crowd that’s worrisome because it could wreak havoc on an area the size of Iowa, depending, of course, on its composition, density, and angle of entry. And even if they don’t make landfall, they’re capable of producing dangerous “air blasts.”

So, in 2005, the U.S. Congress tasked NASA with the job of searching out the bulk of them by 2020. But Paul Chodas, director of NASA’s Center for N.E.O. Studies at the Jet Propulsion Laboratory in Pasadena, California, said in a phone interview that that “goal can’t be reached by that deadline because funding was not provided for the new search facilities that would be needed.”

As they’re smaller and dimmer, they’re more difficult to detect than the huger asteroids, which are brighter. Tracking them down would require more powerful telescopes than the present crop of optical instruments are capable of. So far, only about a quarter have been cataloged, he said.


The need for near-Earth object detection and deflection has gained so much attention lately, that in the last six years, there’s been more than a ten-fold increase in the federal funding for this purpose—jumping from $4 million in 2010 to $50 million in 2016.

In the first week of January, this year, NASA announced that it had consolidated all its ongoing near-Earth object-related projects under one roof—as the Planetary Defense Coordination Office. The new department, which will be headquartered at the agency’s Science Mission Directorate, in Washington, D.C., will be in charge of “planetary defense.”

Once a ground or space telescope detects a near-Earth object, its detail is forwarded to the International Astronomical Union’s Minor Planet Center, housed at the Smithsonian Astrophysical Observatory, in Cambridge, Massachusetts, where it’s put in a comprehensive database. On the opposite coast, at NASA’s Center for N.E.O. Studies at the Jet Propulsion Laboratory, in Pasadena, California, folks pore over the data to determine if it’s on a collision course with Earth.


And what if it is? NASA has partnered with the European Space Agency to gauge whether it has the technology to alter a rogue asteroid’s trajectory to ensure that it misses Earth by a wide margin. The “Asteroid Impact and Deflection Assessment” mission would target Didymos, a binary asteroid system, in which the smaller of the two spins tightly around the other, a short distance away. Two independent spacecraft would be sent to pair: one would orbit the primary, and the other would ram into its “moon.”

The orbiter, operated by the European agency, is set for launch in October, 2020. The second vehicle, guided by John Hopkins’ Applied Physics Laboratory, in Laurel, Maryland, will set off in July, 2021, to rendezvous with its quarry.

In October, 2022, when Didymos is at a close approach to Earth, it’ll pummel its “moon” at about 14,000 m.p.h. (NASA tried out this know-how in 2005, when it ran a spacecraft into comet Tempel-1.) The European vessel will observe closely, and collect data, which would provide a baseline for planning any future such operations. If this procedure works, it’ll be the first time that humankind would have played a role in changing the dynamics of an celestial body in a measurable way.

The other alternative NASA has been working on would attempt to slowly steer a menacing asteroid out of a hazardous orbit by simply, having a heavy spacecraft hover near it. Its Asteroid Redirect Mission would employ a “gravity tractor,” the space equivalent of a tow truck, to gently nudge it without making contact—with only gravity as a towline.

In some cases, an asteroid passes through what’s known as a “keyhole,” a very narrow region of space, where a planet’s gravity would come to act on it—to its own detriment—such that it’d collide with the planet when it swings by it on a future orbital pass. If, however, it could be made to the skip keyhole, it’d safely fly past the planet. That’s where a gravity tractor could do the trick.

Two bodies, in close proximity, “pull” each other with a force equal to their respective masses. Likewise, so does the spacecraft and the asteroid, even though the tug of the former is puny; a David to the latter’s Goliath. But if the vehicle lingered long enough in its vicinity, it’d be able to move it, albeit, imperceptibly slowly.

All that it’d need to do is fire its engines away from it. The feeble, but steady thrust would alter its course, by making it accelerate toward it, and so moving it out of a perilous path. This technique would work with any kind of asteroid, regardless of its size or rate of spin. By making a very minor alteration in an asteroid’s orbit, it’d push the latter out of the keyhole. On the downside, it’d take a long while to be effective.

If the Asteroid Redirect Mission is green-lighted, a spacecraft would be dispatched to an asteroid, in December, 2020. After landing on it, it’d grab a boulder from its surface, with the help of robotic arms and grippers, and then park it in a stable orbit around the Moon, where it’ll not trouble Earth.


The science-fiction writer, Larry Niven, once said: “The dinosaurs became extinct because they didn’t have a space program.” Fortunately, we do—and one that’s been keeping a vigilant eye out for these ominous cosmic gatecrashers.


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