How To Stop an Asteroid
The Chelyabinsk, Russia Meteor
On June 22nd, 2011, astronomers working with the Lincoln Near-Earth Asteroid Research (LINEAR) project at the White Sands Missile Range in New Mexico made a startling discovery: a house-sized space rock hurtling toward our planet at twelve thousand miles per hour (19,000km/h).
The asteroid, designated 2011 MD, was due to make its closest approach just five days later, on June 27th. Some quick calculations of the asteroid's trajectory showed that it was going to miss the Earth by about 7,500 miles (12,000km) - well below the orbit of most communications satellites. The asteroid passed by our planet on June 27th as predicted, giving a few amateur astronomers an exciting show but leaving us unharmed.
The close encounter with 2011 MD illustrates the very real danger of asteroid impacts. Our planet is littered with the scars of previous asteroid impacts - some quite recent on a geologic time frame. The 3/4 mile-wide Barringer Crater near Flagstaff, Arizona was created by an asteroid 150 feet (45 meters) long - at the upper end of the estimated size of 2011 MD.
Incoming meteors can cause damage well before impact. On February 15, 2013, a space rock 55 feet (17 meters) wide and weighing an estimated 10,000 tons entered the atmosphere over Siberia, exploding over Chelyabinsk, Russia and releasing about 500 kilotons of energy. The shockwave from the Russian meteor's entry and disintegration shattered windows and damaged structures all over the southern Russian city, causing more than one thousand injuries - mainly due to flying glass.
The consequences of an asteroid impact can be far more serious than this. Impacts have been linked with multiple mass extinction events, including the massive impact 65 million years ago that wiped out about 70% of all species on the planet.
So far, we have been lucky that an extinction-level impact event has not occurred in the span of human history. Given enough time, however, our luck will run out. We will find a space rock on a collision course with our planet.
However, unlike all of the previous species who died out from the effects of past impact events, we can do something about it.
Likelihood of collision is zero
Routine discovery that poses no unusal level of danger
Merits attention by astronomers, no cause for public attention or public concern
1% or greater chance of collision capable of localized destruction.
1% or greater chance of collision capable of regional devastation
A close encounter posing a serious, but still uncertain threat of regional devastation.
A close encounter by a large object posing a serious but still uncertain threat of a global catastrophe
A very close encounter by a large object posing an unprecedented but still uncertain threat of a global catastrophe
A collision is certain, capable of causing localized destruction and possible tsunami
A collision is certain, capable of causing unprecedented regional devastation
A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it
Step 1. Identify Near-Earth Asteroids
Since the early 1990s, a collaborative effort by NASA, the European Space Agency, and other space agencies informally known as Spaceguard has been surveying the skies for Near-Earth Asteroids (NEAs). Although scientists had been warning about the potential hazards of an asteroid or comet impact for decades, the issue only began to receive serious attention from the U.S. Congress after the Comet Shoemaker-Levy 9 was witnessed impacting Jupiter in July 1994.
Suddenly, asteroid impacts moved from the realm of science fiction to serious threat. Today, telescopes commanded by survey teams in Australia, Spain, Germany, Japan, the United States, and other countries monitor the skies nightly in an effort to catalog the asteroids in Earth's neighborhood. To date, more than 8,700 have been found. Many of these pose no threat to us - their orbits do not intersect with Earth's at any point and thus there is no risk of collision.
The asteroids that do intersect with Earth's orbit and are known as Potentially Hazardous Asteroids, or PHAs. Of the 8,700 Near Earth Asteroids discovered, 1,300 are considered potentially hazardous. Of these 1,300 PHAs, more than 150 are greater than 1 kilometer (.6 miles) in diameter - these asteroids pose the greatest risk to life on Earth.
Step 2; Determine the Probability of Impact
Once a Near Earth Asteroid is discovered, its orbit must be calculated to determine whether or not it poses a threat. This is no easy task, requiring multiple observations to refine estimates of the asteroid's orbital path and size.
One observation tells astronomers very little about the asteroid - merely where in the sky it is and how bright it is. It is not possible to determine either its size or distance from the first observation, as it might be a small asteroid close to us or a giant asteroid farther away.
Repeated observations of the same object allow astronomers to make better and more precise determinations of the asteroid's size and orbit, as well as determine whether the asteroid is a potential threat. This need for repeated observations is why the impact probability and margin of error often fluctuates wildly when a new asteroid is discovered.
Once an Earth-crossing asteroid's orbit has been established, modeling of its future orbits can see if it and the Earth will be in the same place at the same time in the near future. This potential for collision is then rated according to the Torino Scale, with 0 meaning no chance of collision and 10 meaning the end of life as we know it. To date, only one asteroid is rated higher than 0: 2007 VK184 is currently a 1 on the Torino Scale - not yet a cause for concern, but worthy of further study.
If and when an asteroid is discovered that is on a path to collide with Earth in the near future, we will need to use an asteroid deflection strategy in order to change this path. There are currently several proposed strategies being studied.
Good Ideas from the Bad Astronomer
Step 3. Change the Asteroid's Orbit
Scientists have studied a number of potential methods for diverting an asteroid from its collision course with Earth:
The Nuclear Option
In the 1999 sci-fi blockbuster Armageddon, Bruce Willis and a rag-tag bunch of deep-ocean oil drillers are [Spoiler Alert] dispatched to trans-lunar space to plant a nuclear warhead inside an oncoming asteroid and blow it up mere hours before impact. While this movie was plagued with a comical array of scientific errors, the writers were on to something with their asteroid-mitigation strategy. On a short time-frame, a nuclear warhead could be used against an oncoming asteroid.
However, blowing the asteroid up would not necessarily be the best course of action. This may only turn one large impactor into a few dozen small impactors, spreading the asteroid's destruction over an even larger area of our planet.
A more effective use of a nuclear weapon would be a detonation of a fusion warhead near the asteroid's surface. This would vaporize some of the rock on one side of the asteroid, while the injection of neutrons deep into the rock would cause it to eject some surface material. The change in mass and inertial rebound from the ejected material would cause a slight change in the asteroid's orbit. Given enough lead time, it would be enough of a change for the asteroid to miss Earth.
While this option has some potential, it also carries some risks. The timing and placement of the blast would have to be extremely precise - no easy task when trying to place a warhead on a city-block sized rock of unknown composition from several million miles away. There is also the obvious risk involved with launching a nuclear warhead on a deep-space mission, and the consequences of a launch failure could be even more devastating than the asteroid impact itself.
The Preemptive Strike
Another deflection strategy being explored by astronomers woukd give the asteroid a taste of its own medicine. The kinetic impact deflection method would steer a heavy spacecraft onto a collision course with the asteroid, using the force of the impact to change the velocity of the asteroid and bump it into an Earth-missing orbit.
This method has some advantages over the nuclear weapon strategy. Steering a heavy object to crash into an asteroid requires far less precision than detonating a warhead near it. Furthermore, we've already had some practice - the Deep Impact mission crashed an impactor into comet Tempel 1 in July 2003 in order to sample the surface material. A planned mission by the European Space Agency named Don Quijote would serve as a practice run for this type of asteroid deflection strategy, launching a heavy impactor into a Near Earth Asteroid to measure its effect on the asteroid's orbit.
The effectiveness of the kinetic impact strategy may depend on the composition of the incoming asteroid. If the target is a clump of loose rock rather than a solid object, it may merely stir up the boulders a bit without moving them off their collision course.
Which asteroid-deflection method do you prefer?
The Gravity Tractor
A more subtle but also promising strategy would use a heavy spacecraft not as an impactor but a tugboat. By parking a space probe into a heliocentric orbit alongside the target asteroid, this gravity tractor method would use the slight pull of the probe's gravity to ease the asteroid off course.
An ion engine, such as the one used on the Dawn mission to asteroids Vesta and Ceres, would allow the probe to gently push away from the asteroid's gravitational pull. This constant gravitational tugging between the probe and the asteroid would change the asteroid's orbit enough to miss the Earth.
This proposed deflection strategy would require some substantial lead time, as it would take several years for a gravity tractor to change the orbit enough to avoid a collision. In addition, this proposal and the kinetic impact strategy both require a heavy-lift launch vehicle capable of putting a multi-ton probe into deep space. This launch vehicle does not currently exist, but NASA and some private companies have such a vehicle in development.
Solar Sails and Paint Jobs
A few other proposals that have been floated would use even more subtle methods to steer an Earth-threatening asteroid off course. Both of these use the Sun as their energy source. In one proposal, a highly reflective solar sail would focus the Sun's energy on a portion of the asteroid, using this energy to shift its orbit. A similar proposal would paint a portion of the asteroid black with soot or white with titanium dioxide to change its absorption of sunlight.
Both of these proposals exploit a force known as the Yarkovsky effect, discovered in 1900 by Russian engineer Ivan Osipovich Yarkovsky. The constant heating and cooling of a rotating body in space causes slight changes to its orbit over long periods of time. This force has been observed to slightly alter the orbits of asteroids in our solar system, and could be used as a long-term collision avoidance strategy.
Time: The Key Ingredient
Looking over the strategies listed above, the one factor they all require for success is lead time. There is no system that can stop a hazardous asteroid discovered days, weeks, or even months before impact. This is why the ongoing sky surveys are the most crucial element in this effort. The longer we have to plan and launch a mission to deflect an oncoming asteroid, the lower our chances will be of going the way of the dinosaurs.
Sources and Further Information
- NEO Discovery Statistics
The table below shows the total number of NEOs known at selected dates. This table is updated daily using discovery data published by the Minor Planet Center.
- B612 Foundation
The B612 Foundation is an organization dedicated to ensuring the survival of humanity on this planet.
- Earth Impact Database
The Earth Impact Database (EID) comprises a list of confirmed impact structures from around the world. To date, there are 182 confirmed impact structures in the database.
- Current Impact Risks
The following table lists potential future Earth impact events that the JPL Sentry System has detected based on currently available observations. Click on the object designation to go to a page with full details on that object.
- Near-Earth Object (NEO) - Analysis of Transponder Tracking and Gravity Tractor Performance.
A study, requested and funded by the B612 Foundation, was carried out by JPL scientists to detemine the feasibility of using a gravity tractor concept for use in NEO impact mitigation and to build credibility for the concept.
- News Article: Precision NEO Orbits and the Yarkovsky Effect
Radar studies of NEA Golevka have demonstrated the reality of the Yarkovsky effect for the first time.
Recent Impact Craters in North America
160-meter impact crater, formed c. 50,000 years ago
10-meter impact crater, formed c. 1000 years ago.
1.18 km impact crater, formed c.49,000 years ago
40-meter impact crater, formed c.1100 years ago
3.4-kilometer impact crater, formed c.1.4 million years ago.