Questions and Answers regarding Near-Earth Asteroids.
David Rabinowitz,on behalf of the JPL Near Earth Asteroid Tracking (NEAT) team.
1998 Feb 24
Comets and asteroids are small planets, ranging in size from baseball-sized meteors to 1/3 the size of the moon. They are the rocky and icy bodies left over from the formation of the solar system. Originally, the population was much larger. With time, however, most of these bodies have either collided together to form the major planets, were ejected from the inner solar system into the Oort cloud, or were ejected out of the solar system altogether. The comets and asteroids we see today are the residual population.
The Earth, and all the other planets in the solar system, have been continuously pelted by asteroids and comets ever since their formation. Just look at the moon through a small telescope or a good pair of binoculars. The surface is covered by craters that were created by impacting asteroids and comets. Similar craters have been observed on the surface of the Earth. One of the most spectacular is a ~1/2-mile- wide crater near Flagstaff, Arizona. If it weren't for the effects of erosion, and the cover of the oceans, the surface of the Earth would look much like the Moon or Mars. It is now known that an asteroid impact caused at least one mass extinction of life on Earth -- the one that wiped out the dinosaurs. The remnant of the resulting crater lies off the coast of the Yucatan Peninsula in Mexico, mostly buried by ocean sediments.
Any object striking the Earth can be dangerous if it makes its way through the atmosphere and you happen to be standing in the way. Most of the time, of course, this doesn't happen. This is because most of the bodies striking the Earth are too small to make it through the atmosphere. They usually burn up, leaving a beautiful glowing trail, popularly known as a shooting star. Some slow down in the atmosphere and land without making much of an impact. These are called meteorites.
Fortunately for us, bodies that are large enough to make it through without slowing down or burning up are very rare. The threshold size is roughly 100 meters. A body of about this size exploded over the Tunguska forest in Siberia in 1908. It flattened about 700 square miles of trees. We believe that such events occur once a century on average, but this estimate is uncertain. Military sensors in orbit about the Earth, which watch for explosions that might indicate a violation of nuclear proliferation treaties, have detected dozens of high- altitude explosions each year. These are caused by meter-sized asteroids that have impact energies equivalent to tens of kilotons of TNT, comparable to the bomb dropped on Hiroshima.
The most dangerous asteroids, capable of a global disaster, are extremely rare. The threshold size is believed to be 1/2 to 1 km. These bodies impact the Earth only once every 1,000 centuries on average. Comets in this size range are thought to impact even less frequently, perhaps once every 5,000 centuries or so.
JPL's Near-Earth Asteroid Tracking Program (NEAT) is an effort to develop a system capable of detecting, in 10 years, most the Earth-approaching asteroids larger than 1 km. There are believed to be 1,000 to 2,000 such bodies, but only 100 or so have been discovered so far.
What the effects would be from a large-scale impact are very uncertain. Most scientists believe that an object in the 100-m to 500-m range could penetrate the atmosphere and cause local damage if it either exploded in the atmosphere or impacted the land or ocean. If the impact took place near a major city, or if it occurred in the ocean near a major city and caused a tidal wave, it could be very deadly. Fortunately, however, most of the Earth is still unpopulated. The chance of this type of impact is probably lower than once every 1,000 to 10,000 years. The bodies larger than 500 m are of much more concern. A single impact could cause global devastation. Calculations that were used to predict the effect of a global nuclear war have been applied to this problem. It is likely that such a large impact would raise enough dust into the atmosphere to change Earth's climate -- in effect, a "nuclear winter". Agricultural production around the world would be severely disrupted.
Actually, some 100 bodies have already been discovered on orbits which take them so close to the Earth's orbit, that they could hit in the far distant future. This is because the orbits of these bodies change slowly with time. Although their orbits do not intersect Earth's orbit at present, they could hit in a few thousand years or more.
The scenario you have in mind is most likely to unfold as follows. In the course of our search for Earth-crossing asteroids, we could find one that will hit not in the next year, or even in the next ten years, but might hit in the next hundred years. We believe that the chance that we will find such an object is only 1 in 1,000, even after a complete search. If we do find such an object, we will have plenty of time to track it, measure its orbit more precisely, and plan a system for deflecting it from its current orbit (hopefully away from the Earth's). There will be no great hurry, and no great panic. It would be a project for all the world's nations to take part in. It could be a globally unifying event. Because we will have found it long before it actually hits the Earth, it probably would take only a small impulse (chemical rockets, or perhaps mass drivers) to divert it from a threatening path.
There is a much smaller chance that we would find one that could impact in the next 10 years. The chance of that happening is 1 in 10,000. If this were to happen, we would probably still have time to launch a crash program of scientific and technological research, with the goal of characterizing both the structure of the menacing asteroid, and the best means for diverting its orbit.
The least likely scenario is that we would find one that could hit in the next year. The chance is 1 in 100,000. In that case, there is probably little that we could do to divert it.
The most significant efforts currently being made are to find all the potentially dangerous asteroids long before they are ever likely to approach the Earth. JPL has initiated such a program, in cooperation with the US Air Force. The project is led by Eleanor Helin, who has a long record of accomplishments in the discovery of these bodies. She and her team of astronomers use a remote-controlled telescope on the summit of Haleakala crater in Maui. It is normally used by the Air Force to keep track of man-made satellites. Dr. Helin uses it 6 nights a month to look for Earth-approaching asteroids and comets.
Our plans are to scale this project to 18 nights per month on 3 Air Force telescopes, so that 90% of the Earth-crossers larger than 1 km would be found in 10 years. After this survey is complete, and we know more about the potential hazards, we can begin to think about how to divert an Earth-crosser from a threatening orbit.
An international body, called the Spaceguard Foundation, was established in 1996 in Rome. Its goal is to help protect Earth's environment from the impacts of asteroids and comets by promoting and coordinating discovery programs and physical studies of Near-Earth objects (NEOs).
In the last few years, NASA has convened, at the behest of the US Congress, several working groups in order to plan a system for discovery and characterization of the NEOs.
Most of world's scientific community recognizes the serious possibility of an asteroid or comet impact. However, only very limited funds and resources have been directed towards the effort of finding these objects. The only two active programs involved in this effort are JPL's NEAT program and Spacewatch at the University of Arizona. Both are small-scale efforts with funding levels of less than $500K per year.
No one should worry about being struck, personally, by a comet or asteroid. The threat to an average person from disease, car accidents, accidents in the home, and from other natural disasters is much higher. However, people should worry about the long-term threat to the Earth of asteroids and comets. The chances are extremely small that a devastating impact would occur in a single lifetime. But over then next 1,000 years, the odds are no longer negligible.
It is entirely feasible that we could divert a large asteroid from a colliding orbit with existing technologies, but it would depend on the lead time. If we can predict the event long in advance, by at least 10 to 100 years, then conventional rockets and/or explosives would probably be adequate, even for bodies as large as 1 km. However, if we discover the object only a few years before impact, these technologies might not be adequate.
In any case, before we can address the question of how to divert an asteroid, we have to find out if there are any that require diversion. If we conduct a survey to find all the potentially hazardous asteroids now, it is very likely that we will have plenty of time to plan corrective measures before any one object is found to be in a threatening orbit.