Asteroids and comets are believed to be ancient remnants of the earliest years of the formation of our Solar System more than four billion years ago. From the beginning of life on Earth to the recent spectacular impact of Comet Shoemaker-Levy 9 with Jupiter, these so-called "small bodies" play a key role in many of the fundamental processes that have shaped the planetary neighborhood in which we live.

Comets are bodies of ice, rock and organic compounds that can be several miles in diameter. Comets are thought to originate from a region beyond the orbits of the outermost planets. Scientists believe that gravitational perturbations periodically jar comets out of this population, setting these "dirty snowballs" on orbital courses that bring them closer to the Sun. Some, called long-period comets, are in elliptical orbits of the Sun that take them far out beyond the planets and back. Others, called short-period comets, travel in shorter orbits nearer the Sun.

When comets venture into the more intense sunlight of the inner Solar System, the ices in the comet nucleus begin to vaporize and fall away. The evolved gas forms a tenuous atmosphere around the nucleus called a coma, while the dust previously in the nucleus forms a tail that can be thousands of miles long and sometimes can be seen from Earth. While striking the early Earth billions of years ago, comets are thought to have created major changes to Earth's early oceans, atmosphere and climate, and may have delivered the first carbon-based molecules to our planet, triggering the process of the origins of life.

Most asteroids are made of rock, but some are composed of metal, mostly nickel and iron. They range in size from small boulders to objects that are hundreds of miles in diameter. A small portion of the asteroid population may be burned-out comets whose ices have evaporated away and been blown off into space. Almost all asteroids are part of the Main Asteroid Belt, with orbits in the vast region of space between Mars and Jupiter.

Some asteroids pass very close to Earth's orbit around the Sun. Scientists have found evidence that asteroids have hit our planet in the past. Usually, asteroids and smaller debris called meteoroids are too small to survive the passage through Earth's atmosphere. When these burn up on their descent, they leave a beautiful trail of light known as a meteor or "shooting star." Larger asteroids occasionally crash into Earth, however, and create craters, such as Arizona's mile-wide Meteor Crater near Flagstaff. Another impact site off the coast of the Yucatan Peninsula in Mexico, which is buried by ocean sediments today, is believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. Fortunately for us, these big asteroid impacts are rare. A smaller rocky meteoroid or comet less than 100 yards in diameter is believed to have entered the atmosphere over the Tunguska region of Siberia in 1908. The resulting shockwave knocked down trees for hundreds of square miles.

Over the next ten years, NASA expects to spend more than $1 billion gaining a better scientific understanding of asteroids and comets. Major areas of research supported by NASA include detecting and tracking so-called Near Earth Objects (NEOs) that could possibly impact the Earth in the future, and numerous spacecraft missions to learn more about the physical properties and evolution of asteroids and comets, including returning samples of them to Earth.

GROUND-BASED RESEARCH:

DETECTING, TRACKING AND CHARACTERIZING

Earth and all the other planets and moons of our Solar System have been continuously pelted by asteroids and comets ever since their formation -- just look at the Moon's craters through a small telescope or a good pair of binoculars.

NASA supports several ground-based programs and related technology development efforts that use sensitive electronic detectors to scan the skies for undiscovered NEOs. Less than 10 percent of the estimated 2,000 or more NEOs that are larger than about a half-mile in diameter have been detected to date. (Most scientists believe that objects of this size have the potential to cause global effects should they hit the Earth.)

Major examples of these programs include the Near Earth Asteroid Tracking (NEAT) system operated by NASA's Jet Propulsion Laboratory (JPL) in conjunction with the U.S. Air Force on Mt. Haleakala, Maui, HI; the Spacewatch program run by the University of Arizona in Tucson at Kitt Peak, AZ; and the Lowell Observatory NEO Survey (LONEOS) program in Flagstaff, AZ.

In fiscal year 1998, these NASA programs are funded at $3 million, a threefold increase over recent levels; future budgetary requirements and technology research plans are under review. NASA also is discussing greater collaboration in this area with the U.S. Air Force Space Command, as well as solutions to how to improve coverage of the sky as seen from the Southern Hemisphere. The overall goal of this effort is to detect 90 percent of the most hazardous NEOs within the next decade.

NASA also funds a variety of ground-based research designed to characterize the composition, size, shape and other basic properties of asteroids and comets. These scientists use many of the world's most advanced telescopes and most powerful radars at sites ranging from the Keck facility atop Mauna Kea in Hawaii to the massive Arecibo radar in Puerto Rico, which was upgraded recently by NASA and the National Science Foundation in part for this task.

NASA's overall work in this area soon will be coordinated by a new NASA office, bringing a central focus to this diverse activity. NASA-supported researchers also have been asked to follow a new protocol for reporting potentially hazardous NEOs to the public, based on the principles of verification and consensus. Detailed orbital data should be shared with fellow researchers within 24 hours of receipt, with a subsequent effort to reach consensus on the object's long-term orbital path within 48 hours. NASA officials are beginning discussions with related researchers around the world to develop more comprehensive guidelines that can be applied on an international basis in the future.

HOW MUCH OF A HAZARD?

The most dangerous asteroids, those capable of causing major regional or global disasters, are extremely rare. These bodies impact the Earth only once every 100,000 years on average. Comets in this category are thought to impact even less frequently, perhaps once every 500,000 years or so.

The risk from NEO impacts increases with the size of the projectile. The greatest risk is associated with objects larger than a half-mile to a mile (1-2 kilometers), which are large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures and the amount of surface sunlight around the globe, leading to loss of food crops and related problems. An ocean impact could trigger large ocean waves, or tsunamis.

Such global catastrophes are qualitatively different from other more common hazards that we face daily, given that these common events occur with much greater frequency but affect fewer people. No individual person should worry about being struck by a comet or asteroid. The daily threat to an average person from disease, car accidents, home accidents and from other natural disasters is much higher.

It is entirely feasible that we could divert a large asteroid or comet that may collide with Earth from its orbit using existing technologies. The potential response depends on the lead time. If we can predict the event long in advance, by at least 10 to 100 years, then conventional rockets and explosives would probably be adequate, even for bodies as large as a half-mile. However, if we discover the object only a few years before impact, these technologies might not be adequate. Such a response would be coordinated in the United States by the departments of Defense and Energy, and likely would include international partners.