THE SOLAR SYSTEM: NEAR-EARTH OBJECTS
Despite great progress over the past 50 years, there is yet much to learn about our solar system. While the great majority of its mass is contained in the sun and giant planets, by number the overwhelming content of the solar system is relatively small, dark bodies like comets and asteroids.
Such objects are intact samples of the material from which the solar system formed and thus hold important clues to the origin of the sun, planets and, indeed, of life itself. Asteroids and comets reflect little light, and thus, despite their proximity, much about their origin and dynamics remains uncertain and difficult to study. Indeed, it has proven impossible even to provide a reasonably complete census of these objects. Such uncertainty is naturally troubling to scientists. Over the past twenty years, however, this uncertainty has taken on new importance with the recognition that Earth suffers collisions with these objects.
Comets consist primarily of water and carbon monoxide ice that is converted to gas (sublimated) as they approach the Sun. Asteroids are composed primarily of more refractory materials: rocky, solid objects that vaporize only at the high temperatures they experience when well within the orbit of Mercury. Both comets and asteroids may collide with the Earth at extremely high velocities — on the order of 20 to 30 kilometers per second. At such speeds, the kinetic energy of ordinary materials vastly exceeds the chemical energy of an equivalent amount of high explosive. Upon impact, the bulk of this kinetic energy is immediately dissipated in the form of heat and an associated shock, or pressure, wave that can cause devastation on continental and even global scales.
It is now widely believed that such an impact caused the mass extinctions which form the transition from the Cretaceous to the Tertiary period, the so-called K-T boundary, 65 million years ago.
An object about ten kilometers in diameter struck the Earth in what are now coastal waters off Mexico's Yucatan Peninsula. Numerical simulations of this event suggest that the impact created a shock front which spread out across the North and South American continents, heating the atmosphere to incandescence. Directly above the impact site, a hole was blown in the atmosphere and a vast quantity of the Earth's crust was thrown into space. The heat from this material re-entering the atmosphere caused forest fires throughout the world. Soot from these fires and the dust generated by the explosion probably caused a significant global drop in temperature, and nitrous oxide formed during the shock wave's passage through the atmosphere lead to widespread acid rain.
The results were calamitous for life, and not only for the dinosaurs: as much as 85 percent of all species on Earth became extinct within a short time. The geological record shows many such impacts over the past billion years, some considerably larger than the K-T boundary event. 250 million years ago an impact occurred which likely destroyed more than 95 percent of all living species.
Impacts which cause destruction on a global scale are, of course, rare, occurring roughly every 50 to 100 million years in the geological record. This translates to a one-in-a-million chance of one occurring within our lifetime. Smaller, more frequent, impacts can still have global consequences, however. There is a one-in-a-thousand chance of a one- to two-kilometer meteoroid striking the Earth within the century. Such an impact would release ten to one hundred times the explosive energy of all of the nuclear weapons ever produced. The devastation from such an event would be continental in scale; its effects on climate would be global and likely last for centuries.
AS THE SIZE OF THE BODY DECREASES, the frequency of events continues to rise. Fifty thousand years ago, a 30-meter object comprised mostly of iron struck northern Arizona. Because of its high density, the meteor penetrated the atmosphere. The resulting 50-megaton (MT) explosion excavated a crater more than a kilometer in diameter and 540 feet deep, and devastated hundreds of square kilometers. Similar craters are found throughout the world, testifying to the frequency of such events. In 1908, a larger (50 meters) but less-dense object struck over Tunguska, Siberia. The object disintegrated high in the atmosphere, but the resulting 10-20 MT airburst burned and flattened over 1000 square kilometers of forest. If the collision had happened a few hours later, it could well have devastated northern Europe.
Events of this magnitude happen every two to three centuries, so the probability of an impact of this size within the next century is thus 30 to 50 percent. There is a one percent chance of a 250-meter impact during the same period. Such an object would certainly penetrate to ground level. The resulting 1000 MT explosion would cause catastrophic devastation over a large area. If the impact were to occur on land, the resulting crater would be three to five kilometers across. At sea, such an impact would result in a tsunami of unprecedented magnitude, most likely devastating coastal populations.
Numerically, and somewhat paradoxically, the overall risk to human life is greatest from the largest impacts. The cost in human life rises more rapidly with the size of the event than its frequency declines; there would be few if any survivors of a ten-kilometer impact. Thus, the odds of dying in an impact of global proportion are about equal to that of dying in an earthquake or an airplane crash, while those of dying in a city-sized event are somewhat less. Nonetheless, the probability of regional or city-scale calamities is far from zero and merits serious attention.
The U.S., British, and other governments have recognized the threat from asteroid and comet impacts. Congress held hearings to study the NEO impact hazard in 1993, 1998, and 2002, and NASA has formed a NEO program office. To date, however, the funding from governmental sources has been limited. Professional asteroid and comet astronomers have hoped that recognition of the impact threat might spur the U.S. government to provide funds for an early-warning system to identify objects which could be on a collision course with Earth. While Hollywood has responded with popular and spellbinding simulations of collisions in films like "Deep Impact" and "Armageddon," the public and the U.S. government have so far done little to identify such potential hazards.
Contemporary efforts to survey the night sky for comets and asteroids are limited because instruments for this purpose do not have the required light-gathering capacity or area coverage to find all the dangerous asteroids in a reasonable time. It is estimated that half of the most dangerous asteroids capable of striking Earth, those larger than about a kilometer in diameter, have been identified. At current rates of discovery it will probably be two decades until more than 90 percent of these one-kilometer objects are found.
Smaller objects are just too faint for small telescopes to detect with any efficiency, and this has concentrated attention on the largest bodies. The focus on NEOs larger than one kilometer ignores the threat from more numerous smaller objects. While there are thousands of one-kilometer objects, there are estimated to be more than a million objects with diameters greater than 50 meters, and more than ten thousand larger than 250 meters. The vast majority of these are uncharted. Most of those which are detected are subsequently "lost" as they move out of range of current surveys before accurate orbits can be determined.
With its ability to go faint fast, LSST will find virtually all one-kilometer NEOs in less than a year. In a decade of operation, it will find 90 percent of all NEOs down to 140 meters in diameter.
DETECTING NEOs IS ONLY the first stage of the early-warning process. When current surveys detect an object, they provide only a crude estimate of the object's trajectory. This in turn allows only a crude estimate of how closely the object will approach the Earth. At this stage, many NEOs seem potentially dangerous. Follow-up observations over time, usually with larger telescopes, are necessary to provide better orbital data. Better data allow a more accurate risk assessment, and this usually shows that the object is not, in fact, headed for Earth. If the initial orbit is not sufficiently accurate or if time on a larger telescope cannot be obtained, the object is lost before it can be ruled out as a threat.
Because LSST will survey a much larger volume of space more frequently than other systems, its repeated observations will continue to observe detected NEOs, refining our knowledge of their orbits to an extent not currently possible for most objects. LSST automatically does its own follow-up and allows more accurate risk assessments even for objects detectable by other surveys.
LSST can also contribute to understanding what might be done when an object is eventually found on a collision course with Earth. Little is known about the mechanical properties of asteroids, yet these properties are crucial to understanding how to deal with such a threat. For example, if asteroids are solid bodies with high mechanical strength, a rocket motor might be attached and the object's orbit nudged away from collision. If asteroids are instead piles of dust and rubble with little cohesive strength, adding a motor might break the object into several parts, potentially transforming a bullet into grapeshot.
LSST will find a significant number of much smaller bodies, down to a few meters in diameter. Some of these will be found shortly before they collide with Earth to become meteors. Knowing the size and initial velocity of an object and then observing its destruction in the atmosphere will allow us to determine its mechanical properties, knowledge essential to plans for countering larger threats.
The large aperture, wide field, and full-time observing schedule of the LSST system provide a capability unrivaled by any other project for detecting and assessing the risk to Earth presented by NEOs and for the studies of these objects crucial to mitigation efforts. This aspect of its mission will be accomplished automatically and with the same observations that will bring new scientific insights to a wide variety of other fields.