Interlopers in the Inner Solar System

The inner solar system is at first glance a pretty clean place, having long since been swept clean of most interplanetary debris by the gravity of the Sun or that of the planets in the area. Every now and then there will be an interloper that will drop down into the inner solar system after a gravitational interaction with Jupiter or Saturn or out beyond the planets in the Kuiper Belt or Oort Cloud. It has been the secret fear of astronomers for many years that a large asteroid or comet will drop into the inner solar system on an impact trajectory with Earth. The potential for such a cataclysm is sufficiently terrifying that the U.S. government has funded two studies to scan the skies for such objects and inspired Hollywood to produce two movies based on such a disaster called Deep Impact and Armageddon. Such an impact is believed responsible for the extinction of the dinosaurs and many other reptilian forms of life on Earth many years ago.

The two government funded robotic programs are designed to scan the entire northern sky at regular intervals for undiscovered asteroids. These robotic programs called LINEAR (Lincoln Near Earth Asteroid Research) and NEAT (Near Earth Asteroid Tracking) operate every night at desert locations. LINEAR uses two one-meter telescopes based at White Sands Space Harbor in New Mexico. One telescope is dedicated to the LINEAR program; the other is primarily used by the Air Force for space surveillance. NEAT uses two 48-inch telescopes. One is based on Maui and the other is based at Mount Palomar. NASA's Jet Propulsion Laboratory manages the NEAT program. LINEAR is operated by the Lincoln Laboratory of the Massachusetts Institute of Technology. The hope is that any asteroid moving into the inner solar system that might become a threat will be found in time to allow the government to devise a plan to cope with it prior to the disaster striking. Each survey is responsible for finding thousands of these space rocks. Learning about the interior of these bodies was the principal objective of the 2005 Deep Impact mission (which NASA claims was not named for the aforementioned movie) to excavate a crater in the surface of Comet Tempel 1. Through such studies, we hope to learn whether comets and asteroids are solid rocky or metallic objects or primarily loosely associated clumps of ice and dirt. The answers would be critically important to scientists and Defense Department researchers who might one day be forced to deal with answering the question of how to destroy or deflect an asteroid or comet on a collision course with Earth.

The objects of greatest potential concern are asteroids with orbits that are known to cross inside that of Earth, which astronomers classify as Apollos. The two robotic telescopes also survey asteroids from the Amor group, which approach Earth from outside our orbit and Atens, which approach our orbit from the inside. Amors and Atens do not actually cross our orbit in space but can approach close enough to Earth to possibly some day present a danger. These asteroids are normally brought into the inner solar system from the main asteroid belt between Mars and Jupiter after a gravitational interaction with Jupiter. As Jupiter tugs at the asteroid from behind, it loses energy and falls deeper into the inner solar system on its next orbit. Fortunately most of these asteroids are in orbits that are inclined several degrees from that of Earth so even though the asteroids might come closer to the Sun than Earth does, they do not actually cross the plane of Earth's orbit and thus are not a collision threat. But our planet does bear the scars of the occasional hunk of rock that has found its way across our path. Meteor Crater in Arizona is an enormous one-mile across impact scar formed within the last few million years by an object about the size of a railroad car.

NEAT and LINEAR not only sweep up asteroids but also voluminous numbers of comets. Each has found more than 100 comets to date. Most are far too faint to be viewed in amateur telescopes but the two robotic telescopes have found some of the brighter ones in the last decade as well including two naked-eye comets which put on nice displays in 2004. Many comets coming from deep space from the Oort Cloud come not just inside the orbit of Earth but well inside the orbit of Mercury as well. Such comets, appropriately called "Sun-grazers," can become the most spectacular comets of all. The great comets of 1882 and 1876 as well as 1965's Comet Ikeya-Seki all came within one million miles of the Sun and all grew so bright after perihelion that they could be easily viewed in daylight. It had been thought that sun-grazing comets were rare phenomena but the SOHO spacecraft has taught us that they are in fact rather common. Since its launch, SOHO's C2 coronagraph has found more than 100 comets, which were too faint to be detected by conventional means until they flared to life in close to the solar photosphere.

Now having studied the various movements, tricks and surprises of the inner solar system, let's put our new knowledge to the test as Mercury, Venus and the occasional surprise guest spin through space between Earth and the Sun. While Venus moves with great regularity repeating the same eight-year cycle over and over again, Mercury darts in and out in apparitions that vary from one month to another in viewing quality. Then there are the surprise visitors, asteroids that come from both inside and outside of our orbit and from the deepest realms of the solar system, the frozen snowballs called comets that terrorized the ancients and thrilled amateur astronomers for centuries.

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