Asteroid Radar Astronomy: Steven J. Ostro, JPLOstro has provided some background materila here
Plans for VLBA-Radar Imaging of Near-Earth Objects: G. Black (NRAO), D. Campbell (Cornell), B.Butler (NRAO), S. Ostro (JPL)
Radar Observations of Comets; John K. Harmon (Arecibo), D.Campbell (Cornell), S.Ostro (JPL), M.Nolan (Arecibo)
Bistatic Radar Imaging of 6489 Golevka in June 1999: Nolan (NAIC), Margot (NAIC)
Mainbelt asteroids: Results of Arecibo and Goldstone Radar Observations of 37 Objects During 1980-1995: C.Magri et al

Tuesday, July 27: Near-Earth Objects

In two recent blockbuster movies, we have seen asteroids and comets threatening to collide with the Earth, only to be destroyed at the last minute by astronaut heroics. In real life, the hazards due to collisions with comets or asteroids are being considered more and more seriously. It has been estimated that only 15 to 20 percent of the Near Earth Objects larger than 1 kilometer have been detected to date. Several telescopic discovery programs are actively searching for these large near-Earth objects and the discovery rate is increasing. However, it may be at least 10 years before we find 90 percent of the total population. A new analysis by Dave Rabinowitz has shown that there may be only half as many large hazardous objects as previously estimated. Al Harris will present new evidence to refute a controversial theory put forth several years ago, that the Earth is continually bombarded by a population of house-sized comets. Andrea Milani and Paul Chodas will discuss their independent efforts to predict close Earth approaches and impact probabilities farther into the future than has been previously possible.

Daily prediction of Earth close-approach uncertainties for potentially hazardous asteroids: Chodas et al
The number of kilometer-sized NEAs: New results from NEAT: Rabinowitz et al

Tuesday, July 27: Analyzing Asteroids and Distant Comets

In studying the solar system, one of the most fundamental questions that can be asked is "What materials make up the different objects we see?" If we know what any particular planetary body is made of, we can place important constraints on how that body was initially formed, and on the processes that might have changed that object over the age of the solar system. The best way to determine the compositions of the various bodies in the solar system is to collect pieces that can be accurately analyzed in a laboratory. When we don't have these "hand samples" that can be closely examined, we have to rely on other techniques of remote sensing, in particular, spectroscopy, to help us estimate the compositions of the various bodies. Spectroscopy is the technique of breaking light into its component colors, or spectrum, and measuring the amount of each color that is present. Since different materials reflect (or emit) light in different ways, the spectrum of an object is like a fingerprint of the material making up that object. Spectroscopy is particularly important in the study of the small bodies in the solar system (the asteroids and comets) where their numbers are so great that we will never be able to visit them all (by spacecraft) or collect pieces of every one. Spectroscopy helps to reveal how these various small bodies are similar or different from one another, and helps to reveal the much larger picture of what role these asteroids and comets may have played in the evolution of the solar system.

In this press conference, results will be presented for objects in different regions of the solar system. First, new results will be discussed from a spectroscopic survey of asteroids in the main belt (located between the orbits of Mars and Jupiter), where it is found that collisions between the asteroids have played a major role in their evolution. Then, our attention will turn to the outermost observable part of the solar system, to the recently discovered Kuiper Belt (and Centaur) Objects. These objects are thought to be leftover fossils from the early formation of the solar system, and may thus contain some of the most pristine materials that we can study. Due to their small sizes and great distance from the Sun, however, these objects also are the most challenging to observe. Measurements of their colors reveal significant variations between the individual bodies, suggesting that processes, which as yet are not understood, are at work in this coldest region of the solar system.

Compositional structure of asteroid families: S.J.Bus
Physical and composition characteristics of centaurs and EKOs: Barucci
Light curves of centaurs and Kuiper Belt objects: Romanishin, Tegler

Wednesday, July 28: Future Missions

Because they are the remnants of the solar system formation process, the primitive comets and asteroids offer clues to the materials and conditions under which the planets formed some 4.6 billion years ago. Throughout Earth's history, the impact of comets and asteroids have delivered to the early Earth the carbon-based molecules and water that were necessary for the formation of life. Subsequent collisions may have punctuated the evolution of life itself, allowing only the most adaptable species to evolve further. Ironically, those objects that can get closest to the Earth are also the ones that can be reached most easily to exploit their raw materials. In the next century when we begin to colonize the inner solar system, asteroids could provide the metals necessary for space structures and comets could supply the water that is necessary to sustain life and (after being separated into oxygen and hydrogen) provide the fuel to power interplanetary spacecraft. Spacecraft missions to comets and asteroids are necessary to study the origins of the solar system and to measure their strengths and densities in case we need to deflect one from an Earth impacting trajectory. Missions are also necessary to assay these objects to determine which among them are richest in raw materials.

The current era is truly the golden age of comet and asteroid exploration. Within the next dozen years, there will be detailed investigations of 13 different comets and asteroids by at least 7 spacecraft. Within this interval, spacecraft flybys will characterize a diverse group of comets and asteroids, including objects of widely different compositions, ages and origins. Although it was designed primarily to test new technologies in space, the New Millennium Deep Space 1 (DS1) mission will fly closely past, and image, asteroid 1992 KD on July 29, 1999. If the resources are allocated, DS1 will then be targeted to fly closely past asteroid Wilson-Harrington and comet Borrelly in January and September of 2001. Beginning on Valentines day of 2000, the NEAR spacecraft will spend nearly a year in close orbit about the asteroid Eros subjecting it to detailed mapping and chemical analysis. In a Japanese and American cooperative effort, the MUSES-C mission will land upon asteroid 1989 ML in October 2003 and deploy a book-sized rover to study the asteroid's surface and chemical composition. The Muses-C spacecraft will also collect surface samples and bring them back to Earth in June 2006 for study in Earth-based laboratories. A few months earlier in January 2006, the Stardust mission will return a sample of cometary dust to Earth for analysis. The Stardust mission will fly past comet Wild 2 in January 2004, collect dust particles and provide images of the comet's nucleus. Between November 2003 and August 2008, the CONTOUR mission will fly past and scrutinize three widely diverse comets (Encke, Schwassmann-Wachmann 3, d'Arrest) and if a bright new comet is discovered in time, CONTOUR could be diverted to intercept it. The recently selected Deep Impact Discovery mission will release a 500 kg impactor that will strike comet Tempel 1 on the fourth of July 2005 and then study the resulting crater and ejecta as the spacecraft flies by. This will be the first experiment to study the subsurface cometary regions. After flying by two asteroids en route, the European Space Agency's Rosetta spacecraft will rendezvous and land upon comet Wirtanen in November 2011. The nucleus of this comet will then be subjected to a wide variety of scientific studies to determine its nature and composition.

Comet and Asteroid Missions in NASA's New Millenium Program: Weissman, JPL
CONTOUR: A discovery mission to study the nature and diversity of comet nuclei: Veverka et al
The International Rosetta Mission: Gerhard Schwehm

Thursday, July 29: Space Dust

The "Space Dust" press session will focus on spacecraft observations of small interplanetary grains, which are usually particles ejected by comets.

Comets spend most of their lives much beyond the planetary realm, only becoming visible when they reach the inner solar system where their ices sublime, carrying gas and dust outward. Yet, more than fifty comets have been discovered sweeping close to the Sun. These observations were made in the last few years by the SOHO (Solar and Heliospheric Observatory) spacecraft which shielded its telescopes from the Sun's glare to see the region a few solar radii wide. Philippe Lamy (Laboratoire d'Astronomie Spatiale, Marseille; Cornell Ph.D. '75) will discuss the properties of these comets, the majority of which are only a few hundred meters across and display little cometary activity.

Traditionally, comets are thought to exhibit at most two tails, a graceful one made of fine dust and a knotted ionized version. Bill Reach (IPAC/Caltech) will describe another type of tail for comet 2P/Encke, recently observed by the Infrared Space Observatory at infrared (long) wavelengths that are sensitive to much larger particles. These images show that pebble-sized particles, invisible to human eyes from the ground, stream away from the comet's nucleus in a long slender "trail", containing hundreds of times more mass than Encke's previously known tail.

Most meteors, popularly called "shooting stars", are pea-sized bits of comets that slam into the Earth's upper atmosphere at 20-60 km/sec (45,000-135,000 mph). Well known annual meteor showers occur when Earth passes through a comet's path; they can be spectacular if the source comet is nearby. For example, the Leonid shower, which is associated with comet Tempel-Tuttle (orbital period 33.5 years) and happens each November 12, peaked at 9000 meteors in 15 minutes in 1833; 1966 was also a very good year. Peter Jenniskens (NASA/Ames), the leader of a multinational airborne campaign, will discuss the mixed results from last fall's apparition, which some predicted to be very strong, and preparations for this fall's observations.

Very surprisingly, nearly one-third of the largest grains in space are interlopers from the galaxy, and not native to our solar system, according to impacts detected three spacecraft (Galileo, Ulysses and Hiten). Eberhard Grun (Max Planck Institute-Heidelberg), the chief scientist of these instruments, will discuss the identification of these grains and their nature. He will also describe future missions that might capture these pieces of other stars for return to Earth.

Joe Burns
Session Chair

Galactic Dust Measurements near Earth: Gruen et al
The properties of the SOHO sun-grazing comets: P.L.Lamy et al
Results from the Leonid Multi-instrument aircraft campaign: P.Jenniskens et al
Comet Encke's Dust Trail: Reach et al

Friday, July 30: Asteroid Moons and Spins

Until recently, most planetary scientists considered asteroids to be little more than beat-up rocks in space, with solid interiors and lunar-like surfaces. It is difficult to prove or disprove this hypothesis, since remote sensing techniques can, at best, only probe the top layers of an asteroid's surface. Still, using data taken from spacecraft flybys, ground-based observations, and computer modeling, a new view of has evolved which suggests that many large asteroids are "rubble-piles", collections of large and small components held together not by physical strength but by the gravitational attraction of the components themselves. P. Pravec's results help provide support for this hypothesis. For example, Pravec has recently made an extensive study of near-Earth asteroid (NEA) rotation rates using his own observations and data provided by astronomers from all over the world. At ACM'99, Pravec will show that only two sub-100 meter bodies (1996 KY26 and 1995 HM) are spinning so fast that they have to be solid objects -- weakly-bound rubble-piles fly apart if you spin them too fast. These bodies are small enough that they do not present a contradiction to the rubble-pile hypothesis; instead, they provide the "missing link" astronomers have been looking for, filling the gap between between km-sized asteroids, which cannot spin overly fast, and meter-sized meteorites, which are known to be solid. Computer modeling suggests the transistion size where solid objects turn into rubble piles is roughly a few hundred meters in diameter. More support for the rubble-pile hypothesis comes from a surprising place, namely Pravec's report that many near-Earth objects have small moons. If true, these moons are most likely to be a by-product of close encounters between rubble-pile asteroids and planets; when a rubble-pile asteroid passes too close to a planet like Earth, tidal forces can pull it apart, leaving some of the fragments to orbit one another. This scenario, however, does not explain all asteroid moons. Binary asteroids in the main belt, such as (243) Ida and its moon Dactyl, or the new moon reported around (45) Eugenia by W. J. Merline, cannot be produced by tidal forces, since these bodies can never approach a planet. It is believed these binaries may be by-products of asteroid-asteroid collisions, though the details are still unclear. Regardless, these new results show us that asteroids are very different than our preconceived notions, and that further work is needed to understand the true nature of these interesting bodies.

Discovery of a Satellite of (45) Eugenia: Merline et al
Fast Rotating Asteroids: Petr Pravec