From: Ames Research Center
Posted: Tuesday, August 6, 2002
The professional meeting "Asteroids, Comets, Meteors 2002" was held last week in Berlin. Every three years, astronomers and others who study asteroids, comets, meteors, and meteorites get together to report their recent work and discuss the state of the field. More than 300 scientists attended this meeting. Following are a few highlights from the papers that deal with Near Earth Objects (NEOs).
1. NUMBER AND IMPACT FREQUENCY OF NEAS
As the discovery of NEAs (Near Earth Asteroids) has accelerated, we have much more complete data to use in estimating the numbers of NEAs of various sizes. However, these estimates are also complicated by an increasing awareness of the complexities of NEA dynamics. In particular, the objects that are the easiest to find are in general also those that come closest to the Earth and constitute the greater part of the impact hazard. From the perspective of the Spaceguard Survey, for example, we want to focus on these potentially hazardous NEAs. For other purposes, however, we may want to look at the total population.
Alan Harris (JPL) presented the major invited paper on NEA populations, focused on the question "Just how many Tunguskas are there?" He reviewed recent work that indicates that the number of NEAs of diameter 1 km or greater (the primary targets of the current Spaceguard Survey) is 1100 +/- 100. The number of NEAs down to the size of Tunguska (60 meters diameter) is of order 100,000 but considerably more uncertain. Translating into impact frequency, Harris finds that one Tunguska size impact should take place at intervals of roughly 1000 years. Some lunar crater count data actually suggest that these impacts are as much as a factor of ten lower, but this seems to conflict with other data. The millennium timescale derived by Harris is also supported by an estimate from Roger Revelle (Los Alamos) that the average largest atmospheric impact in a year has energy of about 10 kilotons, for an equivalent diameter of about 5 meters.
Robert Jedicke (Lunar & Planetary Lab) and Alessandro Morbidelli (Cote d'Azur Observatory) each discussed the distribution of NEOs from the perspective of meeting the Spaceguard Goal of finding 90% of NEAs larger than 1 km. Jedicke modeled the LINEAR survey, the most successful current NEA search, finding that if all NEAs are taken into account the survey will not be 90% complete until after 2020; however, if we focus on the NEAs in the most dangerous orbits, the goal will be met by 2010 (a result that Harris confirms from his models). Morbidelli and colleagues carried out a reassessment of the impact hazard as originally formulated by Chapman and Morrison (1994), finding in general that the hazard should be lowered by about a factor of 4 - their estimated interval for a 1 million megaton impacts is about 3.8 million years, somewhat lower than other recent estimates.
All these results are generally consistent in their evaluation of the NEA numbers and the associated risk. For Tunguska-size impacts, the Chapman / Morrison estimate of once in 300 years is now down to once in 1000 years. For million-megaton impacts, the Chapman / Morrison estimatIn once in a million years is now down to once in 2-4 million years. And somewhat coincidentally, the Chapman / Morrison estimate of 1500-2000 NEAs larger than 1 km is now down to 1100.
Curiously, while there is general agreement among astronomers, others have been arguing for a hazard a factor of ten or more higher. It is not uncommon to read that Tunguska-size impacts take place once a century, or even more often! One recent claim was for 3 Tunguska-size impacts during the 20th century. These claims are not consistent with the weight of accumulating astronomical evidence as reviewed at this meeting and at the 2001 asteroids meeting in Palermo.
2. FUTURE ASTEROID SURVEYS
Recent United States National Academy of Sciences panels in both astrophysics and planetary science have recommended the construction of the LSST, the Large Scale Synoptic Survey Telescope. One major objective is the extension of the Spaceguard Survey down to 300 meter NEAs. The nominal LSST design considered by the Academy panels is a single 6-8 meter aperture telescope.
At this meeting David Jewitt and David Tholen (University of Hawaii) announced that full funding of $40 million has just been received from the U.S. Air Force to construct an alternative version of the LSST called Pan Stars, based on multiple small telescopes. Although the design has not been finalized, one option would use 4 telescopes of 2-3 meter aperture constructed on Mauna Kea. Depending on how the system is configured and operated, Jewitt estimates that it can survey the entire sky to visual magnitude 24, with the prospect of finding 10,000 NEAs per year, as well as 10,000 members of the Kuiper Belt per year and about 100,000 supernovas per year.
3. ORIGIN OF NEAs
The NEAs are for the most part fragments of main belt asteroids that have been transported into Earth-crossing orbits. Two processes contribute: collisions or impacts among main belt asteroids, and a variety of gravitational processes that can perturb these fragments into the inner solar system. Unfortunately, most recent studies have found that these effects are insufficient to account for the observed numbers of NEAs. Recent application of a third process - the Yarkovsky effect - has now apparently removed this discrepancy.
The Yarkovsky effect was postulated about a century ago as a way to change asteroid orbits by the absorption of sunlight followed by its asymmetric re-emission as thermal radiation. Although the forces are extremely small, they act continuously over many millions of years. The result is to move the asteroids or asteroid fragments until they reach a resonance, where the more conventional gravitational forces take over and complete their perturbation into the inner solar system. William Bottke (Southwest Research Institute) and several others presented highly convincing examples of the Yarkovsky effect at work. In addition to explaining the transport of asteroids into the inner solar system, the Yarkovsky effect also allows us to understand the evolution of families of asteroids in the main belt, and it even contributes to spinning up asteroid rotation and the formation of some binary systems.
A few NEAs may be small dead comets, but this has not been demonstrated. Most comets in the inner solar system are Jupiter family comets. Paul Weissman (JPL) reviewed the size, structure, and density of comet nuclei. He concludes that the Jupiter-family comets are derived from the Kuiper Belt. Weissman argued that they are fragments from a collisionally evolved population, with typical rubble-pile interiors and densities of 0.4 to 1.1 times the density of water. There is an apparent depletion of small comets (diameters less than about 2 km), which suggests that relatively few survive to become dead objects indistinguishable from NEAs.
4. NEA SURFACES
Several ACM papers by Marco Delbo (DLR), Allan Harris (DLR), Rick Binzel (MIT), Mike Nolan (Arecibo Observatory), and others dealt with the physical nature of asteroids, including NEAs. Radar observations of 22 NEAs were made in 2001 - including multiple and binary objects, both fast and slow rotators, and both spherical and highly-elongated objects. A number of mechanisms were suggested that might lead to the accumulation of a loose high-porosity surface - overcoming some previous calculations that small asteroids would not have sufficient gravity to retain a dusty regolith. One of the results of recent work is the suggestion that for small NEAs, the albedo (reflectivity) increases with decreasing size. If this reflectivity trend is correct, then the faint NEAs currently being discovered by LINEAR and other search systems are actually smaller than has been assumed from their brightness - suggesting that there are fewer NEAs larger than 1 km and that we might be closer to achieving the Spaceguard Goal of 90% completeness at 1 km diameter than we have thought.
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