NEO News (08/07/02) NEOs at ACM
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).
David Morrison
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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.
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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.
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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.
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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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NEO News is an informal compilation of news and opinion dealing with
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