Dynamical configuration of binary near-Earth asteroid (66391) 1999 KW4

Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primary's proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.

Binary asteroids in the near-Earth object population

Radar images of near-Earth asteroid 2000 DP107 show that it is composed of an approximately 800-meter-diameter primary and an approximately 300-meter-diameter secondary revolving around their common center of mass. The orbital period of 1.755 +/- 0.007 days and semimajor axis of 2620 +/- 160 meters constrain the total mass of the system to 4.6 +/- 0.5 x 10(11) kilograms and the bulk density of the primary to 1.7 +/- 1.1 grams per cubic centimeter. This system and other binary near-Earth asteroids have spheroidal primaries spinning near the breakup point for strengthless bodies, suggesting that the binaries formed by spin-up and fission, probably as a result of tidal disruption during close planetary encounters. About 16% of near-Earth asteroids larger than 200 meters in diameter may be binary systems.

Asteroid 1950 DA's encounter with Earth in 2880: physical limits of collision probability prediction

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.

Radar detection of the nucleus and coma of comet hyakutake

Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet.

Mortality in a cohort of homeless adults in Philadelphia

BACKGROUND

Homeless people are at high risk for death from many causes, but age-adjusted death rates for well-defined homeless populations have not been determined.

METHODS

We identified 6308 homeless persons 15 to 74 years of age who were served by one or both of two agencies for the homeless in Philadelphia between January 1, 1985, and December 31, 1988. Using a data base that contained all deaths in Philadelphia and listings of all Philadelphia residents during the same period, we compared the mortality rate for this homeless population with the rate in the general population of Philadelphia.

RESULTS

The age-adjusted mortality rate among the homeless was 3.5 times that of Philadelphia's general population (95 percent confidence interval, 2.8 to 4.5). The age-adjusted number of years of potential life lost before the age of 75 years was 3.6 times higher for the homeless people than for the general population (345 vs. 97 years lost per 1000 person-years of observation). Fifty-one of the 96 deaths of homeless persons (53 percent) occurred during the summer months. Mortality rates were higher among the homeless than in the general population for nonwhites, whites, women, and men. Within the homeless cohort, white men and substance abusers had higher mortality rates than other subgroups, but even homeless people not known to be substance abusers had a threefold higher risk of death than members of the general population. Injuries, heart disease, liver disease, poisoning, and ill-defined conditions accounted for 73 percent of all the deaths among the homeless.

CONCLUSIONS

Homeless adults in Philadelphia have an age-adjusted mortality rate nearly four times that of Philadelphia's general population. White men and substance abusers are at particularly high risk. Matching cohorts of homeless people to death records is a useful way to monitor mortality rates over time, evaluate interventions, and identify subgroups with an increased risk of death.