E. F. Helin

Determination of iron, nickel, cobalt, calcium, chromium and manganese in stony meteorites by X-ray fluorescence☆

Abstract The concentrations of total iron, and of nickel, cobalt, calcium, chromium and manganese have been determined by X-ray fluorescence in fifty-seven chondrites, seven carbonaceous chondrites, three enstatite chondrites, fifteen achondrites and in the silicate phase of two pallasites. The high-iron-group chondrites have an average of 27.36 % iron by wt., and the low-iron group has an average of 21.71 % iron by wt. Their nickel and cobalt contents show comparable differences. Significant differences in the concentrations of calcium and manganese are found between chondrites which belong to the low-iron group and those which belong to the high-iron group. The ratios Ca Mn are quite constant between the groups. The ratios Fe:Ca and Ca:Mn for the carbonaceous chondrites of Type II and Type III differ generally from the Fe:Ca and Ca:Mn ratios for the high-iron chondrites as a group. Among the achondrites examined, the Ca-poor achondrites are much more variable in composition than the Ca-rich achondrites.

The Near-Earth Asteroid Tracking (NEAT) Program: An Automated System for Telescope Control, Wide-Field Imaging, and Object Detection

The Near-Earth Asteroid Tracking (NEAT) system operates autonomously at the Maui Space Surveillance Site on the summit of the extinct Haleakala Volcano Crater, Hawaii. The program began in 1995 December and continues with an observing run every month. Its astrometric observations result in discoveries of near-Earth objects (NEOs), both asteroids (NEAs) and comets, and other unusual minor planets. Each six-night run NEAT covers about 10% of the accessible sky, detects thousands of asteroids, and detects two to five NEAs. NEAT has also contributed more than 1500 preliminary designations of minor planets and 26,000 detections of main-belt asteroids. This paper presents a description of the NEAT system and discusses its capabilities, including sky coverage, limiting magnitude, and detection efficiency. NEAT is an effective discoverer of NEAs larger than 1 km and is a major contributor to NASAs goal of identifying all NEAs of this size. An expansion of NEAT into a network of three similar systems would be capable of discovering 90% of the 1 km and larger NEAs within the next 10–40 yr, while serving the additional role of satellite detection and tracking for the US Air Force. Daily updates of NEAT results during operational periods can be found at JPLs Web site (http://huey.jpl.nasa.gov/~spravdo/neat.html). The images and information about the detected objects, including times of observation, positions, and magnitudes are made available via NASAs SkyMorph program.

The Palomar planet-crossing asteroid survey, 1973–1978

Photographic coverage of about 80,000 deg^2 of sky with the Palomar 46-cm Schmidt camera has yielded 12 new planet-crossing asteroids as well as many objects in the main asteroid belt. The estimated population of planet-crossing asteroids includes ∼100 Atens, 700 ± 300 Apollos, 1000–2000 Amors, 10,000 ± 5000 Mars crossers, and ∼5000 Mars grazers.

Discovery of Asteroid 1976 AA

Asteroid 1976 AA was discovered as a result of a continuing systematic search for planet-crossing asteroids. It is the first asteroid to be thoroughly investigated by means of photometry and radiometry on its discovery apparition. It is also the first asteroid found with a semimajor axis and period less than that of the Earth and the first Earth-crossing asteroid which does not cross the orbit of either Mars or Venus. We estimate that there might be several tens of objects to absolute magnitude 18, which are exclusively Earth crossing. Some of these objects might be exceptionally easy to reach by spacecraft.

The discovery of 1982 DB, the most accessible asteroid known

Abstract A small Apollo object was found while photographing the split comet DuToit 2-Hartley on February 27/28, 1982. Designated 1982 DB, this Earth-crossing asteroid passed the Earth at a distance of 4.08 million km about 1 month prior to its discovery. Asteroid 1982 DB has been determined to be the most accessible near-Earth minor planet known. It provides many excellent opportunities for rendezvous and sample return missions, and a rare dual rendezvous mission with 1980 AA as the second target. For a mission to be realized, opportunities to observe 1982 DB during future apparitions must be taken.

Earth-Crossing Asteroids: New Discoveries

A total of 43 Earth-crossing asteroids are now known. Twenty-five were discovered or recovered in the last decade. There were only six numbered Earth-crossing asteroids prior to 1970. Since then, twenty-one have been numbered. The Aten asteroids, a new group of Earth-crossing asteroids, have orbits smaller than that of the Earth. The largest Earth crosser, Hephaistos, has C-type UBV colors and is probably about 10 km in diameter. The smallest, Hathor, has unusual UBV colors and a probable diameter of about 200 m.

1984 AB—A unique Mars-crossing asteroid

Abstract Asteroid 1984 AB, discovered in January 1984, proved to be a unique object with a close dynamical relationship to Mars. A brief history of the discovery and subsequent “evolution” of the orbit as it was refined is presented. The preliminary orbit of 1984 AB indicated that it might be a Mars Trojan, and an extended discussion of this interesting possibility is presented, but this hypothesis had to be dismissed after further observations had refined the orbit. The semimajor axis and orbital eccentricity are very similar to that of Mars. No other known Mars-crossing asteroid exists with an orbit as closely associated to Mars.