Encounters with Comet Halley: the new view begins to emerge.
Encounters with Comet Halley: The new view begins to emerge
The international family of five spacecraft that encountered Comet Halley in March would "rewrite the textbooks,' many researchers said, and this week saw the publication of the first detailed notes for the job. Thirty-eight scientific papers, some with more than 30 authors, appear in the May 15 NATURE, summarizing initial results from the most elaborate space "flyby' ever mounted.
Together, the two Soviet, one European and two Japanese craft photographed the comet, sampled its dust and gases, and measured its electromagnetic properties (SN: 3/22/86, p. 180), revealing, among other findings, that comets are even less understood than had been thought.
The nucleus of Comet Halley, far from being an ordinary ball, is an irregular, "potato-shaped' object, in the words of scientists with the Soviet Vega space-craft team. Its double-lobed shape, resembling a large lump with a smaller lump bulging out of one side, measures about 14 by 7.5 by 7.5 kilometers, according to according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. the Vega analysis. A similar result came from Europe's Giotto craft, which took its last pictures of the comet from only 1,703 km away. Giotto actually got to within about 605 km, but an apparent glitch A temporary or random hardware malfunction. It is possible that a bug in a program may cause the hardware to appear as if it had a glitch in it and vice versa. At times it can be extremely difficult to determine whether a problem lies within the hardware or the software. See glitch attack. in the spacecraft power system ended its observations about 12 seconds earlier.
The nucleus looked extremely dark, reflecting only about 4 percent of the sunlight striking it. The Vega and Giotto groups both conclude that the largely icy nucleus--more than 80 percent of the material coming from it is water vapor--is covered with a thin layer of nonvolatile material, through which numerous bright outbursts often emerged. "If the ice were not protected by an insulating crust on the surface,' writes the Giotto group, "then the total gas and dust production would be about an order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc. higher than observed.'
The close-up photos show striking details, all of which are hidden from earth-based observers by the glare of reflected light. One "large bundle of jets,' for example, is reported by the Giotto group to have its origin near the nucleus's northern end, where there are "scalloped scal·lop also scol·lop or es·cal·lop
a. Any of various free-swimming marine mollusks of the family Pectinidae, having fan-shaped bivalve shells with a radiating fluted pattern.
b. features resembling craters, each of which appears to be the source of a narrow jet.' Study of some of the images, the team reports, indicates that surface features seen "in profile' appear to have slopes of no more than about 15|, suggesting a "rough "crater-type' terrain' in which features measuring 1 km or less in extent can be identified.
Determining the comet's composition was a major goal of the encounters, not just for the study of Comet Halley but also because many researchers have long felt that comets may represent samples of some of the most primitive material in the solar system solar system, the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass. . Most of the dust particles were found to be rich in hydrogen, carbon, nitrogen and oxygen, for example, "suggesting the validity of models that describe cometary dust as including organic material,' according to one Giotto report.
Similar material has also been proposed by some researchers to be a major component of a class of meteorites Meteorites
See also astronomy.
the science of aerolites, whether meteoric stones or meteorites. Also called aerolitics.
the study of meteorites. Also called meteoritics. known as carbonaceous chondrites A carbonaceous chondrite or a C-type chondrite is a type of chondritic meteorite which contains high levels of water and organic compounds, representing only a small proportion (~5%) of known meteorites. , and may also account for the surface appearance of such dark solar system objects
Following is a list of solar system objects by orbit as the rings of Uranus Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter. It was the next ring system to be discovered in the Solar System after Saturn's. and the dark portions of Saturn's moon lapetus. Indeed, one Vega team notes, "it is reasonable to assume that all of this material is carbonaceous car·bo·na·ceous
Consisting of, containing, relating to, or yielding carbon.
of, resembling, or containing carbon
Adj. 1. in composition.' On the other hand, says one Giotto report, mass spectrometer spectrometer
Device for detecting and analyzing wavelengths of electromagnetic radiation, commonly used for molecular spectroscopy; more broadly, any of various instruments in which an emission (as of electromagnetic radiation or particles) is spread out according to some measurements suggest that the cometary particles are "only roughly similar' to a variety of carbonaceous chondrites known as type C1, which some scientists think may be the type most likely to represent unaltered, primitive solarsystem material.
A new finding about the comet, reported by researchers with Japan's Sakigake spacecraft, was the discovery that it seems to be emitting radio waves Radio waves
Electromagnetic energy of the frequency range corresponding to that used in radio communications, usually 10,000 cycles per second to 300 billion cycles per second. . First detected as far as 10 million km away, the signals showed their maximum intensity at the craft's closest approach, "indicating that the emissions are intimately related to the comet.' Several different emission patterns were detected, collectively termed "cometary kilometric radiation' and ranging from intense, sporadic bursts of broad-band noise to continuously rising and falling tones. Though the mechanisms that produce such emissions will--like virtually every other aspect of the encounters--require much additional study, the Sakigake team infers that the signals result from "moving shock or bow waves in the coma region of the comet, due to time-dependent variations in the solar wind solar wind, stream of ionized hydrogen—protons and electrons—with an 8% component of helium ions and trace amounts of heavier ions that radiates outward from the sun at high speeds. .'
Vast amounts of study lie ahead for all of the Halley-encounter scientists, ranging from the nature of the solar wind's interaction with the comet (the presence or absence of a conventional "bow shock' may be too easy an answer) to the results of additional computer-processing of the spacecraft photos. NASA NASA: see National Aeronautics and Space Administration.
in full National Aeronautics and Space Administration
Independent U.S. hopes, meanwhile, to start developing a visit to another comet, this time for an extended look in the 1990s. Rewriting the textbooks will not be easy.
Photo: Comet Halley's irregular, double-lobed nucleus, photographed on March 9 by Vega 2 from 8,030 km away, 1.5 seconds before the spacecraft's closest approach.
Photo: Brightness contours of the above image.
Photo: Five jetlike bursts from the nucleus, and a possible region of lesser activity, observed in the photo's outer portion (not shown).
Photo: The nucleus's solid shape is brought out by computer processing that compensates for differing brightness distributions at and around the nuclear region. Further processing may add topographic detail, possibly leading to reconstruction of the nucleus in three dimensions.