Number 150, November 5, 1993 by Phillip F. Schewe and Ben Stein
CHAINS OF CRATERS ON CALLISTO AND GANYMEDE , two of Jupiter's moons,
are now explained as being mostly due to split comets like Comet Shoemaker-Levy,
which has broken into a chain of 22 fragments and is headed for a smashup
with Jupiter in July 1994. Jay Melosh of the University of Arizona and
Paul Schenk of the Lunar and Planetary Institute cite as evidence the fact
that nearly all of Callisto's crater chains are on the Jupiter-facing hemisphere.
In a separate paper, Melosh and James Scotti use a tidal-breakup model
to calculate the size of the Shoemaker-Levy parent comet as being about
2 km. (Nature, 21 Oct. 1993.) Hubble Space Telescope pictures of the fragments
suggest a size more like 3-4 km. The uncertainty in the size estimates
translates into an uncertainty factor of 1000 for the energy of the collision.
In any case, the ensuing explosions will cause Jupiter to ring, which might
provide some rare seismological information about the planet's interior.
(Nature, 28 Oct. 1993.) Study of reverberating waves in Jupiter's atmosphere
should allow scientists to sharpen their views on the nature of the Great
Red Spot. (Science, 22 Oct.)
HINGED NETWORK CRYSTALS , a hypothetical class of materials, have bizarre
mechanical and thermal properties: when stretched they become thicker (i.e.,
they have a negative Poisson's ratio) and more dense; when heated they
contract. These materials, investigated by Ray Baughman of Allied Signal,
Inc. (Morristown, NJ) and Douglas Galvao of the Physics Institute in Sao
Paulo, Brazil, consist of twisted chains of interconnected polydiacetylene
molecules. Unlike other negative-Poisson materials (generically called
auxetics), these computer-designed materials derive their predicted novel
properties from the interconnectivity of various parallel and non- parallel
bonds in alternating layers, which allows the chains to twist, hingelike
and with a minimum amount of energy expenditure, without changing the network
bond lengths. Such crystals, if they could be synthesized by chemists,
might also have interesting optical and electrical properties. (Nature,
21 Oct.)
ATOM-OPTICS TECHNIQUES FOR MICROCIRCUIT LITHOGRAPHY keep improving.
Using the electric fields of laser light as a lens, scientists at NIST
(Robert Celotta, 301-975-3710) have been able to steer a beam of chromium
atoms onto a silicon substrate with great dexterity; the result is a series
of thin ridges 65 nm wide, about 35 nm high, and spaced about 215 nm apart.
(Science, 5 Nov.)
THE PHYSICS OF BUNGEE JUMPING involves primarily the conversion of gravitational
potential energy into the elastic energy of a stretched cord. Originating
on Pentecost Island in the Pacific, the practice of a person jumping from
a high place harnessed to a flexible attachment was introduced to Western
culture in 1979 by the Oxford University Dangerous Sport Club. An all-important
parameter, the amount by which the cord stretches at the bottom of the
fall, should be accurately known in order to avert death. It is given by
the following equation: extension = mg/K + squ root (m**2 g**2/ K**2 +
2mgl/K), where g is the gravitational acceleration, K is the cord's stiffness,
L is the free length of the cord, and m is the mass of the plummeting object.
(Paul Menz, Cumberland County College, in The Physics Teacher, Nov. 1993.)
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