October 28, 2004A nearby star blast
In 1999, an interdisciplinary team of German scientists led by K. Knie of Munich's Technical University discovered supernova debris in 2-million-year-old ocean sediments. The debris took the form of high concentrations of iron-60 (60Fe, half-life: 1.5 million years) — radioactive atoms produced on Earth in smaller amounts — in three layers on the South Pacific seafloor.
Now, Knie and colleagues report finding the isotope in 28 sediment layers from another site 1,900 miles (3,000 kilometers) away. The new samples, says the German team, represent the remains of a star that exploded 2.8 million years ago (give or take 0.3 million years) less than 300 light-years away. One effect of Earth's scooping up this star-stuff, they argue, might have been a 300,000-year-long increase in the flux of cosmic rays. The work appears in the October 22, 2004, issue of Physical Review Letters. — Francis ReddyTides can prompt earthquakes
UCLA scientists discovered strong ocean tides can trigger earthquakes. The researchers conducted a statistical analysis of more than 2,000 earthquakes worldwide of magnitude 5.5 and higher that occurred between 1977 and 2000. They found a strong correlation between when earthquakes occurred and when tidal stress on faults was high. The gravitational pull of the Sun and Moon on Earth creates tides, which raise and lower the amount of stress on faults twice a day.
"The earthquakes would have happened anyway," explains graduate student and team member Elizabeth Cochran, "but they can be pushed sooner or later by the stress fluctuations of the tides." The scientists studied subduction zones (where tectonic plates overlap) along continental coasts, where tides are strong. For California and most other places, the correlation between tides and earthquakes is minimal — tides may influence the earthquake rate by only as much as 2 percent — because the faults are inland and the tides are much smaller.
The team confirmed its results October 21 in Science Express
, the on-line journal of Science
; the research will appear in a November issue. — Laura BairdChange shape, change orbitEarlier this month,
scientists showed that Earth's rotation swirls nearby space-time — the fabric of our universe, according to Einstein's general relativity — and measurably alters satellite orbits. Last year, scientists proposed that space-time's properties also allow a satellite to make tiny orbital changes just by changing its shape, without using rockets or thrusters. Now, Michael Longo, a physicist at the University of Michigan, predicts a much larger effect from old-style Newtonian physics alone.
Imagine a dumbbell-shaped satellite orbiting Earth. If one mass remains slightly farther from Earth than the other, the outer mass feels slightly less gravity than the inner mass. Pull the two masses together, and the differential force on the separate masses will cause the dumbbell's center-of-mass to move inward slightly. According to Longo, if this were done to a 100-meter-long dumbbell in a highly elliptical orbit, the satellite's closest approach to Earth would move inward 1 millimeter on each pass. Such a satellite could use sunlight or other stored energy to change its shape — and thus its orbit — without the need for propellant. Longo's work appears in the October 2004 issue of American Journal of Physics. — Francis ReddyDART launch delayed
NASA has delayed the launch of the Demonstration of Autonomous Rendezvous Technology (DART) spacecraft due to a possible contamination. The next possible launch date is November 4.
DART team members discovered loose foil on the spacecraft's launch vehicle. It has since been returned to the vehicle-assembly building for closer inspection.
The DART mission will help scientists plan and direct future autonomous spacecraft rendezvous. The unmanned spacecraft will use only computers and sensors to perform all of its docking functions. The project's results will enable NASA to prepare in-space assembly, services, and rendezvous operations better.
The launch has been delayed twice already. — Jeremy McGovern"Marsquakes" shaped Red Planet
Scientists from the Southwest Research Institute (SwRI) in San Antonio, Texas, believe seismic activity on the Red Planet shifted its landscape. This created crater chains that could have served as channels for past water flow.
"These faults could now serve as reservoirs for water or ice, making these locations of potentially great interest to the scientific community searching for signs of life on Mars," says David Ferrill, a senior program manager at SwRI.
SwRI team members compared data and photographs of features on the martian surface with pit chains, strings of depressions, found in Iceland. They conducted laboratory experiments to reconstruct the processes they believe formed the pit chains.
It is more difficult to locate pit chains on Earth than Mars because surface erosion and higher gravity on Earth results in smaller pits that are rapidly erased. — Jeremy McGovern