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Field
Notes
by Laura Wright
Losing Landsat 4
The operational costs of the U.S. Geological Survey’s aging Landsat
4 and 5 Earth observation satellites have surpassed their scientific utility.
The agency has already begun to decommission those satellites and push
forward with Landsat 7, the survey announced May 8.
The Landsat mission was launched in 1972 with Landsat 1. Since then,
successive satellites have gradually collected a global archive of Earth
images exhibiting both natural and human-induced changes in surface cover.
Landsat 4 and 5 were put into operation in 1982 and 1984, respectively.
Congress turned over their operations to the private sector in 1985 in
hopes of spurring outside interest for later missions. The satellites were
expected to last no more than two years, according to original USGS estimates,
but Landsat 4 continued to relay land-surface images at a 100-mile resolution
until just a few years ago when its communication hardware failed. Landsat
5 can still send high-quality image data back to antennas on Earth, but
compared to the cost and benefit of the newer Landsat 7, the Landsat 5
mission is no longer cost-effective and is slated to end, the USGS says.
Signals from space
On April 28, scientists in Madrid received weak signals from what at
first appeared to be the constellation Taurus, but was soon discovered
to be Pioneer 10, launched from Cape Kennedy in 1972. Revolutionary for
its time, Pioneer 10 was the fastest spacecraft ever built and was the
first to travel through the asteroid belt, the first to use a planet’s
gravitational pull to change its course and also the first to reach solar-system
escape velocity. Now, nearly 30 years later, Pioneer 10 has traveled twice
as far from the Sun as Pluto, outliving its life expectancy by 27 years.
The radio signals that reached Earth on April 28 took 11 hours to make
a 7.3-billion-mile journey — that’s 78 times the distance between Earth
and the Sun. Engineers believe that its radio transmitter is still being
powered by electricity generated by heat from decaying plutonium-238. However,
the thermocouple device — the part of the spacecraft that converts the
heat energy to electricity — is decaying more rapidly than the plutonium
and NASA scientists do not expect the radio to transmit signals much longer.
Once its power runs out, Pioneer 10 will continue to travel deeper
into space. In about 300,000 years it will pass within three light years
of a star — the red dwarf Ross 248 in the constellation Taurus. Over the
next million years, if the 570-pound spacecraft is still intact, it will
pass 10 stars within a distance of nine light years. And in about 5 billion
years, Pioneer 10 will most likely be traveling through the Milky Way when
the Sun becomes a red giant and destroys Earth.
Supporting ‘Out of Africa’
Reporting in the May 11 Science, an international team of molecular
biologists rejected the idea that ancient humans in Asia contributed genetically
to modern humans. The team based their findings on analyses of genetic
mutations of the Y-chromosome in 12,127 Asian males.
The Y-chromosome is one of the best tools for tracing human evolutionary
history. The scientists looked for a particular mutation of the Y-chromosome
that molecular biologists believe evolved between 35,000 and 89,000 years
ago — spanning the period of time during which humans migrated out of Africa.
Under that lineage there are three additional mutations that can be used
as markers to determine whether all humans descended from the “Out of Africa”
lineage. In the absence of at least one of those three mutations, it could
be deemed that humans migrating from Africa had not replaced all humans
in Asia, refuting the theory that all modern humans came from Africa. What
the scientists found was that each of the males sampled carried one of
the three mutations under the “Out of Africa” lineage, indicating that
modern humans of African origin completely replaced earlier Asian populations.
This is further evidence for the “Out of Africa” theory.
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