A wind of charged particles that stream constantly from
the sun is at its lowest level ever recorded in the 50 years since spacecraft
have made the measurement possible.
The Ulysses
spacecraft observed the weak solar winds, the constant, high-speed stream
of particles that races from the sun, during a quiet period in the sun's
activity. The solar weather cycle affects Earth and other planets in the solar
system.
"We know that the sun has been this cool before,
this inactive before," said Nancy Crooker, a physicist at Boston
University in Boston, Mass., during a NASA teleconference on Tuesday. "But
that was prior to the Space Age, so we didn't have actual physical measurements
until now."
The solar wind's charged particles blow out from the sun
at a blistering 1 million mph, sweeping away background radiation and colliding
with incoming galactic cosmic rays from distant stars. It effectively encloses
our solar system in a protective bubble called the heliosphere.
For Ulysses, the finding is a swan song. The probe, which
is dying, has spent the past 17 years watching the solar wind rise and fall
during the sun's 11-year cycle of activity. Fast and steady solar winds came
from the upper latitudes, while more unpredictable, slower solar winds blew
from the sun's equator.
But during the sun's latest quiet period, the spacecraft
found that the overall solar wind is 20 to 25 percent weaker, in terms of
pressure and density, than during the previous solar minimum. Weaker solar
winds mean a smaller and leakier heliosphere bubble, a protective sheath that
surrounds the entire solar system. That means more background cosmic radiation
gets through.
The danger to Earth from galactic
cosmic rays remains rare in any case. However, future missions to the moon
or Mars would probably try to plan the best times to travel outside of
Earth's own protective magnetosphere bubble. Astronauts on the International
Space Station and the space shuttle remain safely within Earth's magnetosphere.
When the sun is active,
charged solar particles present risks to astronauts and even satellites.
"During a solar minimum there's relatively little
solar radiation, so the primary risk is more from galactic cosmic rays,"
said Dave McComas, a Ulysses scientist at the Southwest Research Institute in
San Antonion, Texas.
Current studies suggest that astronauts should undertake
long-duration missions during solar maximums, when increased solar activity
still poses a risk but also shields better against the constant threat of
galactic cosmic rays, McComas added.
The solar wind's effects are also felt at the edge of the
solar system, where the Voyager 1 and Voyager 2 probes have entered the heliosphere's
outer envelope layer. Voyager 2 arrived at the solar system edge later than
Voyager 1, yet found that that boundary to be almost a billion miles closer – a
possible sign of the shrinking protective bubble.
The wanderings of Ulysses are drawing to a close with its
latest observations of the sun. The spacecraft can only look forward to an icy space tomb
once its radioisotope thermoelectric generators fail to provide enough heat to
keep the onboard hydrazine fuel from freezing.
However, scientists expressed nothing but satisfaction
with the joint NASA-European Space Agency mission, which has lasted four times
longer than the spacecraft's expected lifetime.
"For the first time we've got a spacecraft that
flies over the poles of the sun," said Ed Smith, a Ulysses scientist at
the Jet Propulsion Laboratory in Pasadena, Calif. "That allows us to see
changes in the heliosphere in three dimensions, or four if you count
time."
At least one lingering question still stands out —
whether the current weak solar winds represent an isolated phase or a longer
term trend in future solar cycles.
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