If humans live on the moon some day, they might turn on the
weather forecast just as they do on Earth. But in space, they won't fear rain
storms, but sun storms.
During a solar
radiation storm, the sun emits huge sprays of charged particles that can
disable satellites and would harm humans in space if they're not properly
protected. Although these storms are notoriously difficult to predict,
a new method of forecasting storms can give up to an hour's warning.
The technique relies on measurements taken by the NASA/ESA
SOHO (Solar and Heliospheric Observatory) spacecraft in orbit around the sun. SOHO,
launched in 1995, has been near death more than once, but clever engineers,
working with the equivalent of electronic duct tape and more than a dash of
luck, have kept it running well beyond its expected lifetime. And now the probe
is making fresh contributions to solar weather forecasting.
The new technique relies on SOHO's Comprehensive
Suprathermal and Energetic Particle Analyzer (COSTEP), which monitors the
radiation coming from the sun.
Scientists analyzed the data COSTEP
recorded from sun storms during the first six years of the spacecraft's launch,
and compiled a matrix that can predict a full-blown storm is coming after the less-dangerous
first wave arrives. Because there is about an hour's lag time between the early
arrival of electrons, and the eventual onslaught of more-damaging protons, this
new forecast system gives people and spacecraft enough time to take cover.
The method was described in the journal
Space Weather, and went online just in time for the recent launch of the
STS-122
shuttle mission.
Radiation risk
On Earth, people are protected from the brunt of solar
radiation by our planet's atmosphere and magnetic field. Even astronauts
orbiting the Earth on the International
Space Station are shielded from much of it since Earth's magnetic field
extends far enough to cover them. The astronauts do retreat to a
radiation-shielded part of the station during severe storms, however.
But when people venture beyond our planet's protective
bubble, they will really be at risk of severe health effects from solar storm
radiation. It was a concern even during brief lunar missions in the Apollo era.
"We know what happens when humans are exposed to a lot
of radiation — Hiroshima and Nagasaki have given us an example," said Arik
Posner, a senior research scientist at Southwest Research Institute in San
Antonio, Texas, who developed the new forecast technique. "Humans are
constantly hit by some sort of radiation, even on Earth, at very low levels. But
what happens when you increase the radiation level slightly? We don't know. But
the best thing to do is limit exposure."
DNA damage
The most damaging solar-storm radiation particles are
fast-moving protons. These energetic particles can destroy human tissue and
break strands of DNA.
The radiation is also dangerous for spacecraft. When a
speeding particle hits electronics, it can cause bits to change from zeroes to
ones or vice versa, prompting program malfunctions. If power is knocked out, a
satellite can fail completely.
Scientists don't fully understand the physics behind solar
storms. They know they are closely related to the cycles of the sun's magnetic
field, and that they emerge from relatively cool, intensely magnetic
regions of the solar surface called sunspots. The magnetic field is always
changing — about every 11 years its magnetic north pole becomes the south pole,
and vice versa. As the polarity cycles and regional instabilities develop, the
sun's magnetic field lines get twisted and tangled, resulting in big knots of
extremely strong magnetism.
Although the exact mechanism isn't known, somehow these
magnetic field events are tied to eruptions of matter and energy.
"We don't have a really good idea of exactly how these
processes happen," Posner said. "It's a very active area of
research."
Huge explosions on the sun accelerate charged particles to
near light-speed. The lighter particles, electrons, can speed up more easily,
so they reach SOHO's radiation detector first. This allows the spacecraft to
give humans about an hour's advance notice before the heavier protons and ions
arrive and wreak havoc.
"The detector can only say that there is already an
event in progress," Posner told SPACE.com. "It cannot predict one before
it starts. But it does give you a tool to forecast these events."
After Posner and his collaborators designed the matrix to
predict a coming storm, they tested it on the sun storm record from 2003, a
year that did not comprise the initial data set on which the matrix was based. When
it measured an increase in high-speed electrons, the program was able to
successfully predict that a solar storm was coming. It forecasted all four
major storms of 2003 with advance warnings ranging from 7 to 74 minutes.
Enduring craft
SOHO is a joint project between NASA and the European Space
Agency (ESA). Three times it has seemed to be a doomed mission. During the most
recent scare, in 2003, a stuck motor drive would not permit the craft's primary
antenna to move. The antenna is used for transmitting pictures and data back to
Earth, and it must be pointed toward the planet. Scientists feared lengthy
blackout periods.
Engineers employed some tricks, including flipping the craft
upside down during portions of its orbit, to get the data flowing again.
In 1998, changes in the spacecraft's software inadvertently
sent it into a flat spin. The electricity-providing solar panels turned edge-on
to the Sun, and SOHO lost power. It could not orient itself or maintain its
temperature — critical in the frigid environment of space.
The mission appeared doomed.
After nearly three months, with much perseverance by the
ground team, contact was re-established and the craft's orientation was fixed.
Sunlight hit the panels, and SOHO was back.
Later in 1998, another problem nearly ended the mission
again. The craft's last navigational gyro failed.
"In a race against time and a depleting
maneuvering-fuel supply," explained SOHO Project Scientist Bernhard Fleck
of ESA, engineers had to develop a software patch to get the craft back in
operation without the gyro. New software was developed in early 1999 to allow a
spacecraft to maintain attitude without gyros. Engineers sent it up,
"making SOHO the first 3-axis stabilized spacecraft to be operated without
any gyros," Fleck said.
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