An artist's impression of the Solar Sentinels mission: six probes used to improve future forecasting of solar storms
Credit: NASA
WASHINGTON, 7 September 2006: With NASA planning to send astronauts back to the Moon by 2020, a new system is required for forecasting weather in space, U.S. scientists say.
In his 1970s book, Space, James Michener depicted a fictional Apollo mission that lost its crew to radiation from a massive solar flare - a violent explosion in the Sun's atmosphere with an energy equivalent to tens of millions of hydrogen bombs.
He based his tale on what might have occurred were it not for some lucky timing: a massive flare on 7 August 1972 occurred between the Apollo 16 and Apollo 17 missions in April and December - mankind's last two journeys to the Moon.
The event still resonates today, as U.S. space agency NASA prepares to send astronauts back to the Moon, and on even longer journeys to Mars. With crews "out there" for extended periods, "the chances go way up that they'll be caught in the middle of a storm," says David Hathaway, a solar physicist at NASA's Marshall Space Flight Centre in Huntsville, Alabama.
"The educated view about the August 1972 flare is that a crew on the surface of the Moon would have gotten really sick." Or worse. Lawrence Townsend of the University of Tennessee in Knoxville and his colleagues calculated that energetic particles from a super flare, like one recorded in September 1859, could kill.
Not only humans are at risk. Miniaturised modern electronics are more susceptible to radiation damage than their predecessors were 40 years ago. A recent example: Japan's Nozomi Mars probe was crippled by an intense solar energetic particle event in April 2002. Future NASA probes are going to be vulnerable, too.
"NASA needs reliable forecasts of space weather," says Hathaway. The problem is, scientists are still learning to make these forecasts. "It's often said that space weather forecasting is 50 years behind Earth-weather forecasting. We need to catch up."
A good "catch-up" opportunity is just around the corner. Solar Cycle 24 is beginning and it is expected to reach maximum between 2010 and 2012. During that time, there will be an abundance of solar flares and coronal mass ejections - sudden ejections of materials from the Sun's atmosphere - for astronomers to study.
"NASA astronauts are scheduled to return to the Moon around 2020," says Robert Lin, a solar physicist at the University of California in Berkeley. "We've got only one solar maximum left to learn what we need to know" to protect those crews.
Lin recently chaired a team commissioned by NASA in 2004 to devise a Solar Sentinels mission that would help scientists learn to predict solar storms in time to warn astronauts. Their report has just been published.
They note that several new spacecraft are already planned for studying the Sun during Cycle 24, including the Solar Terrestrial Relations Observatory (STEREO), Solar-B, and the Solar Dynamics Observatory. These missions will take 3D pictures of solar explosions, map the unstable magnetic fields of sunspots (the source of flares), and probe the sun's inner magnetic dynamo.
But that may not be enough, according to the Sentinels team. Additional eyes and sensors are needed, they say, to help address two key questions: how are solar energetic particles accelerated from the Sun, and how are coronal mass ejections born?
In particular, they recommend the following:
Four identical probes, called Inner Heliospheric Sentinels, which would be stationed inside the orbits of Venus and Mercury. These spacecraft would sample freshly accelerated solar energetic particles close to the Sun.
A Near-Earth Sentinel, which would comprise a single probe orbiting Earth. This Sentinel would carry a coronagraph, a special telescope for observing the Sun's faint corona where coronal mass ejections get their start.
A single probe, called a Farside Sentinel, which would be used to watch the farside of the Sun. Together with other spacecraft, this sentinel would provide a complete picture of the sun - not just the half we see from Earth.
"The Sentinels would be based on existing technology so they could be built and launched in time for the next solar maximum," says Lin. The Sentinels themselves aren't intended as a continuous operational warning system. "But what we learn from it will naturally form the basis for a true operational network."
NASA is considering the Sentinel team's recommendations. Meanwhile, Solar Cycle 24 is beginning.
