Research Profile
Harlan Spence - Tightening Our Understanding of Earth's Radiation Belts
Beginning several hundred kilometers above Earth is a region prone to such severe
“space weather” that astronauts generally avoid it. Not Physics Professor Harlan
Spence, however. Although earthbound, he helps lead a NASA project that has sent two
unmanned probes to explore this region, known as the Van Allen radiation belts.
In late August 2012, the twin Van Allen Probes ‒ named after the astrophysicist who in
1958 discovered these volatile radiation belts ‒ launched from Cape Canaveral to begin
the most in-depth study yet of this harsh and unpredictable environment. This zone,
whose outer ring extends about one-tenth of the way to the moon, is the site of intense
magnetic and radiation storms that can damage spacecraft and harm human health.
The mission will continue for at least two years.
“Having the probes orbiting in the radiation belt environment is like
seeing things through the Hubble telescope instead of a small,
Earth-based telescope,” Spence says. “These probes really take us to
the next level where we get both a big picture and all the details.”
Like many of his other projects supported by NASA and the National
Science Foundation, Spence’s work with the Van Allen Probes builds
on his long-running investigation of space weather. “It has very
practical societal relevance,” says Spence, who directs UNH’s Institute
for the Study of Earth, Oceans, and Space. “We’re a space-faring
nation and world. So much of the world economy depends on
space technologies that can be affected by space weather.”
A graphic depiction of the twin Van Allen
Probes in orbit within Earth's magnetic field.
Credit: The Johns Hopkins University Applied
Physics Laboratory
Indeed, hundreds of commercial weather, communications and navigation satellites ‒ each typically costing
about half a billion dollars ‒ must spend time in the Van Allen radiation belts as they move in their orbits. In
these donut-shaped areas surrounding Earth, the satellites encounter subatomic charged particles, mostly
electrons and protons. Like surfers, the invisible particles “catch” electric and magnetic waves, accelerating to
extremely high speeds. So high, in fact, that some electrons approach the speed of light, or 186,000 miles per
second. Known informally as “killer electrons,” these particles are moving fast enough to penetrate through a
spacecraft’s metal skin and destroy one of its electronic parts, potentially crippling it.
The aim of the Van Allen Probes mission is, essentially, to find out what makes these particles go bad. “The
probes travel to the scene of the crime,” Spence says. “We’re looking at what causes the radiation belts to be
pumped up.”
Spence’s team developed the Energetic Particle, Composition, and Thermal (ECT) Plasma Suite, which
accounts for six of the ten instruments onboard each of the probes. The ECT Suite measures the radiation
particles, with the goal of discovering what triggers their acceleration.
Scientists already know that the drivers of space weather in the radiation belts usually can be traced back to
the sun. Explosions called coronal mass ejections send out blast waves of highly charged particles. In addition,
solar wind carries the sun’s magnetic field into the space between planets. Yet it remains unclear exactly how
these events produce remarkably rapid effects in the radiation belts ‒ or even what those effects will be,
Spence says. After a solar storm, for instance, scientists cannot predict whether activity in the belts will
increase, decrease, or remain the same.
That is now changing, however. Already the Van Allen Probes are relaying
intriguing data to researchers on the ground, thanks in part to a fortuitous
upturn in solar activity. Until recently, the sun was the quietest it had been
since the beginning of the space program more than fifty years ago. After
the launch, Spence says, “The sun woke up as if on cue and threw out some
unbelievable events that are now providing deeper insight into how things
actually work in the radiation belts. We’re seeing changes in dynamics and
spatial structuring that are new and mysterious. It’s challenging our
traditional understanding of this environment, and I have no doubt that
we’ll be rewriting the textbook on radiation belt science.”
NASA's Solar Dynamics Observatory
spacecraft captured this massive
coronal mass ejection erupting from
the sun on Aug. 31, 2012.
That would have important economic implications. Knowing the range of space weather expected over the
long-term would allow for the creation of satellites strong enough to withstand weather extremes while
avoiding costly over-engineering. It’s analogous to getting information on the magnitude of a 100-year flood
before constructing a riverfront home, Spence says. Moreover, being able to predict what space weather will
be in the near future could lead to the safer handling of spacecraft. For instance, operators might choose to
avoid risky maneuvers if they knew that a storm was coming.
The radiation belts are dangerous not only for satellites, but also for humans.
“Learning how to be a space-faring species beyond the cocoon of Earth is a real
challenge,” Spence says. Highly charged particles can penetrate space suits, exposing
astronauts to unhealthy levels of radiation. The knowledge gained by studying the
Van Allen belts could be applied to other radiation environments, helping astronauts
stay safe as they venture to the Moon again, to Mars, or deeper into the solar system.
Ultimately, the probes will provide insight into deep space, since the same processes also are occurring at very
distant stars. Says Spence, “This allows us to understand objects we could never travel to in a spacecraft.”
For more information on Harlan Spence and the Van Allen Probes, visit:
http://www.eos.unh.edu/Faculty/spence
http://rbsp-ect.sr.unh.edu/
http://www.nasa.gov/mission_pages/rbsp/mission/index.html
Story by Sonia Scherr. 6/25/13
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Research Development and Communications, Office of the Senior Vice Provost for Research
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