LAST week, NASA proudly announced that it is expecting twins. It got the
go-ahead for a two-satellite mission to map the plasma that girdles the Earth,
trapped by the magnetosphere.
The satellites will be launched in 2002 and 2004. And with their binocular
vision scientists hope to get the first three-dimensional pictures of the
Earth鈥檚 magnetic 鈥渨eather鈥�. Such pictures could provide early warning of
electrical storms that can knock out satellites and power grids on the ground.
Each satellite will image the Earth鈥檚 magnetosphere, the boundary between the
atmosphere and interplanetary space. This region is filled with a hydrogen and
oxygen plasma that is trapped by the Earth鈥檚 magnetic field.
But as the TWINS satellites cannot look at those charged particles directly,
they will look at neutral ones instead. When one of the charged particles hits a
neutral atom from the upper atmosphere, the charge transfers to the atom, and
the newly neutralised particle shoots out of the plasma because it is no longer
trapped by the magnetic field, explains Los Alamos National Laboratory鈥檚 Ruth
Skoug, the TWINS project leader.
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鈥淣eutral atoms are the key,鈥� says Thomas Moore, a physicist at NASA鈥檚 Goddard
Space Flight Center. 鈥淭hey鈥檙e the only thing that comes out of that volume of
space that you can sample.鈥� Analysing the energy and paths of these particles
will enable the scientists to capture an image of the plasma.
With their 3D picture of the plasma, scientists will be able to infer the
magnetic weather patterns that constrain it. They aim to make a movie of the
Earth鈥檚 magnetic fields as they are battered by solar winds.
The whole project will cost a relatively modest $18 million because
the satellites will use existing detector technology. The detector design is
borrowed from an instrument on the IMAGE satellite, a more complicated
spacecraft due for launch in 2000.
The trick is to allow enough particles into the detector to create a good
picture, while filtering out high energy photons that would otherwise flood the
image. So the detector incorporates a gold grating, or series of microscopic
slots, to filter out most of the photons. Each of the TWINS will also cut costs
by hitching a ride into orbit on previously planned launches of non-NASA, US
government satellites.
The satellites will take a very eccentric 鈥淢olniya鈥� orbit, which will make
them swoop back and forth through the magnetosphere and up to an altitude of
nearly eight Earth radii. 鈥淲e want to get far away from the Earth to get a good
picture,鈥� says Craig Pollock, a physicist at the Southwest Research Institute in
San Antonio and a TWINS coinvestigator.
TWINS will give scientists a way to figure out how solar-powered magnetic
disturbances form. 鈥淚n the late evening, about midnight, there鈥檚 an explosive
acceleration of particles鈥攁 magnetic storm,鈥� says Moore. 鈥淲e鈥檙e not sure
how that works yet. TWINS will be an imaging tool to really learn the answer.鈥�