Berkeley – July 6, 1998 – As part of its move toward “faster, cheaper, better” space missions, the National Aeronautics and Space Administration has given the University of California, Berkeley, total control of a new $72 million satellite scheduled for launch in the year
2000 to study solar flares.
Not only will university scientists design and build the
instruments, but they will choose a company to make the launch
vehicle and even erect a radio antenna in the Berkeley hills to
communicate with the satellite and download data.
No university has been given responsibility for both science and
mission operations since at least 25 years ago, when a small
satellite was controlled by the University of Colorado.
NASA will retain responsibility for the booster and the launch of
the satellite, called HESSI (High Energy Solar Spectroscopic
Imager), but then UC Berkeley will take over.
“We are the first of the Small Explorer science missions to do
the whole thing,” said project leader Robert P. Lin, UC Berkeley
professor of physics and the new director of UC Berkeley’s Space
Sciences Laboratory (SSL). “For universities this is a better way
to do it, and Berkeley is perfectly suited to direct the entire
project.”
The laboratory has served as the science operations center for
two recent satellites: the FAST Explorer, launched in 1996 to
study the aurora; and the Extreme Ultraviolet Explorer, launched
in 1992 to survey the heavens in the extreme ultraviolet.
In addition, more than a dozen scientific satellites now orbiting
Earth or planned for launch contain at least one instrument built
by scientists and engineers at the Space Sciences Laboratory. Lin
himself has instruments aboard the Lunar Prospector and Mars
Global Surveyor. Instruments built at SSL also are mounted on
many terrestrial telescopes, as well as on the Hubble Space
Telescope.
Based partly on the success of these projects, NASA decided to
relinquish even more control to universities which build the
instruments and do the science.
“When we initially put together this mission the estimated cost
was 10 times as much, in part because we had to comply with the
way NASA did things in large projects,” said Lin, who signed the
HESSI contract with NASA this spring. “Now that NASA has changed
its philosophy, we can be lean and efficient — and more
responsible to the public.”
HESSI will carry a single telescope to take both x-ray and gamma
ray snapshots of solar flares — seen in visible light as sudden,
rapid and intense brightenings of the sun’s surface near
sunspots, which are regions of strong magnetic field.
“Solar flares are probably the most powerful explosions in the
solar system, the largest releasing as much energy as several
billion megatons of TNT,” said Lin. “Nobody knows how the sun is
able to release this much energy, or why up to half of that is in
the form of high-energy particles.”
The satellite will be the first to look in detail at hard x-rays
from flares, obtaining both images and spectra in search of clues
as to how the particles are accelerated to such high energies.
The satellite also will be the first to image gamma-ray emission
from the sun.
A. Gordon Emslie, a co-investigator and professor of physics at
the University of Alabama in Huntsville, emphasized that what is
learned about how solar flares accelerate particles will provide
insight into many other astrophysical phenomena.
“HESSI is a flare mission, but what we learn will apply to all
processes where particles are accelerated to high energies, as in
active galactic nuclei, gamma-ray bursters, the magnetospheres
around planets and even terrestrial fusion reactors,” Emslie
said. “We’re using the sun as our ‘laboratory’ because of its
closeness compared to other astrophysical objects, but we are
addressing fundamental questions of physics.”
Flares have been of interest for a long time because the x-rays
they produce disrupt the Earth’s ionosphere, interfering with
radio and TV communications. In addition, the energetic particles
streaming out from flares can endanger satellites and astronauts
in orbit outside the Earth’s protective magnetic field.
HESSI’s launch is timed for the peak of the sun’s 11-year cycle
of solar activity — expected sometime in the year 2000 or 2001 —
when flares are most common. During its two-to-three-year
lifetime it is expected to study 1,000 or so hard x-ray flares
and about 100 gamma-ray flares.
Scientists think flares are caused by magnetic events originating
in the sun’s corona or outer atmosphere, which somehow
reconfigure huge regions of magnetic field, initially formed as
giant loops rooted in the solar surface. These changing fields
accelerate electrons and ions to high speeds. The temperature
inside a flare typically reaches 10 or 20 million degrees
Celsius, and can be as high as 100 million degrees Celsius (180
million degrees Fahrenheit).
“The sun is the most powerful particle accelerator in the solar
system,” Lin said.
As the fast-moving electrons collide with atoms in the atmosphere
of the sun they abruptly slow down, producing x-rays
(bremsstrahlung or braking radiation). In addition, when the
high-energy ions hit the nucleus of an atom, they can stimulate a
nuclear reaction, which produces even more energetic gamma-rays.
“By studying the x-rays and gamma-rays given off during these
processes, we can determine the distribution of energetic
particles and get a pretty good idea of what accelerated them,”
Lin said.
One theory the team hopes to test is whether the particles are
being accelerated like the electron beam in a TV set — by a
strong electric field — or through a process called stochastic
acceleration. Just as a tennis ball bouncing between two rackets
speeds up as the rackets get closer together, so particles
accelerate when bounced between areas of strong magnetic field —
areas that act like magnetic “mirrors.”
A major complication of studying flares in detail is that x-rays,
in particular “hard” or high energy x-rays, and gamma-rays are
impossible to focus into an image with a lens or mirror. Instead,
the scientists will obtain images of the flares using nine
rotating modulation collimators.
Each collimator is composed of two widely separated grids sitting
in front of a detector. The grids operate like pairs of Venetian
blinds, each pair having different spacings and different
orientations. Just as your view through crossed blinds changes as
you move, so does the image seen by the detector as the satellite
rotates, Emslie said. The changing information is converted into
an image of the flare.
One advantage of this method is that, while a high-resolution
image of the flare can be obtained each half rotation, every two
seconds, lower resolution images can be obtained in as short a
time as a tenth of a second. This allows the telescope to take
movies of flares. A spectrometer aboard will also measure the
specific gamma ray emission lines, which indicate the types of
elements comprising the accelerated particles.
Able to image the entire sun at once, HESSI can resolve detail on
the order of about 1,000 miles in size. For comparison, the sun
is 864,000 miles across, while flares can be 60,000 miles long.
HESSI will be placed in an orbit that passes over Berkeley six
times a day, allowing a ten-minute window for communication
between ground and spacecraft.
Lin heads a science team which includes nearly 20 scientists from
the U.S., Switzerland, France, Japan, the United Kingdom and the
Netherlands.
Another Small Explorer satellite also was approved recently by
NASA, for launch in 2001. Called the Galaxy Evolution Explorer
(GALEX), it is to be built and operated by Caltech in
collaboration with the Jet Propulsion Laboratory in Pasadena,
which has controlled many satellites and space probes over the
decades. GALEX will carry an ultraviolet telescope during its
two-year mission to explore the origin and evolution of galaxies
and the origins of stars and heavy elements.
