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Voyages to the Sun
Editor's Note: This is the 16th in a series of essays
on exploration by Steven J. Dick.
Humanity's
epic voyages to the Moon are well known, the stuff of history. But what about
voyages to the Sun? Though they do not involve human spacecraft, those voyages
are no less epic. And on the occasion of the tenth anniversary of the launch of
one of the most successful voyages - the Solar and Heliospheric Observatory,
fondly known as SOHO - it is fitting that we recall those voyages to our
life-giving star.
The Sun is indeed our nearest star, a mere 8 light
minutes away, compared to 4.5 light years for the next star, the Alpha Centauri
system. A nuclear furnace generating prodigious amounts of energy, the Sun
provides the conditions necessary for life on Earth. It is a matter of practical
importance that we know how the Sun works, as well as a matter of theoretical
importance, since its proximity gives us the best information on how other
Sun-like stars work.
Image Above: A solar prominence hovers above
the Sun in this photograph from SOHO October 25, 2002. A prominence, seen here
in extreme ultraviolet light, is a cloud of gas shaped by the Sun's magnetic
field. 40 Earths could fit along the length of this prominence.
Credit :
SOHO - EIT Consortium, ESA, NASA
An object of worship and awe
from the days of ancient civilization, the Sun has been well-studied from the
ground. It has been known since the days of Galileo that the Sun rotates, and is
speckled with spots, though our knowledge of the nature of the spots (at only
6,000 degrees, cooler parts of the Sun's surface) and the 11-year sunspot cycle,
are relatively recent.
After early observations from sounding rockets,
the study of the Sun from space began, naturally enough, from Earth orbit. The
Orbiting Solar Observatory (OSO) was a series of eight orbiting observatories
that NASA launched between 1962 and 1971. Seven of them were successful, and
studied the Sun at ultraviolet and X-ray wavelengths. The OSO spacecraft
photographed the million-degree solar corona, made X-ray observations of a solar
flare, and enhanced our understanding of the Sun's atmosphere among its many
other achievements.
The Apollo Telescope Mount, though inelegantly
named, was an innovative program for astronauts to observe the Sun from Skylab,
the orbiting space station that made use of hardware in the aftermath of the
Apollo program. It was the most important scientific instrument aboard Skylab,
which operated for eight months beginning in May, 1973. Unhampered by the limits
of telemetry, the astronauts brought solar photographs back to Earth, including
x-ray observations of solar flares, coronal holes, and the corona itself.
Attempts to observe the Sun beyond Earth orbit are more recent. Ulysses,
known before launch as the International Solar Polar Mission, was deployed in
October, 1990 from the Space Shuttle Discovery. It was a joint mission of NASA
and the European Space Agency designed to gain a new perspective of the Sun by
viewing its polar regions.
Remarkably, it uses one of the tricks that
space navigators have learned - a gravity assist, whereby it travels first
outward to Jupiter before being hurled back toward the Sun. It reached Jupiter
in 1992, passed the Sun's south pole in 1994 and its north pole a year later. It
repeated these passes in 2000 and 2001, and will do so again in 2006 and 2007.
Quite a voyage!
With the first pass of Ulysses, scientists discovered
unknown complexities of the Sun and its surroundings, including different speeds
of the solar wind. Ulysses - named after Homer's Greek adventurer - did not
carry imaging instruments, and focused on the Sun's environment rather than its
surface. Fifteen years after launch, the spacecraft remains in good health.
This brings us to SOHO, also a joint American-European project, and
another epic solar voyage still underway. Launched Dec 2, 1995, its array of
instruments were designed to study the solar wind, as well as the Sun's outer
layers and interior structure.
In order to do this, it was placed in an
orbit 1.5 million kilometers from Earth, at a point known as the L1 Lagrangian
point, where the combined gravity of Earth and Sun keep it in an orbit locked to
the Earth-Sun line. Though still far from the Sun, this location, about four
times the distance of the Moon in the direction of the Sun, is ideal for
long-term uninterrupted observations with the Earth out of the way.
SOHO's scientific findings have been phenomenal. It has imaged the
structure of sunspots below the surface, measured the acceleration of the wind
from the Sun (streams of protons and electrons traveling at a million miles per
hour!), discovered coronal waves and solar tornadoes, and found more than 1000
comets.
Moreover, it has revolutionized our ability to forecast space
weather, and provided data on the variability of the Sun's energy, both of which
affect us directly on Earth. Some 140 Ph.D. theses have been written using SOHO
data, and almost 300 meetings held to discuss its findings.
And what
images SOHO has returned during its ten years! Both still images and movies
showing the dynamic Sun's prominences, flares, spots, coronal mass ejections,
and otherwise lively gyrations fill the SOHO website at http://sohowww.nascom.nasa.gov/
As with most voyages,
SOHO has remarkable stories to tell, mostly of what it has seen. But there are
other stories: for example, of the day on June 24, 1998 when a ground controller
accidentally turned the satellite to face away from Earth. The satellite was
feared lost, but a month later the enormous 305-meter radio telescope at
Arecibo, Puerto Rico found SOHO.
After a lengthy battery recharge, the
satellite was declared recovered on September 18. By the end of the year its
gyros had failed, but ground controllers found other ways to control the
spacecraft, and it continues to send spectacular images to this day. Designed
for a nominal mission of two years, it celebrates it's tenth anniversary this
month, and is expected to operate at least until 2007.
With its nine
European and three American principal investigators, SOHO is also another
example of international cooperation in space. It was built by companies in 14
European countries, and is operated from Goddard Space Flight Center.
So
here's to SOHO, its predecessors, and the many other spacecraft, including
TRACE, Cluster, Polar, Wind and Geotail, that have contributed to our
understanding of Sun-Earth connections. They are connections most of us take for
granted every day. But solar scientists know that improved knowledge of the Sun
and its effects on Earth can only be a good thing - another of the reasons why
we explore.
Steven J. Dick
NASA Chief Historian
Further Reading
Bester, Alfred. The Life and Death of a Satellite
(Boston: Little, Brown and Co., 1966).
Compton, W. D. and C. D. Benson,
Living and Working in Space: A History of Skylab (Washington: GPO, NASA
SP-4208, 1984)
DeVorkin, David H. "Solar Physics from Space," in
Exploring the Unknown: Selected Documents in the History of the U. S. Civil
Space Program, John Logsdon, ed., vol. 6: Space and Earth Science
(Washington, D.C.: NASA SP-4407, 2004), pp. 1-36.
Hufbauer, Karl.
Exploring the Sun: Solar Science since Galileo (Baltimore and London:
Johns Hopkins University Press, 1991).
SOHO Web site at http://sohowww.nascom.nasa.gov/
Ulysses Web site http://ulysses.jpl.nasa.gov/
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