From chaos to astrophysics to space debris, John Lindner, associate
professor of physics at The College of Wooster, is a valuable
resource of information for members of the college and the community.
Lindner joined the Wooster faculty in 1988 after receiving his
bachelor's degree from the University of Vermont and his Ph.D. from
the California Institute of Technology.
An expert in the field of nonlinear dynamics, including chaos and
fractals, Lindner's recent work involves stochastic resonance,
cellular automata, and self-organized criticality. His research has
been featured on the covers of Nature, Physics Today, Science News,
and on the annual calendar of the American Physical Society.
A member of the American Physical Society, Lindner received an
honorable mention award for his work, "Chaotic Flows" in the
Computers in Physics software competition in 1997.
 Past
Q&A's
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Space Junk
Space flight is a dangerous undertaking, and the risks are even greater
due to the prevalence of space debris. Micrometeoroids and orbiting human-made
objects are a serious threat to astronauts and a possible danger to people
on Earth. John Lindner, professor of physics at The College of Wooster,
describes the differences between these objects and the kind of threats
they pose.
What is the difference between an asteroid and a meteor?
Also known as "minor planets" or "planetoids", asteroids are
rocky, mountain-sized bodies that revolve around the sun, typically
between the orbits of Mars and Jupiter. If two asteroids collide (as
commonly occurred in the early solar system), they may fragment into
smaller chunks of rock called meteoroids or be ground into even
tinier pieces called micrometeoroids. If a meteoroid enters Earth's
atmosphere, friction will heat adjacent air molecules causing them to
glow and creating a bright streak in the sky, called a "shooting
star" or meteor. If a piece of the meteoroid survives its fiery
descent to the ground, it is called a meteorite.
These terms are frequently misused. For example, the famous Barringer
Crater near Winslow, Ariz., is often erroneously referred to as
"Meteor Crater." It was created when a large nickel-iron meteoroid
crashed into the desert, completely disintegrating during impact,
leaving no meteorite at the bottom. The meteor that briefly
accompanied its entry into Earth's atmosphere played no part in the
creation of the crater.
What is a comet?
Comets are relatively small, icy bodies, known as "dirty
icebergs." They revolve in highly elliptical orbits that bring them
close to the sun where sunlight evaporates volatiles and dust from
their surfaces. The solar wind, a stream of matter and radiation
escaping the sun, sweeps these jets of gas and dust into long tails
pointing away from the sun. Unlike "shooting stars" or meteors, which
are short-lived phenomena in Earth's atmosphere, comet tails are
long-lived phenomena in interplanetary space.
What are the chances of a large asteroid colliding with Earth?
Astronomers can estimate the chances of a body - asteroid or
comet - striking Earth by observing the number of such objects
crossing Earth's orbit. The probability of collision decreases with
the size of the body. Larger collisions, inducing planet-wide
catastrophes like the impact that may have caused or accelerated the
extinction of the dinosaurs 65 million years ago, are very rare and
might occur only once every 10 million years. However, smaller
collisions, equivalent to a megaton of TNT, might occur once a
century. The last such recorded event was the Tunguska explosion in
Siberia in 1908. Fortunately, Earth is mainly ocean, and such impacts
are likely to occur in uninhabited locations.
Asteroids and comets are more likely to collide with more massive
planets. In 1994, astronomers witnessed the collision of the
fragments of comet Shoemaker-Levy 9 with the giant outer planet
Jupiter. The largest resulting fireball was bigger than the planet
Earth, and disruptions in Jupiter's atmosphere lingered for months.
What are the various forms of human-made space debris?
Orbital debris includes everything from derelict spacecraft
to tiny flecks of paint released from satellites by thermal stress or
small impacts. The greatest source of debris is rockets that have
exploded in orbit. The U.S. Space Surveillance Network tracks over
9,000 objects larger than 10 centimeters (4 inches) in size, and the
number of smaller particles probably exceeds tens of millions.
How fast does space junk and natural debris travel?
In low Earth orbit, a few hundred kilometers (miles) high,
the space shuttle and the International Space Station (ISS) move at
speeds of 7.5 kilometers per second (17,000 m.p.h.) with respect to
the ground. Because the debris may be moving in different directions,
maximum relative collision speeds will be twice this, and average
relative speeds will be about 10 kilometers per second (22,000
m.p.h.). At such speeds, even a small object has large momentum and
kinetic energy. For example, an orbiting aluminum sphere 1 centimeter
(3/8 inch) in diameter has the damage potential of a 400-pound safe
traveling at 60 m.p.h.
What is the likelihood that space debris could damage a
spacecraft or space station?
This happens all the time. NASA routinely replaces the outer
panes of the space shuttles' triple-paned windows after they are
struck by even tiny debris. The solar panels of the Mir space station
suffered significant pitting and other degradation during its 15-year
lifetime. The orbits of space shuttles and of the ISS are regularly
adjusted to avoid large debris.
Consequently, the ISS is designed to be the most shielded spacecraft
ever flown. Critical components will be able to sustain hits by 1-cm
(3/8-inch) diameter debris. For example, a debris shield blanket made
of a material similar to that used in bulletproof vests covers the
Destiny Laboratory Module.
Does the overwhelming presence of space junk pose any threat
to people on the ground?
Most of the debris does not survive the severe heating, due
to atmospheric friction, during re-entry. The debris that does
survive will likely fall harmlessly in the oceans or on sparsely
populated areas like the Russian and Canadian tundra or the
Australian outback. I am not aware of any injury resulting from
re-entering orbital debris. The largest pieces to ever re-enter were
probably from the Skylab space station in 1979 and the Mir space
station in 2001. These returned to Earth harmlessly.
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