A Space Oddity (from the news)
Jun 17, 1997 03:12 PM
by Eldon B Tucker
I found the following on the news lately. It's interesting from
a symbolic aspect. Here ve have a satellite that is, in a way,
in an orbit partly around the sun and partly around the earth.
Space Oddity Space Oddity Thursday, June 12, 1997
By K.C. COLE, LOS ANGESES TIMES Science Writer
Astronomy: Scientists find asteroid's orbit of sun is uniquely in
tune with, yet different from, Earth's.
The earth has a tiny companion that orbits the sun exactly in
tune with our planet, astronomers at York University in Ontario,
Canada, announced Wednesday.
Calling the 6-mile-diameter rock the "Mona Lisa of asteroids" for
the subtle beauty of its sympathetic vibrations with Earth,
astronomer Paul Weigert said it was a "thrilling discovery" akin
to "finding a diamond in your own backyard."
Similar odd couples orbit within the solar system in harmonic
resonances, including several moons of Saturn and Jupiter.
However, out of hundreds of asteroids that orbit near Earth, only
the rock called Asteroid 3753 is held captive by our planet's
gravitational pull in a way that keeps it pulsing in a regular
Unlike the Earth's most familiar companion, the moon, this
asteroid orbits the sun--and at times finds itself 1,000 times
farther away from Earth than the moon, Weigert reports in today's
issue of the journal Nature. However, it is still considered a
"companion" because of the exact mathematical one-to-one
relationship of its orbit to Earth's.
The mystery is: How did it achieve such an unlikely orbit?
"That's one of the big questions," Weigert said. "We're working
The asteroid was discovered 11 years ago, but its unusual
horseshoe-shaped orbit (as viewed from Earth) was only unraveled
recently with the help of powerful computers. "It was waiting to
be discovered, but it took someone to have the intuition to check
it out," said planetary scientist William Bottke, a postdoctoral
researcher in planetary science at Caltech. "It's an interesting
The asteroid, scientists say, poses no threat to Earth--at least
for 100 million years, when it could stray from its present path
and careen unpredictably. Before that time, hundreds of other
asteroids would be more likely to smash into the planet. "There
are plenty of other enemies out there," said astronomer Brian
Marsden of Harvard University, who called the discovery
"interesting," especially because "there are no other asteroids
Only because the asteroid orbits in resonance with the Earth does
it avoid colliding with our planet, the astronomers said. Two
things "resonate" when they vibrate in the same period, or in
tune with each other. Asteroid 3753's "year"--the period it
takes to orbit the sun--is about the same as the Earth's. The
asteroid gains stability from its resonance with Earth just as
air blown into a flute produces a tone that gains stability when
its vibration matches the natural resonant frequency of the
However, the path that the asteroid takes around the sun is
wildly different from Earth's. At its closest approach to our
planet, it's still about 40 times farther away than the moon; at
its farthest point, about 25 times that distance.
Moreover, while the Earth and the vast majority of bodies orbit
the sun in a flat plane, like the ridges on a phonograph record,
Asteroid 3753 dips far under the plane, and then far over it.
Still, the asteroid remains enthralled in Earth's gravitational
grasp through a complicated celestial choreography. As the
asteroid begins to approach the planet, Earth's gravity flings it
out to a larger orbit. Since objects orbiting far from the sun
orbit more slowly than objects nearby--just as an ice skater
spins more slowly when she extends her arms--the asteroid slows
When the asteroid gets too far behind Earth, the planet pulls it
back onto an inside track, where it can orbit faster, and catch
up to Earth again.
"It just keeps pingponging around the Earth," Weigert said. "The
Earth and the asteroid chase each other around." The dance is an
adagio, taking place over many centuries.
Many other pairs of objects in the solar system orbit in
resonance--for example, Pluto makes two orbits around the sun for
every three of Neptune's--a resonance that keeps them from
colliding. However, the one-to-one orbital relationship between
the asteroid and Earth allows for one of the most complex paths
ever seen, Weigert said.
Called a "horseshoe" orbit because the asteroid appears to shift
direction in mid-swing, it is a phenomenon seen only in one other
place in the solar system--on two moons of Saturn. Astronomers
have known since the 1980s that the moons Janus and Epimetheus
pull on each other in a similar pattern. But as Marsden pointed
out, that system is different because the two moons are
relatively equal in mass and each pulls equally on the other.
In contrast, while the Earth changes the orbit of Asteroid 3753,
the rock's mass is too tiny too affect the Earth. "The Earth
doesn't care about that object at all," Marsden said.
Before Weigert and his York colleague Kim Innanen figured out the
asteroid's orbit, scientists thought it would be impossible for
such strange behavior to remain stable. Three-body problems, as
scientists call them, are notoriously complex. While it is easy
enough to figure out the effects of two bodies on each
other--say, the Earth and sun--throwing a third into the equation
makes it all but impossible to solve.
In essence, Weigert got around the problem by brute force,
letting a computer calculate the future orbit of the asteroid
based on information about its present motion through the sky.
Astronomers at Turku University in Finland corroborated the
Just how the asteroid got there in the first place remains an
enigma. Since asteroids orbiting near Earth usually hang around
for no longer than 100 million years, it's hard to imagine how
3753 could have formed with the Earth at the birth of the solar
system almost 5 billion years ago.
On the other hand, for the combined gravitational influences of
the planets and sun to nudge the asteroid into its present orbit
at a later time would be as unlikely an operation as "threading a
needle," Innanen said.
Asteroid 3753 should be visible in the Southern Hemisphere this
fall at its closest approach to Earth, but only through
telescopes. Like most asteroids, this most unusual rock around
the sun is exceedingly dim.
* * *
The asteroid is held captive by Earth's gravitational pull in a
way that keeps it locked in a kidney-shaped danced tied to
1. The asteroid's path as it would appear if the Earth were
2. A view of its orbit compared to the plane of the Solar
System--the spiral is an additional motion.
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