Today, we discover Pluto. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
Before we had telescopes the
solar system held only six planets: Mercury, Venus,
Earth, Mars, Jupiter, and Saturn. Long after the
telescope there were still only six. Galileo
spotted Neptune with his
telescope in 1612 but didn't realize it was a
planet. William Herschel saw another wandering star
in 1781. It took a while, but he finally realized
it was a seventh planet. He
called it Uranus.
Accurate measurements of Uranus' orbit eventually
revealed that it was being influenced by another
even more distant planet. European
astronomers found that one in 1846. They called it
Neptune. Meanwhile, the search for planets
was being muddied by asteroids that were showing up
between Mars and Jupiter.
Then an even closer study of Uranus' orbit
suggested yet another planet outside Neptune's
orbit. A young astronomer named Clyde Tombaugh
finally found it in 1930 -- a tiny little speck
seen through a thirteen-inch lens. It was
Pluto -- some three billion miles away, with
a diameter only two-thirds that of our moon.
During the 1990s, Pluto's status as a planet was
seriously called into question by the discovery of
another belt of small objects between Neptune and
Pluto. None of these so-called Transneptunian
Objects is nearly as large as Pluto. But Pluto,
with only one five-hundredth of Earth's mass, is
still so small that many astronomers want to call
Then a 1978 discovery gave Pluto a unique place
among orbiting bodies. What we took to be a single
planet turned out to be two nearby planets orbiting
one another. We keep the name Pluto for the
larger one. The other, with over half its diameter,
we call Charon. Charon is almost too large
to qualify as a mere moon.
So far we've found nothing further from the
sun than Pluto, but we recently put
something there. We launched the small spacecraft
Pioneer 10 in 1972. Its mission was to
explore the Asteroid Belt and then fly past
Jupiter. No one thought we'd still hear from it
after a year or so. But NASA did etch images of a
man and a woman on it and included a drawing of our
location in the solar system -- just in case
distant aliens ever found it.
Pioneer 10 made it through the asteroids without
being hit. It sent back photos as it passed within
80,000 miles of Jupiter. It kept on going, and it
kept on broadcasting. It passed the orbits of
Saturn, Uranus, Neptune, and finally Pluto. Now
it's seven billion miles away, and we can still
make out its signal. It's far off in the cold of
space -- twice as far from us as Pluto.
But Pluto is also in a cold place where the sun is
only a distant star. Its temperature is 37 degrees
Kelvin. That's over four hundred Fahrenheit degrees
below zero. If it has any atmosphere, it's a thin
vapor given off by methane ice. This is the last
stop in our vast solar system -- unless you want to
count Pioneer 10, which has now left the warmth of
the sun entirely behind.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Much good information on Pluto may be found on the
web. NASA maintains this excellent site:
For information on Pioneer 10, see: Wolverton, M.,
The Spacecraft That Will Not Die. Invention
& Technology, Winter 2001, pp. 46-58.
I am grateful to Sherron Lux, UH Library, for
suggesting this topic.
Image of Pioneer 10, courtesy of NASA
The Engines of Our Ingenuity is
Copyright © 1988-2000 by John H.