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August
23, 2006: When Mariner 4 swooped over Mars on
July 14, 1965, it was a moment of high drama. Six other
probes had already tried to reach Mars and failed--most malfunctioning
before they even left Earth. People had been hearing about
life on Mars since the days
of H.G. Wells (The War of the Worlds, 1898), and
they were ready to see the canals and cities. But the wait
was becoming excruciating.
The wait was over: With flawless precision, Mariner 4 dipped
less than 10,000 km above the planet's surface and took 22
pictures. Mars was covered with desert sand and ancient craters.
No cities. No canals. No Martians. No one would ever look
at the red planet the same way again.
Right:
Mariner 4's flyby of Mars. [More]
Most histories
of the mission end right there, with Mariner 4 buzzing Mars—"the
first spacecraft to visit the red planet"-- and throwing
cold water on a lot of good science fiction. But there's more
to the story. After the flyby, something strange happened
to Mariner 4, setting the stage for a 40-year mystery:
Fast-forward
to September 15, 1967. Mariner 4 was cruising the dark emptiness
between Earth and Mars. Having shot past Mars in '65 without
enough fuel to turn around and go back, there was nothing
else to do. All was quiet. Fuel was running low. Soon, Mariner
4 would fade into history.
That's
when the meteor storm hit.
"For
about 45 minutes the spacecraft experienced a shower of meteoroids
more intense than any Leonid meteor storm we've ever seen
on Earth," according to Bill Cooke, the head of NASA's
Meteoroid Environment Office in Huntsville, AL. The impacts
ripped away bits of insulation and temporarily changed the
craft's orientation in space. "It was a complete surprise."
Think
about it. Out in the "emptiness" between Earth and
Mars, a region of space astronauts are going cross one day
if NASA's Vision for Space Exploration comes to fruition,
lurks a dark stream of meteoroids capable of producing a shower
more intense than anything we've seen in centuries of sky
watching on Earth. "Until Mariner 4 stumbled onto it,"
says Cooke, "we had no idea it was there."
For
almost 40 years the source of the shower remained a mystery.
But now, meteor expert Paul Wiegert of the University of Western
Ontario may have cracked the case. The culprit, he believes,
is a "dark comet" named D/1895 Q1 (Swift) or "D/Swift"
for short.
"Comet
D/Swift was first seen in August 1895 by the prolific comet
hunter Lewis A. Swift," says Wiegert. Swift discovered
or co-discovered more than a dozen comets, including 109P/Swift-Tuttle,
the source of the well-known Perseid meteor shower. Unlike
his other comets, however, "D/Swift quickly vanished.
The comet was last spotted in February 1896 heading out of
the inner Solar System, and it has never been seen since,
even though its orbit indicates it should come back and brighten
every 5 years or so."
 (Note
that the prefix D/ indicates a lost or broken-up comet, one
that was well-observed on one or more occasions, but which
failed to reappear as expected.)
What
happened to D/Swift? "The comet may have disintegrated,"
says Wiegert. Comets are notoriously fragile and sometimes
a little sunlight is all it takes to make them crumble. Comet
D/Swift probably overheated when it passed by the sun in 1895
and later fell apart.
Right:
The Hubble Space Telescope took this picture of Comet 73P/Schwassmann-Wachmann
3 falling apart in April 2006. Perhaps the same thing happened
to D/Swift in the 19th century. [More]
D/Swift
was mostly forgotten until last year when Bill Cooke wondered
if "some old D/ comet" might be responsible for
the Mariner 4 episode. Comets, especially disrupted comets,
leave a stream of debris in their wake as they orbit the sun.
If Mariner 4 passed through such a stream, "it would
have been sandblasted."
He
asked Wiegert, a friend and colleague, to look into it. Wiegert
began to examine old comet data and—voilà—"Mariner 4
was close to the orbit of Comet D/Swift at the time of the
meteor encounter."
Amazingly,
Mariner 4 was not merely close to the comet's orbit, it may
have been close to the comet itself. "According to our
calculations, the [possibly shattered] nucleus of D/Swift
was only 20 million kilometers from the spacecraft."
As distances go in the solar system, that's nearby.
"It's
like in Star Trek when Enterprise stumbles across a comet
in the middle of deep space. Of course, that's crazy,"
says Cooke. "Space is so big, the chances of running
across a comet are almost nil." Yet this may be what
happened to Mariner 4.
 Right:
An artist's rendering of Mariner 4. [More]
Mariner's
cameras weren't turned on at the time, so a comet could've
passed by unnoticed—except for the jostling of comet dust.
Telescopes on Earth saw nothing, but that's no surprise. An
old, shattered nucleus wouldn't necessarily glow. It all makes
sense.
Case
closed?
Wiegert
still has doubts. "The complicating factor is that, because
D/Swift was seen for only a short time in 1895-96, its orbit
is not terribly well-known. Our extrapolations could be wrong.
We're in the process of collecting more observations from
19th century archives and re-analyzing them. Soon, I hope
there will be enough information to convict or acquit Comet
D/Swift."
This
investigation may lead to others. "The space between
Earth and Mars is probably criss-crossed by old debris streams,"
says Cooke. Wiegert's methods can be used to find some of
them, "so the next meteor storm won't be such a surprise."
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Author: Dr. Tony
Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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