[meteorite-list] Mariner 4 Meteor Mystery, Solved?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 23 16:20:58 2006
Message-ID: <200608232018.NAA23224_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2006/23aug_mariner4.htm

Mariner Meteor Mystery, Solved?
NASA Science News
August 23, 2006

August 23, 2006: On July 14, 1965, Mariner 4 swooped over Mars. It was a
moment of high drama. Six other probes had already tried to reach the
red planet and failed. Since the days of H.G. Wells (War of the Worlds,
1898), people had been hearing about life on Mars, and they were ready
to see the canals and cities. The wait was becoming excruciating.

Finally, all was revealed. 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.

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 Weigert 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 Weigert. 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
Weigert. 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.

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 Weigert, a friend and colleague, to look into it. Weigert began
to examine old comet data and - voila - 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.

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?

Weigert 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. Weigert's
methods can be used to find some of them, "so the next meteor storm
won't be such a surprise."
Received on Wed 23 Aug 2006 04:18:05 PM PDT