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(meteorobs) Hunting for Halley's Comet
Hunting for Halley's Comet
Marshall Space Center
http://science.nasadot gov/newhome/headlines/ast07may99_1.htm
A high-flying weather balloon takes off this weekend in search of meteoroids
from comet Halley
May 7, 1999: A group of educators and ham radio operators known as the
Kansas Near Space Project plan to launch a weather balloon this weekend
destined for the stratosphere. The balloonists hope to set a new world
altitude record and, with a little help from NASA, catch a piece of a famous
comet.
The 8 foot helium-filled balloon will lift off from the Johnson 'Near Space'
Center south of Manhattan, Kansas at 7 a.m. CDT on May 8th, just 36 hours
after the peak of the eta Aquarids meteor shower. The payload includes a
lightweight dust collector developed at the NASA/Marshall Space Flight
Center that scientists hope will capture micron-sized eta Aquarid meteoroids
120,000 ft above Earth's surface. If the experiment succeeds then the
balloonists will have captured bits of Halley's Comet, the parent of the eta
Aquarid meteor stream.
Attempts to catch meteoroids with weather balloons have recently taken off,
beginning in November 1998 when NASA/Marshall scientists flew a helium
balloon during the Leonids meteor shower. The balloon carried a xerogel
meteoroid catcher to 90,000 ft altitude where it captured at least eight 30
- 50 micron sized particles. Are they extraterrestrial? The jury is still
out. Scientists are analyzing the composition of the impactors for specific
chemical signatures of cometary and asteroidal debris. If the particles are
found to be members of the Leonids meteoroid stream, they will be the first
known samples of material from beyond the orbit of Uranus where the Leonids
parent comet Tempel-Tuttle orbits.
While NASA scientists continue to analyze the Leonids meteoroid candidates
from 1998, the Marshall Space Flight Center is working with the Kansas Near
Space Project in an attempt to capture particles from Halley's comet.
"The Kansas balloon is going up two days after the peak of the eta Aquarids.
The meteor shower will be dying down by then, but it may be the best time to
catch comet dust," explained Dr. John Horack of NASA's Marshall Space Flight
Center. "Eta Aquarids hit the atmosphere at 140,000 mph and conventional
wisdom says they lose most of their kinetic energy right away, about 80 km
above the stratosphere. Then they drift slowly to the ground, kind of like a
feather thrown from the top of a skyscraper. High altitude winds and the
bouyancy of the particle affect how quickly meteoroids descend. They might
not pass through the stratosphere until days after the shower's peak."
The meteoroid catcher scheduled to fly this Saturday includes 300 square
centimeters of polyacrylic films, plus 30 square centimeters of silica
xerogel. The xerogel is lower density than the polyacrylic material and
probably more sensitive to lower velocity particles drifting slowly downward
from higher altitudes. The polyacrylic films are denser and better suited to
the capture of compact, higher velocity particles.
"The chances of catching an eta Aquarid are slim," says Dr. David Noever, a
NASA/Marshall scientist. "The meteor shower will be 20 to 50 times weaker
than the '98 Leonids, and the debris stream hits the earth slightly south of
the equator. But, if we do catch something it should be relatively easy to
tell whether or not it came from Halley. In 1985 Giotto [a European Space
Agency spacecraft] flew right by Halley's nucleus through a cloud of debris
bubbling off the comet. It got data on particle sizes, density, chemical
composition -- just about everything you need to ID a meteoroid from
Halley."
"Chemically speaking, a Halley meteoroid caught in the stratosphere won't
look exactly like one from deep space," continued Noever. "All meteoroids
experience a thermal spike (5-15 s) during deceleration in the atmosphere.
Simply put, the fragmentary dust gets hot and some volatile elements boil
away. This changes the internal chemical ratios. How long the heating lasts
depends unpredictably on the particle size, density of both particle and
atmosphere, entry velocity and entry angle. Despite these complications we
think we know enough about volatile evaporation to make reasonable chemical
comparisons with pristine Halley particles."
This weekend's adventure won't be the first science experiment carried aloft
by the Kansas Near Space Project (KNSP). Since its inception in 1996 the
KSNP has flown payloads to study cosmic rays, to test the response of
microorganisms to the environment of the stratosphere, and to gather
high-altitude meteorology data, among others. All the experiments were
designed and executed by students and hobbyists.
"Above 100,000 ft the air pressure is only 1% of that at ground level and
air temperatures are approximately -60 degrees F," explains Lloyd Verhage,
the KNSP project manager at Kansas State University. "These are conditions
closer to the surface of Mars than to the surface of Earth. The air up there
is too thin to refract or scatter sunlight, so the sky looks black rather
than blue. What we see at these altitudes is close to what shuttle
astronauts see from orbit."
"My favorite experiment so far was the Flight of the Roachanauts," continued
Verhage. "We sent several roaches in a plastic habitat to 84,000 ft. We know
roaches can survive the winter, so felt that temperatures wouldn't be a
problem for a short 3 hour flight. Unfortunately the insects returned dead.
We're planning to repeat the experiments with improved habitats and
monitoring to better understand what happened. Who says there is no risk to
expanding the frontiers of science?"
In February 1999 Verhage and collaborators set a new altitude record for
amateur launched balloons -- 114,600 ft. The previous record of 111,000 ft,
established by Sky Science Over Kansas, had stood since 1994. The KNSP
benchmark didn't last as long. Three weeks later, in March 1999, a Colorado
group set a new standard by sending a balloon to 121,000 ft.
"We would like to recapture the record this weekend," said Verhage. "I want
to reach 122,000 ft at least, and I'd say we have a better than 50% chance
of making it."
This weekend's flight will begin just after dawn when the meteoroid flux
over the launch site is greatest (see the figure below). The balloon will
take about 2.5 hours to reach 120,000 ft. At that altitude the balloon,
which measures 8 ft across at sea level, will have grown in size to over 25
feet and it will pop. The descent by parachute takes about 1 hour. If all
goes as planned the meteoroid collector will be recovered by Kansas Near
Space Project personnel and returned to the Marshall Space Flight Center for
analysis.
NASA/Marshall scientists plan to launch two more weather balloons later this
year to capture meteoroid samples. In August, xerogel will be used to take
samples of the Perseid meteoroid shower. This November, the Leonid shower is
predicted to be an even better show than the much-hyped 1998 Leonid shower,
and scientists will again fly collectors to the stratosphere in an attempt
to "catch a falling star."
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