NEW SCIENTIST ISSUES PREMATURE COMET IMPACT ALERT

NewScientist.com, 1 June 2005
http://www.newscientistspace.com/article/dn7449.html

Comet put on list of potential Earth impactors

13:17 01 June 2005
NewScientist.com news service

David L Chandler

A comet has been added to the list of potentially threatening near-Earth objects
maintained
by NASA's Jet Propulsion Laboratory. Comet Catalina 2005 JQ5 is the largest -
and therefore
most potentially devastating - of the 70 objects now being tracked. However, the
chances of
a collision are very low.

The listing of Comet Catalina underscores the uncertainty in the knowledge of
whether comets
or asteroids pose a greater threat to Earth. Previous estimates of the
proportion of the
impact risk posed by comets have varied widely, from 1% to 50%, with most recent
estimates
at the lower end.

But comets are larger and faster-moving, on average, so their impacts could be a
significant
part of the overall risk to human life. And, unlike asteroids, they lie on
randomly-oriented
and usually highly elongated orbits. This makes them much more likely to remain
undiscovered
until they are very close to Earth.

Comet Catalina was found by the Catalina Sky Survey, one of the six current,
large-scale and
automated search programmes for near-Earth asteroids. It was initially
designated as an
asteroid when first spotted on May 6. But was reclassified as a comet when
observers saw
characteristic fuzziness in the image, indicating ice and dust streaming off.

Its size is estimated at 980 metres, but Steve Chesley of JPL told New Scientist
that the
size determination is based on the assumption it is a dark-bodied asteroid, and
so the bright
coma of a comet would cause the estimate to be high. "It's really an upper
limit," he said.

Collision course?

On 26 May, JPL's unique orbital calculation software determined that Comet
Catalina was on
what could possibly be a collision course with Earth, though the odds of such an
impact were
small: just 1 chance in 300,000 of a strike on June 11, 2085. Based on the
980-metre size estimate, that would produce a 6-gigaton impact - equivalent to 6
billion tonnes of TNT.

Astronomers expected the addition of further observations to the calculations to
rule out
any possibility of a collision, as happens with most newly-seen objects.

But that did not quite happen. The comet's predicted pathway actually drew even
closer to
making a perfect bull's-eye with the Earth - its predicted path passes within
1000 kilometres
of the where the centre of our 12,700-km-diameter planet will be around that
time.

However, uncertainty in the exact timing of the comet's pass through the line of
Earth's orbit
dropped the odds of an impact to about 1 in 120 million. That is very low, but
the observations
so far cannot categorically rule a collision out.

Forceful outgassing

Chesley adds that even the slim 1 in 120 million odds are an overestimate,
because comets,
unlike asteroids, can move in unpredictable ways because of the forceful
outgassing that
creates their dusty comas and tails. "The uncertainty is much larger than we're
modelling,"
he said. "But I haven't come up with a good way of dealing with this."

The only other comet placed on the JPL list of near-Earth objects with possible
collision
paths was added in 2003. But additional observations ruled out a possible impact
- that comet
was removed from the list after less than a week.

Just one other comet, Swift-Tuttle, has been recorded with a non-zero
possibility of impact.
It was rediscovered in 1992 - after more than a century's absence - before the
JPL list was
created.

Additional observations during Swift-Tuttle's passage, thanks to the publicity
surrounding
the possible impact, made it possible to rule out the possibility of an Earth
impact anytime
in this millennium. However, Swift-Tuttle is on an orbit that will almost
certainly cause
it to crash into the Earth or the moon eventually.

Copyright 2005, New Scientist

EDITOR'S NOTE: Let's see how fast it will be before this latest scare is going
to be
cancelled. I'd give it a few weeks max. I'm not sure either why the temporary
listing
of Comet Catalina "underscores the uncertainty in the knowledge of whether
comets or
asteroids pose a greater threat to Earth." As far as I can see, the listing
doesn't
have any impact on current cometary impact risk estimates. BJP.

=============
(2) "NEW SPACE PROBE MAY SILENCE CLIMATE SCEPTICS" - OR SIMPLY CONFIRM THEIR
FINDINGS

New Scientist, 1 June 2005
http://www.newscientist.com/article.ns?id=mg18625023.600

NewScientist.com news service
Duncan Graham-Rowe

THOSE who deny global warming is happening (sic) often rely on somewhat
error-prone satellite information about our planet. But a proposed probe could
dramatically improve the accuracy of such readings and put an end to the climate
change debate - at least as far as satellite evidence is concerned.

Climate change sceptics have often cited a 1992 analysis by John Christy of the
University of Alabama in Huntsville, which showed that the troposphere had
warmed only negligibly since satellite records began in 1979. But many have
argued that Christy's analysis ignored, among other things, uncertainties in the
satellite data. "The uncertainty allows the sceptics to exist," says Nigel Fox
of the UK's National Physical Laboratory near London. "As long as they can point
to uncertain results, we won't be able to argue convincingly for climate change
treaties," he says.

Some of the uncertainty stems from the calibration of the so-called
hyperspectral sensors that are mounted on satellites, says Fox. These sensors
monitor radiation from the sun and the Earth over a broad range of frequencies.
They can directly or indirectly measure many parameters, including the average
temperature of our planet, the amount of aerosols in the troposphere, and the
health and life cycle of crops, all of which are used to assess climate change.

But despite the great care and precision that goes into calibrating these
sensors on Earth, they inevitably undergo an extremely violent launch that can
knock them out of true. Once the satellite is in orbit, the only way to
recalibrate the devices is to compare them with those on other satellites. But
these sensors have also experienced a punishing launch and, in addition, may
well have become less sensitive or lost their calibration over time.

This means there is a not so insignificant uncertainty in observations of the
Earth from space. For instance, the margin of error in measurements of the total
solar radiation reaching Earth is about 0.3 per cent. While this might not seem
much, a swing of 0.3 per cent could induce a 2°C global temperature change -
enough to trigger a mini ice age of the sort Europe experienced in the 17th
century.

"The way to tackle this problem is to calibrate in orbit," says Fox. He and his
colleagues have designed a satellite, called Traceable Radiometry Underpinning
Terrestrial and Helio Studies (TRUTHS), which could reduce such uncertainties by
a factor of 10. It would be the first unmanned probe to calibrate its
instruments in space. For example, TRUTHS would filter specific wavelengths of
light directly from the sun and use these to fine-tune its sensors. The
satellite could then provide a baseline against which other Earth observation
satellites could be calibrated. Fox will present his proposal at a NASA workshop
on solar irradiation in Washington DC in July.

The need for accurate satellite data to quantify climate change is becoming
urgent, says Joanna Haigh of the department of space and atmospheric physics at
Imperial College London. Fox says that the accuracy of such measurements could
become crucial in settling international disputes over carbon trading. For
instance, countries could plant trees to absorb carbon dioxide and so be
permitted to emit more of it. But the trees would have to remain healthy in
order to qualify for the CO2 credits, and checking that would require
well-calibrated satellite sensors.

Copyright 2005, New Scientist

EDITOR'S NOTE: Improving the accuracy and reliability of satellite climate data
would be a
welcome development. However, Duncan Graham-Rowe seems to ignore that there
exists another
source of observations of atmospheric temperature: weather balloons. The
weather-balloon
data is independent of that generated by the satellites and serves as a
different measurement
of the same quantity (the lower atmosphere). So far, the balloon data seem to
confirm the
Christy-Spencer analysis. A new space probe that aims to solve the riddle of the
apparent
discrepancy between the warming recorded by many of the surface weather stations
and that
measured in its lower atmosphere might indeed falsify the sceptics' view.
However, chances
are that such a scientific venture might just as well confirm them. BJP