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CCNet 43/2002 - 5 April 2002
----------------------------


"The Palermo Scale measures the impact probability, date and energy
to assign a hazard rating against the statistical background of such
events. The rating given to 1950 DA (+ 0.17) indicates that the risk of it
hitting Earth is about 47.9% greater than the expected statistical average
chance of being hit by an asteroid or comet. Lest there be any confusion,
let's be clear about what this number means. There is a 99 and 2/3rds
percent chance that (29075) 1950 DA is going to pass a couple
hundred thousand miles from Earth and not, repeat, not, enter our
atmosphere, crash into water or land, and cause havoc and disaster.
Should we be doing something right now to deal with the danger from (29075)
1950 DA? The consensus is no."
--Mark Perew, Space Daily, 5 April 2002


"You could attach a rocket engine to it, like an outboard," added
Clark R. Chapman, a senior space scientist at the Boulder, Colo., office
of the Southwest Research Institute. "When you have a lead time of
centuries, almost anything will do." There is, however, "the problem that
you can make it worse," said Brian G. Marsden, director of the International
Astronomical Union's Minor Planet Center. Altering the asteroid's orbit
indiscriminately to avoid a 2880 catastrophe could set up an earlier or
later collision."
--Washington Post, 5 April 2002



(1) ASTEROID'S FAR-OFF DANGER DETAILED
    The Washington Post, 5 April 2002

(2) ASTEROID COULD HIT EARTH IN 2880
    BBC News Online, 4 April 2002

(3) THERE'S A ROCK HEADED OUR WAY
    Space Daily, 5 April 2002

(4) POSSIBLE IMPACT IN 2880
    David Morrison <dmorrison@arc.nasa.gov>

==============
(1) ASTEROID'S FAR-OFF DANGER DETAILED

>From The Washington Post, 5 April 2002
http://www.washingtonpost.com/wp-dyn/articles/A63550-2002Apr4.html
 
By Guy Gugliotta
Washington Post Staff Writer

An asteroid nearly a mile wide could be headed for an apocalyptic collision
with Earth. That's the bad news. The good news is that it won't arrive for
878 years, and it might be pretty easy for our descendants to move it out of
the way.

A team of researchers determined that an asteroid known as 1950 DA, a
gigantic, near-spherical boulder hurtling through space on an elliptical
orbit around the sun, has a one-in-300 chance of smacking into the Earth on
March 16, 2880. One-in-300 is as close as the odds have ever been for an
asteroid collision.

"The orbits will meet up," said senior engineer Jon D. Giorgini of NASA's
Jet Propulsion Laboratory in Pasadena, Calif. He led the team that reported
the calculation in today's issue of the journal Science. "The question is,
'Will the Earth and the asteroid be there at the same time?' "

But even if further study determines the asteroid and Earth are on a
collision course, another researcher argues that asteroids such as 1950 DA
could be thrown off line relatively easily by manipulating their ability to
absorb sunlight and translate it into thermal energy. Once absorbed, solar
energy radiates from the asteroid's surface like a tiny thruster engine.
That's meaningless in the short-term when compared with the gravitational
forces that give orbits most of their size and shape. But it's potentially
decisive in moving a small celestial body a few degrees off-course over a
period of centuries.

"You just want to change something about the surface" of the asteroid to
alter the way it processes sunlight, said University of Arizona planetary
scientist Joseph N. Spitale, who outlined the approach in an accompanying
article in Science. And it doesn't take nuclear warheads, as were used in
the 1998 movies "Armageddon" and "Deep Impact."

"There are a lot of ways," Spitale said, such as roughening the asteroid's
surface with conventional explosives or covering it with dirt. Giorgini
suggested coating it with charcoal or chalk, or "shrink-wrapping" a large
piece of it with Mylar.

"You could attach a rocket engine to it, like an outboard," added Clark R.
Chapman, a senior space scientist at the Boulder, Colo., office of the
Southwest Research Institute. "When you have a lead time of centuries,
almost anything will do."

There is, however, "the problem that you can make it worse," said Brian G.
Marsden, director of the International Astronomical Union's Minor Planet
Center. Altering the asteroid's orbit indiscriminately to avoid a 2880
catastrophe could set up an earlier or later collision.

"A kilometer is where you start thinking about global catastrophe," Spitale
said. A kilometer is six-tenths of a mile; the asteroid 1950 DA is
seven-tenths of a mile wide. Something smaller than a kilometer can wipe out
a metropolitan area or devastate a coastline with tidal waves. Something
bigger throws up a huge cloud of dust that can dim the sun for years,
causing a massive die-off of species.

Astronomers say there are between 1,000 and 1,200 objects six-tenths of a
mile in diameter or larger whose orbits intersect that of Earth; about half
of these objects have been detected. Among those identified, only 1950 DA is
a potential threat.

Giorgini is looking for more information on 1950 DA, and fortunately there
is plenty of time to get it. He and his colleagues have already gathered
enough data about 1950 DA to create what Chapman called "the most definitive
study of an asteroid orbit that's ever been done."

This is fortunate, for premature warnings based on sketchy information about
"Earth-crossing" asteroids and comets have a remarkable ability to alarm the
public.

In 1998, Marsden predicted that asteroid 1997 XF11 was "virtually certain"
to pass relatively close to Earth in 2028 and had a slight chance of hitting
the planet. He based his warning on three months of observation, but after
further research, it became apparent that the asteroid had "zero chance" of
collision. By that time, however, the first prediction had created an
enormous explosion of publicity.

"If you say something, you're accused of crying wolf," said Marsden with a
wry chuckle. "If we don't say anything, we get accused of coverups. You
can't win in this business."

Astronomers first sighted 1950 DA in February 1950, but it was lost to
observers until 2000, when it was reidentified. Giorgini said scientists
were able to observe it for four days with radar and with optical telescopes
until August 2001.

What they found was a rounded body with craters "but no large dents,"
traveling on a 2.2 year-long elliptical solar orbit, Giorgini said. The
asteroid comes to within 77 million miles of the sun -- inside Earth's orbit
-- and reaches its farthest point 238 million miles away in the asteroid
belt between Mars and Jupiter.

The team could examine the range of effects of many secondary forces on the
orbital shape.

But an assessment of the thermal energy thrusters, known as the Yarkovsky
Effect, for the Russian engineer who discovered it a century ago, was
difficult. The asteroid rotated once every 2.1 hours, but the Giorgini team
did not know where the poles were, crucial in assessing the direction of the
thruster push.

"We've got two possibilities," Giorgini said. "In one position, the
probability of collision is nearly .33 percent. In the other position, it is
zero." But either way, he added, "there's nothing to worry about . . . in a
few hundred years, there will be ways of dealing with this we can't even
imagine."

© 2002 The Washington Post Company

==========
(2) ASTEROID COULD HIT EARTH IN 2880

>From the BBC News Online, 4 April 2002
http://news.bbc.co.uk/hi/english/sci/tech/newsid_1910000/1910518.stm

By Dr David Whitehouse
BBC News Online science editor 
 
A one kilometre-wide chunk of space rock could strike the Earth in 2880, say
astronomers.
The asteroid, designated 1950 DA, has a one in 300 chance of colliding with
our planet, according to calculations. The situation will become clearer
once more precise data on the rock's orbit in the Solar System are obtained.


Scientists say this is not something people should get too worried about.

If an impact became a real possibility, humanity would have several decades
to work out how to give the rock a gentle nudge to take it away from the
planet.

Lost and found

1950 DA was first detected on 23 February, 1950, by astronomers at the Lick
Observatory in California, US. But after just 17 days of observations the
rock was lost from view.

It was picked up again on 31 December, 2000, by the Lowell Observatory Near
Earth Object Search (LONEOS) program.

And subsequent radar observations from the Arecibo and Goldstone radio
telescopes during the asteroid's most recent close approach to Earth - about
20 lunar distances from the planet - allowed astronomers to refine the
rock's orbit.

The calculations indicate that 1950 DA has a one in 300 chance of striking
the Earth in about 900 years' time.

Quick rock

The data also suggest the asteroid is travelling at a velocity of about 15
km (nine miles) per second relative to the Earth.

This means that if 1950 DA were to collide with the planet, it would do so
with an explosive force of approximately 44,800 megatonnes of TNT.

If it struck land, it would produce a crater about 22 km (14 miles) across,
with a blast radius of intense damage of around 300 km (190 miles).

If the impact occurred in the ocean, a tsunami 25 metres high would be
generated 1,000 km (620 miles) from the impact site.

Small but deadly

Currently, 1950 DA has been rated as a level two event on the Torino warning
scale, which means it is an event "meriting concern".

Experts say the size of the asteroid is close to that which could affect the
Earth's global climate.

However, it is not thought a collision with 1950 DA would threaten the
continued existence of our species.

By comparison, the impact 67 million years ago, which has been implicated in
the extinction of the dinosaurs, is thought to have been caused by an object
10 times the diameter of 1950 DA.

Small force

Given the relatively small size of 1950 DA and the 878 years' advance
warning, it would be possible to alter the orbit of the object if it was
considered necessary.

One method would rely on the Yarkovsky effect. This describes a tiny force
exerted on the asteroid by reradiation of the Sun's rays.

Making one side of the rock dirty, by disrupting the surface with explosives
for instance, would give rise to the force which would, over the course of
many years, change the asteroid's orbit into a safe one.

Research data on 1950 DA are published in the journal Science.

Copyright 2002, BBC

=============
(3) THERE'S A ROCK HEADED OUR WAY

>From Space Daily, 5 April 2002
http://www.spacedaily.com/news/deepimpact-02f.html

by Mark Perew

Pasadena - Apr 5, 2002

We all know the story of David and Goliath. Little David picks a stone,
whirls it around and fells the giant Goliath. Nature, however, decided that
our big, huge Goliath of a solar system would whirl a stone and send it
hurtling toward the tiny David of our planet Earth. Don't start heading for
the hills just yet, though. The dramatic event is 878 years away and there's
a strong chance that the rock will whiz by and never touch us.

The stone coming in our direction is an oblong, lumpy hunk of rock a bit
over a kilometer long. As yet nameless, the only designation it carries is
(29075) 1950 DA. But, its orbit makes it a very interesting hunk of rock.

Carl Wirtanen, an astronomer at the Lick Observatory in central California,
first spotted this object in May of 1950. It was lost until December 31,
2000 when another group found what they thought was a new object. Conrad
Bardwell at the Minor Planet Center quickly determined that the new object
and Wirtanen's object, 1950 DA, were one and the same.

>From that conclusion it was possible to find other photos of the asteroid
taken during the 1980s. This provided a large number of data points, spread
over several orbits, making it possible to calculate a precise orbit and
earning it a number, 29075, as a "minor planet."

That orbit, it was promptly discovered, intersects the orbit of the Earth
and on March 16, 2880, both bodies will be at that intersection. The
question is, will it be a hit or a miss? The initial calculations showed a 1
in 10,000 chance of an impact. Very low, but still not zero.

A few times a year an asteroid is discovered that has some chance of hitting
the Earth in the coming days, weeks, months or years. The NASA "Current
Impact Risks" web site risk lists the objects and their relative risk of
hitting earth.

An early prediction of a possible impact prompts astronomers to make
additional observations. Those observations improve our understanding of the
orbit and in almost every case the chance of a collision is reduced to some
number very close to zero.

Enter Dr. Jon Giorgini and a group of 13 other scientists from the Jet
Propulsion Laboratory (JPL), Washington State University, the Arecibo
Observatory in Puerto Rico, the Lick Observatory and the California
Institute of Technology (Caltech).

Using the large Goldstone radio astronomy dish in California along with the
huge Arecibo facility, they were able to get radar data of 1950 DA. This not
only revealed information about the size and rotation of the asteroid, but
also allowed the orbit to be determined to an even higher degree of
precision.

This time around that greater precision didn't rule out the chance of a
future impact. In fact, the original estimate was way too optimistic. The
original 1:10,000 odds now stand at 0.0033, one-third of one percent, or 1
chance in 300. People have been known to spend money buying lottery tickets
with far slimmer chances.

"Asteroid 1950 DA is a very interesting object," said Dr. Benny Peiser, a
spokesman for Spaceguard UK. It's interesting, "because it is the first Near
Earth Object that scores higher than zero on the Palermo Impact Hazard
Scale."

The Palermo Scale measures the impact probability, date and energy to assign
a hazard rating against the statistical background of such events. The
rating given to 1950 DA (+0.17) indicates that the risk of it hitting Earth
is about 47.9% greater than the expected statistical average chance of being
hit by an asteroid or comet.

Lest there be any confusion, let's be clear about what this number means.

There is a 99 and 2/3rds percent chance that (29075) 1950 DA is going to
pass a couple hundred thousand miles from Earth and not, repeat, not, enter
our atmosphere, crash into water or land, and cause havoc and disaster.

Should we be doing something right now to deal with the danger from (29075)
1950 DA? The consensus is no.

Dr. Alan Harris, an internationally recognized expert in asteroids and a
senior scientist at JPL, gives three reasons for not doing anything more
than studying the object for now.

First, we could still find out that it isn't going to hit us in 2880. We'll
have two more close approaches in the 21st century. Those close approaches
will give us chances to study it in more detail. "Leaving us with a generous
800 more years," according to Dr. Harris, "to deal with it if it is on a
collision course."

Second, we could move it the wrong way. While we have a very good idea of
where it's going, it isn't a perfect picture. There's still room for error
and we could nudge it into a collision instead of out of one.

Third, our descendants may have better ways of dealing with it than we can
imagine.

"Would you expect William the Conqueror, " Dr. Harris quipped, "to
anticipate and solve any of the major problems of our modern society?"

Dr. Harris is pretty emphatic that 1950 DA isn't something we need to get
excited about right now. "To further put this in perspective," he said,
"there is zero chance of an impact from 1950 DA between now and 2880."

There's a story to be told just in being able to compute that degree of
accuracy that far in advance.

Because there are so many objects in space and each one exerts a
gravitational pull on all the others, computing the future location of any
one of them requires calculating where each of them will be at some future
time. Since this process has a known uncertainty, the farther out in time
the orbits are projected, the greater the uncertainty becomes.

1950 DA is a different case, though. In computing its future position, the
uncertainties expand for a time and then contract. Over the course of nine
centuries this process repeats several times. This contraction of the
uncertainty appears to be caused by some unique gravitational interactions
(resonances) with other objects in space, including the Earth itself.

"Resonance is well known," said Dr. Giorgini, "but the effect of it on
modulating the uncertainty region seems not to have been recognized before."

The researchers, whose results will be published in the journal Science on
April 5, 2002, also investigated the precision loss that can occur when
computers use numbers that have a large number of digits to either side of
the decimal point.

When dealing with such numbers, results can sometimes be lopped off or
values rounded up or down. Even with that considered, the positional
uncertainty is less than the diameter of the Earth.

Some other, very small, very subtle and very obscure effects were also
factored into the calculation of a possible impact in 2880. The shape of the
sun, for example, is not perfectly round.

For an object such as 1950 DA that has a highly inclined orbit, that means
that the sun's gravity has an almost negligibly different tug at different
parts of the asteroid's orbit.

Also, the sun is constantly shedding matter and energy into space. This has
two effects on objects in heliocentric orbit. The first is that it produces
a solar wind that pushes objects away from the sun.

It's a tiny force, but over time the cumulative effect can make a big
difference. The second is that as the sun losses mass, its gravitational tug
becomes smaller. Again, the effect is tiny, but over time the effects can be
measured.

When all these pieces, and a few other arcane factors, were all considered,
the chance of an impact could not be eliminated.

One huge factor that can't be well determined is changes in the asteroid's
orbit due to effect of thermal radiation on the daylight side of the
asteroid. This is known as the Yarkovsky effect.

As sunlight heats up the asteroid, some of that heat is radiated back into
space. Since heat is energy and energy is mass, that radiation has a "push"
just like a rocket engine. It's small, but over nine centuries it may
literally make all the difference in the world.

There are many things we don't know well about asteroids in general and 1950
DA in particular. We need to know more about its rotation (we aren't even
sure yet where it's pole of rotation lies), mass, optical and thermal
behaviors, and the depth of its regolith (the powdery "dirt" covering the
rocky core). Our ability to better predict an impact depends on
understanding the Yarkovsky effect and how that affects this asteroid.

"1950 DA," Dr. Giorgini's paper states, "is the only known asteroid whose
predicted impact potential depends primarily on its physical properties, not
on positional measurement uncertainties. A satisfactory assessment of the
collision probability of 1950 DA may require direct physical analysis with a
spacecraft mission."

All the researchers agree that the important thing about 1950 DA is that it
demonstrates that what we don't know about asteroids is more dangerous than
what we do know.

"We need to know more about the physical characteristics of the asteroid,"
Dr. Giorgini said. We're making progress, but we still have a lot to learn.

Mark Perew is a freelance science writer and can be contacted via perew @
addr.com - removes blanks space around @ sign.

=============
(4) POSSIBLE IMPACT IN 2880

>From David Morrison <dmorrison@arc.nasa.gov>

NEO News (04/05/02) Possible impact in 2880

Dear friends and students of NEOs:

Science magazine for April 5 is publishing two interesting articles on Near
Earth Asteroids and collision hazards. The fact that these are being
published in Science is something of an occasion in itself: papers about
NEAs rarely appear there, and 8 years ago Science rejected the original
Chapman/Morrison paper on the impact hazard without even submitting it to
referees. More interesting, however, is the detailed analysis by Giorgini
and others in the first of these papers, of the orbit of NEA 1950 DA. This asteroid
has the highest calculated probability of impact with the Earth for any NEA yet: about one
part in 300. That is the bad news. The good news is that the impact, if it
happens, will not take place for nearly a millennium.

Here is the abstract of the paper:

---------------------

Asteroid 1950 DA's Encounter with Earth:
Physical Limits of Collision Probability Prediction.

J. D. Giorgini, S. J. Ostro, L. A. M. Benner, P. W. Chodas,
S. R. Chesley, R. S. Hudson, M. C. Nolan, A. R. Klemola,
E. M. Standish, R. F. Jurgens, R. Rose, A. B. Chamberlin,
D. K. Yeomans, and J.-L. Margot.

Science 296, 132-136 (April 5, 2002).

ABSTRACT:

Integration of the orbit of asteroid (29075) 1950 DA, estimated from optical
and radar astrometry, reveals a 20-min interval in March 2880 when the
asteroid might collide with Earth.  The uncertainty in the closeness of the
approach is set primarily by uncertainty in the nature of accelerations
arising from time-delayed, anisotropic thermal  re-radiation of absorbed
solar energy.  Those "Yarkovski" accelerations are determined by the
object's mass, shape, pole direction, and global distribution of surface
optical and thermal properties.  Satisfactory assessment of the collision
probability may ultimately require direct inspection of the asteroid by a
spacecraft.

---------------------

This paper provides one of the most complete orbital analyses of any
asteroid done to date. In the text of the paper, the authors note that the
probability of a collision in the year 2880 is approximately equal to the
probability of an impact by an unknown asteroid of comparable size sometime
between now and 2880. As noted by Al Harris of JPL, there is *zero* chance
of an impact from 1950 DA between now and 2880, and only a small probability
then. For all unknown asteroids of that size collectively, there is about
one chance in a million of an impact in any given year, and it could be
*any* year, including tomorrow. In terms of impact hazard over the next
century or two (which is all we usually consider), 1950 DA joins the ranks of
"certified safe" objects. The interesting possibility of an impact in 2880
arises only because of the exceptionally good orbit determinations
(primarily from radar data) that allow us to calculate the position of the
asteroid for the next millennium.

Harris comments that if we wish to find out more about this asteroid, the
next close approach, in 2032, will undoubtedly provide plenty of opportunity
to refine the orbit and most likely eliminate any possibility of an impact.
If not, there is a chance in 2074, still a generous 800 years before any
active intervention would be needed in the very unlikely event that it
really is on a collision course. With this timescale and probability, any
further study should be based on
scientific curiosity, not fear.

Harris has also been asked if he thinks we should take any action now toward
deflecting the asteroid or otherwise considering the mitigation of the
hazard. His answer is "absolutely not", for the following reasons: (1) With
the only possible impact 878 years in the future and complete certainty that
an impact can be positively ruled out (or in) by further radar observations
within the next century (leaving a generous 800 more years to deal with it
if it is on a collision course), it would be a waste of resources to do
anything until those definitive observations are taken. (2) Until we know
the trajectory within an Earth diameter in 2880, a gentle shove could as
well put the asteroid on a collision course as push it off one. Until you
know exactly where it is heading, you don't know which way to push to make
it miss. (3) The technology of our descendants 800 years from now is likely
to be so superior to our own that it is entirely appropriate to let them
worry about it. Our responsibility to that generation is no more than to
take and preserve the observations that tie down the "orbital arc" from
which more definitive orbits can be calculated with the addition of future
observations. Would you expect William the Conquerer to anticipate and solve
any of the major problems of our modern society? Or for Christopher Colombus
to design a vehicle to transport several hundred people across the Atlantic
in 6 hours?

The orbit of 1950 DA is unusual only in that the radar observations coupled
with the long arc of 50 years since its discovery have yielded an orbit of
such precision that we can actually follow it a millennium into the future.
Only 1950 DA and a few other radar-observed asteroids have orbits good
enough to say anything more than probabilistic generalizations that far in
the future. So, what 1950 DA does, and the fact that a possible collision
that far out depends on non-gravitational forces, is not unique to 1950 DA. What is
unique about 1950 DA is that we are in a position to start monitoring these
subtle effects. That's scientifically interesting, even exciting.

Steve Ostro, one of the authors of the paper, notes that most of the
uncertainty in the impact probability comes from uncertainty in the
asteroid's physical properties. Right now the collision probability is 1/300
or less. The geometry of the intersection of the asteroid's orbit and the
Earth in 2880 are very well known, and there is a 20-min interval 2880 when
there could be a collision. Virtually all the uncertainty is in the
along-track component and arises from Yarkovsky accelerations, which depend
on the object's physical properties, including not just the gross ones like
mass, shape, and pole direction, but the detailed global distribution of
optical and thermal properties. This paper establishes that trajectory
prediction and physical characterization are closely connected, because
long-term orbit calculations require knowledge of physical properties.

Following is the press release from JPL for this paper on 1950 DA, followed
by a University of Arizona press release on the second asteroid paper in
this issue of Science.

David Morrison

==============================

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY

FOR IMMEDIATE RELEASE April 4, 2002

RADAR PUSHES LIMITS OF ASTEROID IMPACT PREDICTION

Applying unprecedented refinements to the analysis of celestial hazards,
NASA astronomers have identified a potential close encounter with Earth more
than eight centuries in the future by an asteroid two-thirds of a mile (one
kilometer) wide.

What will most likely be a miss, even without preventive measures, will come
on March 16, 2880, said Jon Giorgini, a senior engineer at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. Odds for a collision are at most one
in 300, and probably even more remote, based on what is known about the
asteroid so far.  Still, that makes this space rock, named 1950 DA, a
greater hazard than any other known asteroid.

"This is not something to worry about," said Giorgini, leader of a team
reporting about the asteroid in the April 5 edition of the journal Science.
"We're showing that searches with optical telescopes and follow-up observations with
radar telescopes can provide us centuries of advance notice about potential close encounters of
asteroids with Earth. That's plenty of time to consider the options -- 35
generations, in fact."

"This report is a success story for our efforts to identify potential
troublemakers," said JPL's Dr. Don Yeomans, manager of the NASA Near Earth
Object Program. "Radar observations are helping us push predictions 5 to 10
times further into the future."

This report differs from previous ones about other asteroids' Earth-impact
potential. Estimates of impact risks in earlier cases came from a few
nights' optical observations of newly found asteroids. Astronomers soon
ruled out the possible impacts after a few more observations narrowed
uncertainties about the asteroids' orbits.  The current orbit of 1950 DA has
been mapped with great accuracy using precise radar data and a 51-year span
of optical data. Uncertainty about how close it will come to Earth in 2880
stems from gaps in knowing physical details of the asteroid that could
subtly alter its course over the centuries.

"How close 1950 DA will approach Earth turns out to depend on the asteroid's
physical attributes -- it's size, shape and mass, and how it spins, reflects
light and radiates heat into space," Giorgini said. These things are
unlikely to be known any time soon. The way the asteroid radiates energy
absorbed from the Sun back into space has the biggest potential effect, he
said. Releasing heat in one direction nudges the asteroid in the opposite
direction. The resulting acceleration is tiny, but over the centuries acts
like a weak rocket and could make the difference between a hit and a miss.

Asteroid 1950 DA was discovered from Lick Observatory, Mount Hamilton,
Calif., in 1950. It faded from view for five decades then was found from
Lowell Observatory in Arizona in 2000. Astronomers used large dish antennas
of NASA's Deep Space Network site at Goldstone, Calif., and the Arecibo
Observatory in Puerto Rico to examine the asteroid with radar when it passed
at a distance 21 times farther away than the Moon in March 2001.

"Once an asteroid is discovered, radar is the most powerful way to find its
exact orbit and, apart from sending a spacecraft, the only way to see what
it looks like," said JPL's Dr. Steve Ostro, who led the radar observations
of 1950 DA.

Giorgini refined calculations of future orbits by including factors such as
the push from sunshine and the potential gravitational tug from 7,000 other
asteroids and nearby stars. Effects of each small influence on the
asteroid's movement could be amplified by 15 gravitational tugs during close
approaches to Earth and Mars -- none of which have any chance of an impact
-- prior to 2880. "It's like predicting a 15-bank shot in a pool game,"
Giorgini said. "We know the cue stroke extremely well because it is right
now and we can measure it. But at each future bank, small variations
accumulate and change the next bounce, which changes the following one and
so on. What we've done is find the range of changes possible due to tilt,
imperfections and fuzz on the table, the bounce of the cushions, and wind
blowing across the room.  We need to know more about the 'cue ball' to
really be sure of how the last three banks in 2809, 2840 and 2860 will line
things up for 2880."

If future generations' studies of 1950 DA indicate it ought to be diverted
to prevent a collision, the subtle influences that its physical properties
have on its motion might be manipulated to advantage. For example, Giorgini
suggested, its surface could be dusted with chalk or charcoal to alter the
way it reflects light, or a spacecraft propelled with a solar sail could
collapse its reflective sail around the asteroid.  In any event, determining
asteroids' physical properties will be important for long-term calculations
of impact hazards.

In addition to Giorgini, Ostro and Yeomans, authors of the report include
Dr. Lance Benner, Dr. Paul Chodas, Dr. Steven Chesley, Dr. Myles Standish,
Dr. Ray Jurgens, Randy Rose and Dr. Alan Chamberlin, all of JPL; Dr. Scott
Hudson, Washington State University, Pullman; Dr. Michael Nolan, Arecibo
Observatory; Dr. Arnold Klemola, Lick Observatory; and Dr. Jean-Luc Margot,
California Institute of Technology, Pasadena.

Images and additional information are available at the JPL NEO website:
http://neo.jpl.nasa.gov/1950da/  This section includes images of the asteroid, a radar movie,
simulations of the asteroid's orbit, and a video segment narrated by Jon Giorgini.

Arecibo Observatory is operated by the National Astronomy and Ionosphere
Center at Cornell University, Ithaca, N.Y., under an agreement with the National Science Foundation.
NASA's Office of Space Science, Washington, D.C supported the radar
observations.  JPL is managed for NASA by the California Institute of Technology.

======================================

TWEAK TEMPERATURES OF SMALLER ASTEROIDS TO DEFLECT THEM FROM EARTH, UA
SCIENTIST SUGGESTS

>From Lori Stiles, University of Arizona News Services, 520-621-1877
April 4, 2002

Humans could deflect small but dangerous asteroids from Earth by changing
how much sunlight the asteroids reflect, a University of Arizona planetary
scientist suggests in the current issue (April 5) of Science.

Possible schemes might include covering the upper few centimeters of the
asteroid with dirt, or painting its surface white, or fusing part of its
surface with a spaceborne solar collector ' all technically feasible and
civically preferable to launching a nuclear warhead to blast an incoming
asteroid off course.

Changing how much heat a space rock radiates will change how it drifts in
its orbit because of the Yarkovsky effect, said Joseph N. Spitale of the UA
Lunar and Planetary Laboratory in his article, "Asteroid Hazard Mitigation
Using the Yarkovsky Effect."

The Yarkovsky effect is a long-known but long-obscure phenomenon named for
the Polish engineer who first described it around 1900. The effect is caused
because when an unevenly heated body re-radiates heat, hotter spots are
subjected to a stronger recoil force than are cooler spots. As I.O.
Yarkovsky noted, the differences in momentum nudge the object so that it
drifts slightly in its orbit, Spitale said. The effect is a relatively small
force, but it accumulates through time.

Not until the mid-1990s did planetary scientists begin to realize how
important the Yarkovsky effect is in calculating motions of asteroid
fragments in the belt between Mars and Jupiter. These include Cornell
University's William F. Bottke Jr., David P. Rubincam of NASA Goddard Space
Flight Center, Paolo Farinella of the University of Pisa in Italy, David
Vokrouhlicky of Charles University in the Czech Republic, and William
Hartmann of the Planetary Science Institute in Tucson.

The mechanism explains why more asteroid fragments than otherwise can be
predicted are launched from the main asteroid belt toward Earth, hitting as
meteorites, according to their papers. And it explains how space rocks can
drift for millions of years before arriving at main belt asteroid
"resonance" zones from which they're flung to the inner solar system, they
conclude.

"It's pretty clear that this is an important effect when it comes to getting
material from the asteroid belt to the inner planets," Spitale said in an
interview.

He's working to develop a sophisticated thermal model to use to precisely
calculate Yarkovsky drift for specific asteroids. Asteroid shape, spin,
composition and surface details all must be factored in to get a precise
orbit for a specific asteroid.

In his Science article, Spitale describes his calculations of Yarkovsky
drift for three stony near-Earth asteroids, 6489 Golevka (300 meters
diameter), 1566 Icarus (one kilometer diameter) and 1620 Geographos (2.5
kilometers diameter).

The idea then is to change a threatening asteroid's surface temperatures so
that, over decades or centuries, its orbit veers away from Earth.

"You might take one of the smaller bare-rock bodies and put a lot of dirt on
it, for a dramatic change in thermal conductivity," Spitale said.
"Blanketing the asteroid with a centimeter of dirt is technically feasible,
but it would be expensive.

"Another way you could do it would be to paint it. If you could cover the
surface with a millimeter of white material, you could 'turn off' the
Yarkovsky effect altogether. That could produce a fairly big change in where
the body would be in another century or so.

"This would be effective in another approach, suggested by Jay Melosh (UA
professor of planetary sciences). It is to use a solar collector - basically
just a big dish that focuses sunlight on a body ' to fuse a region of the
surface and blast off mass, so the object changes course because of its
different mass. But in the process of this, you'd also change the thermal
conductivity of the asteroid, giving it a new orbit also because of the
Yarkovsky effect."

Spitale said the proposed technique would be useless for a large asteroid or
an asteroid less than decades away from Earth.

"This technique will work best on objects the size of Golevka or smaller
(300 meters, about 1,000 feet, or smaller). An object that size could do
damage to the better part of a country. Even a 100-meter or 50-meter object
can take out a good part of a city."

"The biggest technical problem right now with this approach is just doing
the calculations to understand how we'd actually be affecting the orbit by
doing something to an asteroid surface," Spitale said.

If the orbit is miscalculated, an object on course to deliver Earth a
glancing blow may be "mitigated" into an object on course to deliver a
direct hit.

The flip side of that is, you need a good model to compute Yarkovsky effect
perturbations even to know which asteroids pose real hazards, Spitale added.
"That may be the most important use of all for this model, to predict which
are going to hit in the first place," he said.
--
+++++++++++++++++++++++++++++++++++++++++++

NEO News is an informal compilation of news and opinion dealing with Near
Earth Objects (NEOs) and their impacts. These opinions are the
responsibility of the individual authors and do not represent the positions
of NASA, the International Astronomical Union, or any other organization.
To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. For additional
information, please see the website: http://impact.arc.nasa.gov. If anyone
wishes to copy or redistribute original material from these notes, fully or
in part, please include this disclaimer.
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CCNet is a scholarly electronic network. To subscribe/unsubscribe, please
contact the moderator Benny J Peiser < b.j.peiser@livjm.ac.uk >. Information
circulated on this network is for scholarly and educational use only. The
attached information may not be copied or reproduced for any other purposes
without prior permission of the copyright holders. The fully indexed archive
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DISCLAIMER: The opinions, beliefs and viewpoints expressed in the articles
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*

CCNet 44/2002 - 5 April 2002
----------------------------

"Asteroids in our Solar System may be more numerous than previously thought,
according to the first systematic search for these objects performed in the
infrared, with ESA`s Infrared Space Observatory, ISO. The ISO Deep Asteroid
Search indicates that there are between 1.1 million and 1.9 million `space
rocks` larger than 1 kilometre in diameter in the so-called `main asteroid
belt`, about twice as many as previously believed. However, astronomers
think it is premature to revise current assessments of the risk of the Earth
being hit by an asteroid."
--European Space Agency, 5 April 2002


(1) NEW STUDY REVEALS TWICE AS MANY ASTEROIDS AS PREVIOUSLY BELIEVED
    European Space Agency, 5 April 2002

(2) MORE ASTEROIDS IN MAIN BELT, NEW INFRARED SURVEY SAYS
    Space.com, 5 April 2002

==============
(1) NEW STUDY REVEALS TWICE AS MANY ASTEROIDS AS PREVIOUSLY BELIEVED

>From European Space Agency, 5 April 2002
http://www.alphagalileo.org/index.cfm?fuseaction=readRelease&Releaseid=9162

For further information, please contact:
Clovis De Matos
ESA Science Programme Communication Service
Clovis.De.Matos@esa.int
00 31 71 565 3460

Posted By:
European Space Agency    05 April 2002

 
Asteroids in our Solar System may be more numerous than previously thought,
according to the first systematic search for these objects performed in the
infrared, with ESA`s Infrared Space Observatory, ISO. The ISO Deep Asteroid
Search indicates that there are between 1.1 million and 1.9 million `space
rocks` larger than 1 kilometre in diameter in the so-called `main asteroid
belt`, about twice as many as previously believed. However, astronomers
think it is premature to revise current assessments of the risk of the Earth
being hit by an asteroid.

Despite being in our own Solar System, asteroids can be more difficult to
study than very distant galaxies. With sizes of up to one thousand
kilometres in diameter, the brightness of these rocky objects may vary
considerably in just a few minutes. They move very quickly with respect to
the stars - they have been dubbed `vermin of the sky` because they often
appear as trails on long exposure images. This elusiveness explains why
their actual number and size distribution remains uncertain. Most of the
almost 40,000 asteroids catalogued so far (1) orbit the Sun forming the
`main asteroid belt`, between Mars and Jupiter, too far to pose any threat
to Earth. However, space-watchers do keep a closer eye on another category
of asteroids, the `Near Earth Asteroids` or `NEAs`, which are those whose
orbits cross, or are likely to cross, that of our planet.

The ISO Deep Asteroid Search (IDAS), the first systematic search for these
objects performed in infrared light, focused on main belt asteroids. Because
it is impossible to simply point the telescope at the whole main belt and
count, astronomers choose selected regions of the belt and then use a
theoretical model to extrapolate the data to the whole belt.

Edward Tedesco (TerraSystems, Inc., New Hampshire, United States) and
François-Xavier Desert (Observatoire de Grenoble, France) observed their
main belt selected areas in 1996 and 1997 with ESA`s ISO. They found that in
the middle region of the belt the density of asteroids was 160 asteroids
larger than 1 kilometre per square degree - an area of the sky corresponding
to that covered by four full moons as seen from Earth. Then, a model
developed by Tedesco and the astronomers Alberto Cellino and Vincenzo
Zappala (Osservatorio Astronomico di Torino, Italy), allowed them to
estimate the whole asteroid population in the main belt: between 1.1 million
and 1.9 million asteroids with a diameter larger than 1 kilometre.

"If you consider the average value of 1.5 million asteroids, the ISO result
is about twice as high as estimated by two other recent studies in visible
light," Tedesco says.

The study by Durda et al., published in 1998, gave an estimate of about 860
000 asteroids larger than 1 kilometre in the main belt. In 2001, Ivezic et
al. obtained an even lower figure of 740 000 asteroids based on preliminary
data from the Sloan Digital Sky Survey.

Why the discrepancy?

The fact that visually dark objects - such as asteroids - are better
detected in the infrared might explain the discrepancy between visible and
infrared results. For an optical telescope, the brightness of an asteroid
depends on the visible light it reflects from the Sun. Observations with
infrared telescopes, on the other hand, detect the `heat` of the asteroid,
which does not depend that much on the reflected sunlight, but on the
absorbed sunlight.

As an example, let`s consider two spheres of the same size, and located
close to each other in the asteroid belt, one of which reflects ten times as
much of the visible light striking it as the other. As seen by an optical
telescope, the sphere which reflects more appears ten times brighter than
the other sphere which might be even invisible. However, for ISO both
spheres would be visible. Actually, the `dark` sphere would appear brighter
in the infrared because it would have a higher temperature (as it has
absorbed more sunlight).

Expert`s `best estimate`

Tedesco assumes that both visible and infrared searches might have their own
biases, which is the reason why the given results have an error margin.
Considering both the visible and infrared results, the `best estimate` would
be "1.2 million asteroids larger than 1 kilometre in the main belt, give or
take 500,000," Tedesco says.

The best strategy for finding the asteroid size distribution, according to
this expert, is to combine near-simultaneous observations at infrared and
visible light. "They provide different kinds of information and therefore
play a complementary role in the search for the asteroid population`s size
distribution," he says.

The `impact hazard`

A better knowledge of the number and size distribution of asteroids in the
main belt is essential to understand the population of Near Earth Asteroids
(NEAs), since most NEA are believed to be former main belt asteroids. In the
main belt there are four `special` regions where Jupiter`s gravitational
influence is especially disruptive; originally, most asteroids currently
known as NEA suffered collisions which resulted in them ending up in one of
those four key regions, and because of Jupiter`s gravitational influence
their orbits quickly evolved into Earth-crossing orbits. Therefore, by
studying the asteroids near these so-called `source regions` in the main
belt astronomers can learn about NEA. About 500 NEAs have been found so far,
and none of them pose any threat to Earth in this century.

The generally accepted impact rate by objects larger than 1 kilometre in
diameter is one every 100,000 to 300,000 years. The new `best estimate` of
about 1.2 million asteroids of 1 kilometre or larger in the main belt will
not change the current estimates of impact hazard, the IDAS astronomers say;
at least not yet.

"IDAS has contributed to our knowledge of main belt asteroids. And, although
we did not observe any NEAs, the ISO data will be used to improve our
knowledge regarding asteroids currently near the NEA source regions. This,
in turn, will allow us to better understand the population characteristics
of the NEAs and so ultimately enable us to refine our estimates of the NEA
impact frequency and the magnitude of the impact hazard," Tedesco says.

(1) 39,462 main belt asteroids were catalogued as of 28 March 2002. This
number increases by about 2,000 per month at present. 

Notes for editor
The European Space Agency`s infrared space telescope, ISO, operated from
November 1995 until May 1998. As an unprecedented observatory for infrared
astronomy ISO made nearly 30 000 scientific observations.

This note is based on the paper "The Infrared Space Observatory Deep
Asteroid Search" by Edward F. Tedesco and François-Xavier Desert, published
in the April 2002 issue of The Astronomical Journal.

For more information please contact:

Science contacts:

Edward Tedesco
TerraSystems, Inc. (Lee, New Hampshire, United States)
Tel: +1 603 659 5620
E-mail: etedesco@terrasys.com; eft@mediaone.net

Leo Metcalfe, ISO project scientist
European Space Agency, Vilspa, Spain
Tel: +34 91 8131372
E-mail: lmetcalf@iso.vilspa.esa.es

PR contact:

Clovis De Matos
ESA Science Programme Communication Service
Tel: +31 71 565 3460
E-mail: Clovis.De.Matos@esa.int

For more information about ISO visit the ESA Science website at:
http://sci.esa.int/iso

More information on the ESA Science Programme can be found at:
http://sci.esa.int.

Information on ESA can be found at http://www.esa.int

==============
(2) MORE ASTEROIDS IN MAIN BELT, NEW INFRARED SURVEY SAYS

>From Space.com, 5 April 2002
http://www.space.com/scienceastronomy/solarsystem/more_asteroids_020405.html


By Robert Roy Britt
Senior Science Writer

A systematic survey of the asteroid belt done in infrared wavelengths
indicates that there could be two or three times more large asteroids,
those more than 1 kilometer (0.6 miles) in diameter, than had been
thought.

Based on observations of portions of the main asteroid belt, between
Mars and Jupiter, the new study estimates there are between 1.1 million
and 1.9 million of these large space rocks. Many millions or perhaps
billions of smaller asteroids travel around the Sun in the same belt.

Asteroids in the main belt are not generally considered to be a threat
to Earth anytime in the near future, so the new estimate is not likely
to alter calculations of the risk of our planet being hit by one. But it
does highlight how little is known about the contents of our solar
system.

The new survey was made using the European Space Agency's Infrared Space
Observatory (ISO).

"If you consider the average value of 1.5 million asteroids, the ISO
result is about twice as high as estimated by two other recent studies
in visible light," said Edward Tedesco of TerraSystems, Inc. in New
Hampshire. Tedesco conducted the study with François-Xavier Desert at
the Observatoire de Grenoble, France.

Counting in circles

"I think the ISO results are significant in that they show, once again,
just how little we still know about the population of asteroids in our
cosmic environment," said Benny Peiser, a researcher at Liverpool John
Moores University who tracks asteroid studies.

How true. Just last November, another group of astronomers released a
study based on data from the Sloan Digital Sky Survey, projecting that
the main asteroid belt held some 700,000 space rocks larger than 1
kilometer (0.6 miles). Prior to that study, estimates had ranged up to
around two million -- similar to the new estimate announced today.

Here are the firm facts:

As of March 28, exactly 39,462 asteroids had been visually detected and
officially catalogued. More have been spotted but not yet put in the
books. Of these, about 500 large asteroids have been found nearer to
Earth. These so-called Near Earth Asteroids (NEAs) are watched more
closely, since over time gravitational tugs from the planets and other
factors could change the asteroid's courses enough to send them toward
us.

Experts figure there are probably somewhere between 900 and 1,200 total
Near Earth Asteroids larger than 1 kilometer (0.6 miles).

No asteroids are currently known, with any certainty, to be on a
collision course. Just yesterday, however, scientists announced that an
asteroid called 1950 DA has a maximum 1-in-300 chance of hitting Earth
in the year 2880. But researchers said that based on their lack of data,
the risk could actually be zero. Yet they won't know for sure for years
or possibly decades.

"We are still far away from a firm knowledge about the real number of
and risks from the asteroids of the solar system," Peiser said.

More about the survey

The new survey, called the ISO Deep Asteroid Search (IDAS), detected
heat given off by asteroids, rather than optical light.

Because it is impossible to actually count all asteroids with present
technology, the astronomers chose selected regions of the asteroid belt
and then used a theoretical model to extrapolate the data to the whole
belt. The observations were actually done in 1996 and 1997. Only
recently, however, was the data processed.

The infrared survey is better able to spot dark asteroids that might be
missed by optical telescopes, said the ISO researchers.

Yet Tedesco said both visible and infrared searches might have their own
biases, which is the reason why the new study's estimate was given as a
range.

Scientists generally agree that the chance of a large NEA hitting Earth
is about 1-in-5,000 over the next century.

The new results do not change these odds, researchers agree.

But Tedesco and Desert said that improved knowledge of the number and
size distribution of asteroids in the main belt is essential to
understand the population of NEAs, since most are believed to be former
main belt asteroids. As understanding improved, odds could change, they
indicated.

Copyright 2002, Space.com

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CCNet is a scholarly electronic network. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser < b.j.peiser@livjm.ac.uk >.
Information circulated on this network is for scholarly and educational
use only. The attached information may not be copied or reproduced for
any other purposes without prior permission of the copyright holders.
The fully indexed archive
of the CCNet, from February 1997 on, can be found at
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DISCLAIMER: The opinions, beliefs and viewpoints expressed in the
articles and texts and in other CCNet contributions do not necessarily
reflect the opinions, beliefs and viewpoints of the moderator of this
network.

CCCMENU CCC for 2002