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(meteorobs) NEO News (3/20/98) [LONG]
NEO News (3/20/98)
A number of items follow, mostly dealing with 1997XF11.
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WHAT IS NEO NEWS?
My distribution lists for NEO News have recently gotten a bit confused, and
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=46or a web posting of background information and news on the asteroid and
comet impact hazard, or as a reference when people ask for additional
information, you can always check the NASA website at
http://neo.arc.nasadot gov.
David Morrison
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TRUNCATION OF ARTICLE BY OLIVER MORTON
In my posting of 3/17/98 I included just the first two-thirds of the
article by Oliver Morton from The Financial Times, which therefore ended
abruptly in the version you received. The remainder of the article dealt
primarily with asteroid and space resources and not with the NEO impact
issue. But if any of you would like the full text please let me know and I
will send it.
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IMPACT AND SIGNIFICANCE OF THE XF11 SCARE
One of the most frequent questions I was asked at the annual NASA Lunar and
Planetary Conference this week in Houston was "Do you think the XF11 affair
was a net positive or negative for public interest in the impact hazard (or
for the future of Spaceguard)?" After discussing this with scientists and
the press, I don't know the answer. On the positive side, a lot more
people now know about the impact hazard. On the negative, quite a few of
them accuse the asteroid scientists of crying wolf or of being unreliable
sources of information. I do think it is important that we not make the
same mistakes again. Concerns have been expressed by both NASA, which is
the largest funding source for NEO searches, and the International
Astronomical Union, which is responsible for the Minor Planet Center. I
believe that everyone agrees that in similar future cases. we should take
more time to check the orbit and look for other observations before any
announcements are made to the press.
The two leading US weekly newsmagazines, Time and Newsweek, both feature
the asteroid story in their current (March 23) issues, and both had the
time to present an integrated story (unlike the daily media, which were
whiplashed by the rapidly changing story last week). It is interesting to
compare the treatments in Time and Newsweek. The Time story, which at four
pages is almost as long as some of their cover stories, is a serious
discussion by Leon Jaroff, who has supported us in the past (and who
criticized the US government for canceling Clementine 2 last summer). The
story quotes Jack Hills, Brian Marsden, Greg Canavan, Jim Scotti, Eleanor
Helin, Don Yeomans, Gene Shoemaker, Clark Chapman, Tom Gehrels, Edward
Teller, and David Morrison -- quite a comprehensive list of American
sources. The emphasis is almost entirely on the reality of the impact
threat, with strong implications that we should be paying more attention
(and spending more money). There are no recriminations over the affair of
last week, with the revised 2028 miss distance for XF11 attributed to new
data, not to mistakes. Also, there is an editorial elsewhere in the
magazine that uses the asteroid issue for a humorous essay suggesting that
it might be good for the earth to wipe out most of humanity and what we
call civilization.
The Newsweek story is shorter and less serious in tone (but still a
substantial 3 pages long). It discusses more of the history of the
discovery and orbit for XF11 and less on the general impact hazard. It
also plays up the conflicts, for example quoting Yeomans in accusing
Marsden of "crying wolf". There are a few factual errors: (1) The claim is
made that an impact from an asteroid as large as Ida (identified as 36
miles across) would send us back the stone age, when in fact it would very
nearly sterilize the entire planet in an extinction as large as the
Permian, 250 million years ago. (2) It is claimed that asteroids and
comets "refuse to stay in their orbital lane" implying that they
erratically shift orbits, a common public misperception. (3) Jay Melosh is
misquoted concerning the depth of penetration of a colliding mile-wide
asteroid. (4) It identifies the Tunguska projectile as a comet rather than
as asteroid and says it ignited the clothes on a man 60 miles away. (5) It
says Clementine 2 was a NASA mission rather than Department of Defense. In
spite of these technical lapses, however, most of the Newsweek story is
good, and it clearly communicates the long-term impact threat just as does
the story in Time.
Judging by Time and Newsweek, as well as the coverage in the major US
dailies such as the Washington Post and the New York Times, the XF11 story
was handled accurately and placed in its proper context of the long-term
threat of impacts. This was certainly a plus for public communication.
Elsewhere, on local TV and talk radio, there was a greater tendency to make
fun of the story. Some also characterized it as "the IAU got it wrong, but
NASA got it right". Many also suggested that somehow this was tied to the
release of the two films Deep Impact and Armageddon, even suggesting that
the astronomers were in the pay of the films. I hope this sort of thing
was not intended (or taken ) seriously, but who knows, considering how
cynical much of the American public is these days.
My favorite headline: KISS YOUR ASTEROID GOODBYE.
I have heard very little of how the story was presented by the non-US
press, or received by the public internationally.
And finally, a very interesting question raised to me by George Wetherill:
suppose the miss distance for XF11 had held or dropped, and there really
was a small but significant chance of an impact, with 30 years of warning.
That could have been a wonderful thing, inspiring the nations of the Earth
to work together to save the planet and perhaps also to devolop the
technical infrastructure for economical commercial spaceflight. Thus
defending our planet could untimately have been a turning point in human
history -- and now an opportunity lost.
David Morrison
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APOCALYPSE POSTPONED
=46rom: NEW SCIENTIST, 20 March 1998
After a day-long drama in which it seemed there was an outside chance that
civilisation might end 30 years from now with a catastrophic asteroid
impact, astronomers declared the all clear last Thursday. Revised
calculations based on data from 1990 show that
on 26 October 2028 asteroid 1997 XF11 should miss the Earth by 960 000
kilometres--2=855 times farther away than the Moon.
1997 XF11 was discovered by Jim Scotti of the University of Arizona in
Tucson on 6 December last year. Gareth Williams and Brian Marsden of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts,
soon added it to their list of "potentially hazardous objects" which might
hit the Earth.
Initial calculations indicated that 1997 XF11 would miss the Earth by about
800 000 kilometres in 2028. But the inclusion of observations made on 3 and
4 March showed 1997 XF11 skimming just 42 000 kilometres above the Earth's
surface--with a small chance that it would hit us.
Marsden announced the bad news in an electronic circular sent out on 11
March, trying to encourage more observations. He also asked astronomers to
check their archives for any sightings from the previous occasions 1997
XF11 came within viewing distance: in 1990, 1983, 1976, 1971 and 1957. "But
we didn't think the chances were that large of finding something," says Dan
Green, who works with Williams and Marsden.
So the team was surprised when Eleanor Helin of the Jet Propulsion
Laboratory (JPL) in Pasadena promptly unearthed images of the 1990
encounter on photographic plates. While Marsden talked with reporters who
had arrived to cover the story on 12 March, Williams recalculated the
orbit, and found the asteroid would miss the Earth by 960 000 kilometres.
Don Yeomans of JPL has confirmed that result.
The impact of an asteroid the size of 1997 XF11, which is 2 kilometres
across, would release about half a million megatons of explosive energy.
That could devastate global agriculture, warns Scotti.
With sufficient warning, it could be possible to nudge an asteroid away by
exploding nuclear weapons a kilometre or so above its surface. Identifying
all the potential threats from outer space will require a ten-year,
$50-million programme, says David Morrison of the NASA Ames Research Center
in California. That's less than the budgets of the two asteroid disaster
movies scheduled to reach the screen this summer--but the money has not yet
been forthcoming.
Jeff Hecht, Boston
(c) 1998 New Scientist
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GENE SHOEMAKER'S VOICE IS MISSING, BUT HIS LEGACY IS FELT
Excerpted from the New York Times Science Section, Tuesday, March 17,
1998 page F4
Asteroid Expert's Voice is Missing, but His Legacy is Felt
by Henry S.F. Cooper
In all the discussion about Asteroid 1997 XF11 and its chances of hitting
Earth, and what to do about it, one voice badly missing was Dr. Eugene M.
Shoemaker's. He was the geologist who first brought the study of rocks into
space and whose work, first with impact craters and then with the asteroids
and comets that blasted them, has made most people accept the notion --
revolutionary in the 1950s and 1960s -- that big
rocks from space can make big holes in planets, with catastrophic results.
The Palomar Asteroid Search, which he initiated in 1973, was the first
systematic search for asteroids (and later, comets) whose trajectories
crossed Earth's and might hit the planet.
Dr. Shoemaker was killed in an automobile accident in July near Alice
Springs, Australia, where he and his wife, Carolyn Shoemaker, hispartner in
the asteroid and comet search, were studying impact craters.
In the spring of 1996, I spent a couple of weeks with the Shoemakers at
their house in Flagstaff, Arizona, interviewing them for a book I am
writing...
By good fortune, I happened to ask Dr. Shoemaker to unfold for me the
scenario of how we could deal with an asteroid one-kilometer wide -- about
five-eighths of a mile -- if we learned early enough that it was going to
hit Earth. I taped his answer, so that in fact we are not without Dr.
Shoemaker's voice on this subject after all. It is a soothing and
gently persuasive voice, making whatever he said sound reasonable, however
outlandish or devastating.
"If you could manage to get the nations of the world organized, or at least
the lead nations, then you could think in terms of sending up a nuclear
device," he said. "In fact you would send several devices on several
separate spacecraft, with which you have stand-off explosions that would
result in gentle pushes on the asteroid.
"We want to do the maneuver when it is near the Sun, near the asteroid's
perihelion, to get the maximum effect. If you can do it, you can change
the the semi-major axis of the orbit, you can change its period, so that
the Earth is somewhere else when the object goes by. It turns out you
don't have to push very hard. If you do the maneuver 10 to 20 years before
the predicted collision, then the impulse that must be delivered to the
asteroid -- the change in velocity -- is only of the order of one or two
centimeters a year, to deviate a center hit on Earth to a clean miss.
Clearly you don't do just one big shot; you give it a series of smaller
shots; you very carefully herd it into the new orbit that you want." He
discussed other ways of accomplishing this, such as using conventional
explosives.
"Whatever you do, you don't want to break it," he said. "You push on it
gently -- that is why you do a stand-off explosion. If you break it up,
then it becomes an uncontrolled problem. You wouldn't be able to herd it if
you suddenly got pieces going off in various directions -- it's out of
control. You have multiple objects hitting the Earth, and they could
actually cause more damage rather than less."
Although Dr. Gene Shoemaker is no longer on this planet, he, along with
Carolyn Shoemaker and another observer, Dr. Eleanor Helin, may have hadthe
last word on Asteroid 1997 XF11. Photographic plates that the Shoemakers
took in April 1990 as well as plates that Dr. Helin took in March 1990,
which were re-examined last week, revealed extremely faint images of the
asteroid on an earlier orbit. The pictures provided a longer arc of
information on the asteroid's trajectory that had been available before,
enabling astronomers at the Jet Propulsion Laboratory in Pasadena to
determine with authority that in 2028 the comet would miss Earth by a
distance very likely greater than that of Earth from the Moon.
As a result of the Shoemakers' search and a handful of others', about 1,500
Earth-crossing asteroids five-eighths of a mile or more in diameter are
known and their orbits are plotted. Dr. Shoemaker believed there could be
three times as many. Each is capable of taking out a city, a country, or
much more. Following the Shoemaker-Levy impacts, an obviously impressed
Congress asked NASA to study what would be needed for a full survey, and a
NASA committee led by Dr. Shoemaker recommended a program called
Spaceguard, requiring an initial five-year effort that would cost $24
million, followed by an additional $3 million a year to continue the
search. The proposal was shelved.
Mrs. Shoemaker hopes the recent scare will revive Spaceguard. At the very
least, expanding the library of plates might prevent future falsealarms.
And as surely as the Moon follows the Sun across the sky, the alarm some
day will be real. (c) 1998 The New York Times.
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GENE SHOEMAKER MIS-QUOTED?
=46rom: Duncan Steel <dis@a011.aonedot net.au>
In that interesting article (above) from the NY Times, Gene Shoemaker is
quoted as saying:
>If you do the maneuver 10 to 20 years before the predicted collision,
>then the impulse that must be delivered to the asteroid -- the change
>in velocity -- is only of the order of one or two centimeters a year,
>to deviate a center hit on Earth to a clean miss.
In fact the required speed change required to get a potential impactor to
miss, given a lead time of decades, is one or two centimeters PER SECOND.
This is easy to see. Let the lead time be 20 years and the impulse be such
that the change in speed is 1 cm/sec. Then the difference in distance
moved between the unperturbed (by our intervention) and the perturbed (we
kicked it) orbit is 20 x 365.25 x 24 x 60 x 60 x 0.01 metres which equals
6312 kilometres, near as dammit. The Earth's radius is 6371 km. Thus an
induced speed change of just over 1 cm/sec would be sufficient to get an
asteroid whose path w a s due to pass through the centre of the Earth to
miss our planet: just.
I don't know, of course, whether Gene was mis-quoted, or perhaps he
mis-spoke himself. Easily done. Gene undoubtedly knew the impulse required
(which is still small!) but may have slipped (although I'd need to hear the
tape to be convinced of that!).
Another part of the article states:
>As a result of the Shoemakers' search and a handful of others', about
>1,500 Earth-crossing asteroids five-eighths of a mile or more in
>diameter are known and their orbits are plotted. Dr. Shoemaker believed
>there could be three times as many. Each is capable of taking out a city,
>a country, or much more.
That is nonsense. The known population of Earth-crossers larger than 1 km
(which is what that 'five-eighths of a mile' derives from) is ten times
less, only about 150, especially at the time of the interview in 1996. This
is undoubtedly an error on the part of the journalist, not Gene. And Gene
knew that we had so far found only a b o u t 5 to 10% of the
Earth-crossers above that canonical 1 km size. On the other hand, there c
o u l d be three times 1,500 at sizes >1 km needing discovery, although
most estimates are lower (1500-3500, say; depends on what sorts of albedos
you think they've got). And the damage potential of 1 km asteroid is rather
understated in the final sentence quoted above, and again I must ascribe
the error to the journalist. Gene knew better.
Regarding:
>He discussed other ways of accomplishing
>this, such as using conventional explosives.
Again, I'd like to hear the tape. Gene knew damn well that there's no point
in using 'conventional explosives'. You would do as well using just lead in
a hypervelocity impact, with less chance of an accident along the way! At
a speed of 3 km/sec a kg of lead (or a n y t h i n g) has the same kinetic
energy as the chemical energy of a kg of TNT. So there's no point in using
conventional explosives. One needs things with high specific energies, and
that implies nuclear.
Duncan Steel
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THE SKY MAY NOT BE FALLING AFTER ALL
Copyright ) 1998 Nandodot net
Copyright ) 1998 The Associated Press
HOUSTON (March 19, 1998 8:12 p.m. EST http://www.nandodot net) -- A week after
coming off like Chicken Little with a Ph.D., some astronomers have resolved
to make sure they're right the next time they announce the sky might be
falling.
At a meeting this week in Houston, 15 astronomers from around the country
agreed to form a committee that will use its combined expertise to
calculate the risks to Earth when an asteroid looks like a threat.
"This group would be charged with assessing the threat and reaching a
consensus and a clearer plan for defining the nature of the threat," said
Donald Yeomans, a scientist with NASA's Jet Propulsion Laboratory.
"We don't cry wolf. If it's a real threat, the announcement will be made
and steps will be taken to mitigate the threat."
Last week, it appeared a group of astronomers, the International
Astronomical Union, had cried wolf when they issued an alert saying that an
asteroid would pass within 30,000 miles of Earth -- and might even collide
with it -- on Thursday, Oct. 26, 2028, around 1:30 p.m. The next day,
Yeomans -- citing new data -- said the asteroid would pass no closer than
600,000 miles and had no chance whatsoever of hitting the planet.
All parties seem to agree that the gaffe could have been avoided had the
International Astronomical Union and NASA communicated earlier.
"It's in our best interest to try to get harmonious again," said Brian
Marsden, the distinguished Harvard astronomer who made the IAU calculations.
Marsden and Yeomans were among the astronomers who met Tuesday at Houston
and decided to form the peer review committee.
When an astronomer discovers that an asteroid could threaten the Earth, the
committee will review the data and do its own calculations to determine how
serious the threat is.
"Within a matter of a day or two, the situation will become far more clear
and it will either become a nonevent or some appropriate announcement will
be made -- but not until this committee's had a chance to chew on it for a
bit," Yeomans said.
The committee members have not yet been selected, but they are likely to
include both Marsden and Yeomans.
Marsden admitted the entire asteroid episode "left a nasty taste in my mouth=
=2E"
Marsden said he made his calculations based on all the data available at
the time. In Marsden's 40 years of tracking asteroids, the space rock was
the first with the potential of coming so close to the Earth. He said he
decided to make an announcement to try to obtain additional data.
Eleanor Helin of the Jet Propulsion Laboratory immediately called Marsden
and said that based upon his data, she had found 1990 telescopic images of
the asteroid that could be helpful.
Using those pictures and recent observations, Helin's group calculated the
asteroid's new position and forwarded the information to Marsden and her
colleague, Yeomans. Yeomans simply beat Marsden to the punch by releasing
the information, she said.
"I'm very disappointed in how this unfolded," Helin said. "It appears that
people are trying to pit (Marsden's group) against NASA and vice versa
when, really, we're all friends."
By PAULINE ARRILLAGA, The Associated Press
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