Asteroid 4179 Toutatis (formerly 1989 AC) was discovered by C. Pollas on
January 4, 1989, at Caussols, France, on photographic plates taken on the
0.9-m Schmidt telescope by Alain Maury and Derral Mulholland. The images revealed
Toutatis to be a several-kilometer-long (about 5km by 2km) object with a nonconvex shape
dominated by two components in contact, one approximately twice as large as
the other. The highest-resolution images, show craters with diameters ranging
from about 100 m to about 600 m. Toutatis will make its closest planetary approach since at least 1353 and
until at least 2562 on Sep. 28, 2004, when the closest
COM-to-COM separation of Earth and Toutatis will be 0.009 of an AU ( 4 lunar
distances). But because Toutatis will be coming up from behind Earth and then going around it just at its most distant position from the sun, as well as slowing down to start its jorney of being pulled back to the sun... the Earth and its unchanging orbit and speed will catch up to Toutatis and the fact is that the Earth will hit Toutatis, not Toutatis will hit the Earth. Because of this slow pass (about 1 week) gravity will have plenty of time to pull on Toutatis and curve its orbit into the Earth! lets not forget that Toutatis has gravity of its own and so it may be able to change course by a few degrees on its own, which is plenty enough to get it heading directly into Earth. One thing is certain, of all the natural forces that control the orbit of Toutatis, are the same forces that might cause it to impact the Earth.
These three views of the Toutatis computer model show shallow craters,
linear ridges and a deep topographic "neck" whose geologic origin is not
known. It may have been sculpted by impacts into a single, coherent body, or
this asteroid might actually consist of two separate objects that came
together in a gentle collision. Toutatis is about 4.6 kilometers (3 miles)
long.
Toutatis's Rotation State.
Toutatis has one of the strangest rotation states yet observed in the solar
system. Instead of the spinning about a single axis as do the planets and the
vast majority of asteroids, it "tumbles" somewhat like a football after a
botched pass. Its rotation is the result of two different types of motion with
periods of 5.4 and 7.3 Earth days that combine in such way that Toutatis's
orientation with respect to the solar system never repeats.
N.E.O. threat.
For a future asteroid impact, given our current level of insight into the
situation in space, the expected warning time before impact will be zero.
Well, say, five or six seconds, since there will be a bright flash that a few
people will notice before being pulverized. Programs now in preliminary stages will be able to catalog more and more of the "Near Earth Objects" to smaller and smaller sizes, providing in most cases
longer and longer advance notice of impact hazards -- but not for a few
decades yet. Among all the dangers that nature has dished out for Earth,
there's a silver lining to the asteroid impact threat. The most likely objects
to hit Earth are in orbits that repeatedly pass close enough to Earth to be
spotted, tracked, and catalogued far in advance. Their orbital inclinations
are close -- ten or twenty degrees off, at most -- and their orbital periods
are within a factor of two of Earth's.
These objects constitute 99% or more of the impact threat, because the
eccentric comets and deep-space interlopers -- while they exist -- usually
have only one shot at Earth as they pass through the inner solar system. In
contrast, these "NEO's" keep making passes again and again and again UNTIL
they hit, or are flung clear by a very close approach.
The bigger objects -- the ten kilometer rocks -- are the dinosaur killers,
the millions of megatons of explosive force. They are pretty well all
catalogued and all look safe on a time scale of tens of millions of years. The
one-kilometer rocks, like 1997 XF 11 and a few thousand others, are the
continent-killers, the thousand megaton exploders. About 130 of them have been
catalogued, and NASA hopes to discover 90% of the rest over the next
decade.
The 100-meter objects are the kind that made the 20-megaton Tunguska impact
over Siberia in 1908. These are the city-busters, and we should expect them
every few decades -- every century perhaps. There are hundreds of thousands of
these out there and very, very few have ever been detected visually.
Even smaller objects hit more frequently, as you would expect. In 1965, a
tens of kilotons mid-air blast over Revelstoke, Canada, scattered black dust
across miles of new-fallen snow. A similar-sized object barely missed Earth,
but entered our atmosphere and streaked across the Rockies -- not far from
Colorado Springs -- and was videotaped by vacationers. Just a few years ago, a
100-kiloton-sized midair explosion over the western Pacific was startling
enough that President Clinton was awakened to be informed of it -- it might
have been somebody's nuclear weapons test. More will occur, and not merely
over arctic waste, mountains, and open ocean.
The obvious response to an approaching asteroid is to "deflect" it sideways
to miss Earth. But this "common sense" idea fails to appreciate the unearthly
nature of out-of-plane dynamics in space.
Assuming a long-enough lead time, the last kind of impulse you would ever
want to impart to an asteroid is perpendicular to its motion. This would
merely make it wobble in its orbit, but it would for the most part still
arrive at future points close to the original predictions. Instead, the
impulse should be directed ALONG its flight path -- slowing it down (from in
front) or speeding it up (from behind) would work equally well. This would
alter the energy of the orbit and cause it to arrive at predicted future
intersection points at a different time. When it got there, the fast- moving
Earth wouldn't be there -- and the impact would be avoided.
But try and explain this to someone unfamiliar with orbital operations.
Tell them that in order to make it miss Earth, you want to SPEED UP the
approaching asteroid, and see the reaction! The US Defense Department may
still be discussing for decades whether or not it wants to get involved in
this business -- and in the end, the assignment may be dropped in its lap
whatever its own desires may be. But already there have been very practical
DoD interests in asteroids, and one of them is the probe Clementine-2.
Although Clementine-2 was line-item-vetoed last year by President Clinton,
apparently on the advice of policy wonks that it might be misinterpreted as a
space weaponization scheme that might upset the Russians, the recent Supreme
Court decision overturning the constitutionality of the whole line-item-veto
idea may allow the project to resume. It was always a good idea from the DoD's
point of view -- test an autonomous microsatellite sensors and controllers --
but it is also very good idea from an asteroid deflection point of view, since
it addresses the key unknown about asteroids, what is their internal structure
and how would they respond to external forces (such as the US Space
Command!).
Not long ago, astronomers thought of asteroids as rocks, perhaps rubble
covered, but still mainly single bodies. But evidence has accumulated that
asteroids are rubble piles all the way through, loosely bound together by what
is generously called "gravity" (escape velocity is 11,000 meters per second on
Earth but less than 1 meter per second on a typical small asteroid). The
Shoemaker-Levy object was torn apart by a close brush with Jupiter in 1992 so
when it fell back onto Jupiter two years later it was a string of smaller
objects. Crater chains on the moons of Jupiter, on Earth's moon, and on Earth
itself also point to the gravity-induced disintegration of many asteroids
prior to impact. Asteroids which rotate fast enough to fling pieces clear are
extremely rare -- only two are known -- which suggests that these are the rare
single-rock objects.
What this means is that big impulses -- say, from another asteroid
collision or from a nuclear detonation -- would more likely disperse the
material than deflect it. Pushing an asteroid has been likened to clearing a
landslide off a road, rather than rolling a rock. So other techniques --
gentle pushes over long periods -- may prove to be required.
By the way: If there's one class of modern intellectuals who do not find
the idea of deliberate environmental modification to be incredible, it's the
international lawyers and diplomats. There already is a treaty -- the
Convention on the Prohibition of Military or Any Other Hostile Use of
Environmental Modification Techniques -- which was signed in 1977 and ratified
in 1980. It defines "environmental modification techniques" to be "any
technique for changing -- through the deliberate manipulation of natural
processes -- the dynamics, composition or structure of the Earth, including
its biota, lithosphere, hydrosphere and atmosphere, or of outer space." But
all that is forbidden is doing this in order to create "widespread, long-
lasting or severe effects as the means of destruction, damage, or injury." So
peaceful uses are explicitly allowed.
The US Air Force's last major involvement with climate engineering was
Project Stormfury, a quarter century ago. Attempts were made to steer
hurricanes by preferential cloud seeding, to increase localized rainfall and
heating. This was supposed to lead to alteration of the 'steering currents'
which naturally and randomly direct a storm's motion. Results were ambiguous,
except for the discovery that once you "touch" a hurricane, everybody blames
you for where it eventually goes (of course, those living in areas the storm
avoids do NOT come out to thank you). That's another reason for governments to
do this -- so you won't be sued for damages. In any case, future projects to
steer hurricanes -- or at least, mitigate their wind speeds when they do reach
populated areas -- are only a matter of getting up the nerve to try it.
Equally daunting in a political sense is the question of earthquake control
and the "geologic engineering" involved. The problem is not the slippage of
tectonic plates, but their stickiness. They grab and hold as tension builds up
(tension perhaps measurable in terms of magnetic field distortions observed by
very high altitude sensor arrays), then break free all at once. Geologists
have long known that near-surface fault lines can be "greased" or "fixed" by
artificially varying the amount of water in the rock, and it has been proposed
that known fault lines be massaged by fixing two end points and then
deliberately slipping the inside region. This would allow a constant and low-
force release of the energies. But for deeper faults lines, who in the
government is going to suggest taking action to deliberately trigger an
earthquake -- even though such an effort, at a predetermined time, would be
far safer and probably much less damaging than simply waiting for it to happen
at random? Here again it is not the science and technology but the politics
and philosophy that stand in the way.
The Tunguska object that hit Russia in 1908 could have easily wiped out a
city. In fact, if the Earth had been advanced in its rotation by about 71
degrees (4 hours and 45 minutes) the object would have vented its 15- to
30-megaton blast over Leningrad (then St. Petersburg). Considering that the
actual downward-directed blast scorched and leveled 2,000 square kilometers
(700 sq. mi.) of dense Siberian forest, cremating the wildlife within, and
produced a shock wave that traveled completely around the world twice, it is
safe to assume that few if any of the city's two million inhabitants would
have survived. Such an event would have changed recent history. Clube and
Napier have developed a very credible scenario that supports Plato's
contention. They postulate that around 5,000 years ago a large comet, perhaps
20 km in diameter, was perturbed by Jupiter into a short period orbit which
intersected our own planet's orbit. A comet of this size will inevitably break
up, leaving in its path debris of varying sizes and shapes. Since this comet
was crossing the orbit of Earth, at least once a year our planet would have to
careen its way through this debris path producing a spectacular meteor shower
about the same time every year. Most of the larger comet pieces (100 to 1,000
meters across) would be found not too far away from the original comet at
first, but the force--be it rotation or gas pressure--that separated them
initially would still be with them, so they would continue to drift away from
the main berg. From the standpoint of the Earth, what had been a rifle bullet
became a load of buckshot. Clube and Napier speculate that while streaking
through this dense swarm close to the comet, the Earth could have encountered,
in the course of half an hour, thirty impacts in the range of 10 to 100
megatons with perhaps a few in excess of this!
By combining astronomical facts with archaeological evidence, such as
ancient calendars and astronomically aligned megalithic structures, Clube and
Napier further speculate that the object responsible for this mischief was the
progenitor of the comet Encke. Kenneth Brecher, an astrophysicist at Boston
University, sees a link between comet Encke, the Tunguska event of June 30,
1908, and the June 25, 1178 impact on the Moon reported by the monk Gervase of
Canterbury. This lunar impact is thought to have produced a crater (Giordano
Bruno) 20 kilometers in diameter! To account for both these energetic events
occurring on almost the same day of the year, Brecher postulates that a large
piece of comet Encke broke away prior to 1178, producing a swarm of objects,
some of which could be a kilometer across. He believes this swarm will be
entering the Earth-Moon system again in the year 2042.
Cosmic debris has a size distribution somewhat like pebbles on a beach--the
small outnumber the large Currently the population of Earth-orbit-crossing
asteroids a kilometer or larger in diameter is estimated to be around 2,000.
If we go down in size to our 350-meter-across civilization cruncher, this
number would at least double, and if all football-field-size city smashers
such as the Tunguska object were included, the population would jump into the
tens-of-thousands range. Including comets, less than 100 Earth-orbit-crossing
objects (EOCOs) have been discovered to date. The limitations of a telescope
looking through an ocean of air favors the detection of the larger objects, so
most of the EOCOs detected so far have a diameter of a kilometer or more.
Our vast ignorance with respect to the whereabouts of these objects means,
in the words of active EOCO hunter-geologist Eugene Shoemaker, "until we have
tracked all of them, something could sneak up on us." Tracking all EOCOs is
going to take some time. Planetary scientist Eleanor Helin and colleagues have
found 20 of the known EOCOs; this represents 13 years of seeking. Even if
improved equipment allows a discovery rate of 20 EOCOs per year we are
speaking of perhaps 100 years just to locate the objects a kilometer or more
across. The point is this: because there is at present absolutely no way to
predict when the next major impact will occur, we are fools if we do not
effect a defense against these objects as soon as possible.
Asteroid 433 Eros
First, the good news: Asteroid 433 Eros is not on a collision course with
the Earth. At roughly twice the size of Manhattan Island, Eros is huge
compared with other known near-Earth asteroids. A collision by an object this
size would be more devastating than the impact that is thought to have
finished off the dinosaurs 65 million years ago. Eros is in the news because after a torturous four-year journey the NEAR spacecraft attempted to become an artificial moon of Eros. The successful NEAR mission to
Eros shows that we have the ability to rendezvous with an asteroid, orbit it and then even to land on it. This ability is crucial if -- some scientists would say "when" -- an
asteroid is discovered to be on a collision course with Earth.
Space missions to asteroids and comets might not seem as exciting as a
landing on Mars, but the social, scientific and commercial benefits from these
missions could be great. An asteroid or comet impact with Earth is the only
type of natural disaster that could instantly wipe out human civilization, and
yet -- unlike earthquakes, floods and volcanoes -- it is within our grasp to
prevent the collision.
