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PLUTO, COMETS, METEORS and SPACE DEBRIS
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1) Pluto is only 20 per cent the diameter of Earth but is still over 2300 km across.
2) Recently, additional objects like it, but smaller, have been found in the outer reaches of the Solar System
3) Besides the planets and their moons, many other objects are in the family of the Sun. The most
spectacular, as seen from Earth, are comets. Comets have long been seen as omens.
4) Asteroids, which are minor planets, and chunks of rock known as meteoroids are the other residents of
the Solar System.
5) It is becoming apparent that more and more collisions of these with the Earth can be devastating. Every
few tens of millions of years, one large enough to do very serious damage should hit.
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Pluto’s Mass and Size
1) Even such basics as the mass and diameter of Pluto are very difficult to determine. Pluto has made less
than one revolution around the Sun since its discovery, thus providing little of its path for detailed study.
As recently as 1968 it was mistakenly concluded that Pluto had 91 per cent the mass of the Earth.
2) In 1978 Pluto was discovered to have a satellite. The moon was named Charon, after the boatman who
rowed passengers across the River Styx to the realm of Pluto, god of the underworld in Greek
mythology.
3) The presence of a satellite allows us to deduce the mass of the planet by applying Newton’s form of
Keppler’s third law. Charon is 5 or 10 per cent of Pluto’s mass, and Pluto is only 1/500th the mass of the
Earth, ten times less than had been suspected just before the discovery of Charon.
4) Pluto’s rotation axis is nearly in the ecliptic, like that of Uranus. This is also the axis about which Charon
orbits Pluto every 6.4 days. There are two five-year intervals during Pluto’s 248 year orbit when the two
objects pass in front of (occult) each other every 3.2 days, as seen from Earth.
a) Measuring occultations were the case in 1985 through 1990
b) When the apparent brightness was measured, light from both Pluto and Charon were received.
c) Their blocking each other led to dips in the total brightness we received. From the duration of
fading, their sizes were deduced.
d) Pluto is 2300 km in diameter and Charon is 1200 km in diameter. Charon is therefore ½ the size
of Pluto.
e) Charon is separated from Pluto by only about 8 Pluto diameters, compared with the 30 Earth
diameters that separate the Earth and the Moon. Therefore Pluto/Charon is almost a “doubleplanet” system.
5) The rate at which the light from Pluto/Charon faded gave us information that revealed the reflectiveness
(albedos) of their surfaces, since part of the surface of the farther object remained visible most of the
time. The surfaces of both vary in brightness. Pluto seems to have a dark band near its equator, some
markings on that band, and bright polar caps.
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PLUTO
1) Its orbit is the most out of round (eccentric) and is inclined by the greatest angle with respect to the
ecliptic plane, near which the other planets revolve.
2) Pluto’s elliptical orbit is so eccentric that part lies inside the orbit of Neptune. Pluto was closest to the
Sun in 1989 and moved further away from the Sun than Neptune in 1999. Therefore Pluto is still
relatively near its closest approach to the Sun out of its 248 year period, and it appears as bright as it
ever does to viewers on Earth.
3) It hasn’t been as bright – about magnitude 13.5 – for over 200 years. It is barely visible through a
medium-sized telescope under dark sky conditions.
4) The discovery of Pluto was the result of a ling search for an additional planet, that, together with
Neptune, wan believed to be slightly distorting the orbit of Uranus.
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6) The latest Hubble Space Telescope views show Pluto has a dozen areas of bright and dark, the finest
map every made of Pluto. It is unknown whether the bright areas are bright because they are high
clouds near mountains or low haze and frost. What is known is that there are extreme contrasts on
Pluto’s surface.
7) If you were standing on Pluto, the Sun would appear over 1000 times fainter than it does to us on Earth.
Consequently, Pluto is very cold, infrared measurements show that its temperature is less than 60K.
8) From Pluto a telescope would be needed to see the solar disk, which would appear about the same size
that Jupiter appears from Earth.
Pluto’s Atmosphere
1) Pluto occulted - (passed in front of and hid) – a star one night in 1988. Astronomers used this occultation
to learn about Pluto’s atmosphere. If Pluto had no atmosphere, the starlight would wink out abruptly.
Any atmosphere would make the starlight diminish more gradually. The observations showed starlight
diminished gradually and evenly.
2) Therefore Pluto’s atmosphere has layers in it. Astronomers were also able to conclude that the bulk of
Pluto’s atmosphere is nitrogen. A trace of methane must also be present, since the methane ice on
Pluto’s surface must be evaporating (this was determined using the spectrum of the surface of Pluto)
3) Pluto’s atmospheric pressure is very low, only 1/100,000 that of Earth’s.
4) As Pluto moves further from the Sun (as is now happening), the atmosphere is predicted to freeze out
and snow onto the surface.
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Kuiper-Belt Objects
1) Beyond the orbit of Neptune (trans-Neptunian orbit), a population of ice objects with diameters of a few
tens or hundreds of kilometers are increasingly being discovered. The planetary astronomer Gerard
Kuiper suggested a few decades ago that these objects would exist and should be the source of many of
the comets that we see. As a result of his work, these objects are now known as Kuiper-Belt Objects or
Trans-Neptunian Objects.
2) The Kuiper-Belt is probably about 10 A.U. thick and extends from the orbit of Neptune about twice as
far. A few dozen objects have been found so far, but tens of thousands of objects larger than 100 km
across are thought to exist.
3) The objects may be planetesimals left over from the formation of the Solar System. They are very dark
with albedos of only about 4 per cent.
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What is Pluto?
1) From Pluto’s mass and radius, Pluto’s density was calculated to be about 2 g/cm3 – twice the density of
water and half the density of the Earth.
2) Since ices have even lower densities on Pluto, Pluto must be made of a mixture of Ice and rock.
a) Its composition is more similar to that of the satellites of the giant planets, especially Neptune’s
large moon Triton, than to that of Earth or the other inner planets of the Solar System.
3) Now that Pluto’s mass is known, it has been calculated to be far too small to cause deviations in
Uranus’s orbit that originally led to Pluto’s discovery. The discrepancy wasn’t real: The wrong mass had
been assumed to Neptune when predicting the orbit of Uranus.
4) Pluto, with its moon and atmosphere, has some similarities to the more familiar planets.
5) Pluto is strange in that it is so small next to the giants, and that its orbit is so eccentric and so highly
inclined to the ecliptic.
6) The new values of Pluto’s mass and density revived the thinking that Pluto may be a former moon of
one of the giant planets, probably Neptune, and escaped because of a gravitational encounter with
another planet. Some evidence, such as the existence of Charon close by suggest that Pluto was never a
moon.
7) With this evidence, the discovery of similar Pluto-like objects, and the IAU scientific definition of a
planet, Pluto is more closely identified with newly discovered Kuiper-Belt Objects.
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a) By contrast Pluto has an albedo of 60 per cent
4) Pluto is one of the larger Kuiper-Belt objects known along with recently discovered Eris, and they are
unique in that they are covered in frost.
5) Triton may have initially been a similar object, subsequently captured by Neptune.
6) A few objects may have once been Kuiper-Belt objects but now come somewhat closer to the Sun,
crossing the orbits of the outer planets. About 100 of these “centaur” objects a few hundred kilometers
across may exist. Since they are larger and come closer to the Earth and Sun than most Kuiper-Belt
Objects, we can study them better.
a) These centaurs are intermediate between comets and asteroids
New Horizons Mission to Pluto and the Kuiper Belt
1) Link to New Horizons Mission web-site http://www.nasa.gov/mission_pages/newhorizons/main/index.html
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The Composition of Comets
1) At the center of the comet’s head is its nucleus, which is composed of chunks of matter.
a) The most widely accepted theory of the composition of comets, advanced in 1950 by Fred L.
Whipple of the Harvard Smithsonian Observatories, is that the nucleus is like a dirty snowball.
b) It may be made of ices of such molecules as water (H2O), carbon dioxide (CO2), ammonia (NH3),
and methane (CH4), with dust mixed in.
2) The nucleus is so small that we cannot observe it directly from Earth. Radar observation have verified in
several cases that it is a few kilometers across.
3) The rest of the head is the coma, which may grow to be as large as 100,000 km across. The coma shines
partly because its gas and dust are reflecting sunlight toward us and partly because gases liberated from
the nucleus get enough energy from sunlight to radiate.
4) The tail can extend 1 A.U. (150,000,000 km or 93,000,000 miles), so comets can be the largest objects in
the Solar System.
a) But, the amount of matter in the tail is very small – the tail is a vacuum, better than any that can
be duplicated on Earth in a laboratory
5) Many comets actually have two tails
a) The dust tail is caused by dust particles released from the ices of the nucleus when they are
vaporized. The dust particles left behind in the comet’s orbit, blown slightly away from the Sun
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COMETS
1) Nearly every decade, bright comet appears in our sky. From the small bright area called the head, a tail
may extend over one-sixth (30o) or more of the sky.
2) The tail of a comet is always directed roughly away from the Sun, even when the comet is moving
outward through the Solar System
3) Although the tail may give the impression of motion because it extends out only to one side, the comet
does not move noticeably with respect to the stars as we watch. With binoculars or a telescope an
observer can accurately note the position of the comet’s head and after a few hours can detect the
comet moving at a slightly different rate from the stars.
a) Both comets and stars rise and set more or less together
4) Most comets are much fainter than the one just described. About a dozen new comets are discovered
each year, and most become only known to astronomers.
5) If you should ever discover a comet, and you are among the first three people to report it to the
International Astronomical Union Central Bureau for Astronomical Telegrams, at the Smithsonian
Astrophysical Observatory in Cambridge, Massachusetts, it will be named after you.
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by pressure of sunlight hitting the particles. As a result of the comet’s orbital motion, the dust
tail usually curves smoothly behind the comet.
b) The gas tail is composed of gas blown out with high speed, more or less straight behind the
comet by the “solar wind” of particles expanding outward from the Sun. As puffs of gas are
blown out and as the solar wind varies, the gas tail takes on a structured appearance. Each puff
of matter can be seen.
c) As puffs of gas are blown out and as the solar wind varies, the gas tail takes on a structured
appearance, each puff of matter can be seen.
6) A comet – head and tail together – contains less than a billionth of the mass of the Earth.
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Halley’s Comet
1) In 1705, the English astronomer Edmond Halley applied a new method developed by his friend Isaac
Newton to determine the orbits of comets from observations of the positions in the sky.
2) He reported that the orbits of the bright comets that had appeared in 1531, 1604, and 1682 were about
the same.
a) The intervals between appearances were approximately equal, so Halley suggested that we
were observing a single comet orbiting the Sun.
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The Origin and Evolution of Comets
1) It is now generally accepted that trillions of tail-less comets surround the Solar System in a sphere
perhaps 50,000 A.U. (almost 1 light year) in radius.
2) This sphere, far outside Pluto’s orbit, is the Oort comet cloud (named after the Dutch scientist Jan Oort).
3) The total mass of matter in the cloud may be only 1 to 10 times the mass of the Earth.
4) Occasionally one of these comets leaves the comet cloud, perhaps because the gravity of a nearby star
has tugged it out of place.
a) Generally the comet gets directly ejected from the Solar System, but occasionally, the comet can
approach the Sun.
b) The comet’s orbit may be altered, sometimes into an elliptical orbit, if it passes near a giant
planet, most frequently Jupiter.
c) Because the comet cloud is spherical, comets are not limited to the plane of the ecliptic, which
explains why the longer-period comets come in randomly in all directions.
5) The short-period comets, those with periods of 200 years or less, are in orbits much more limited to the
plane of the Solar System.
a) These comets seem to come from the Kuiper belt, a flatter distribution of objects beyond the
orbit of Neptune.
b) Comets on highly eccentric orbits spend most of their time far away from the Sun, an excellent
example of Keppler’s second law.
6) As a comet gets closer to the Sun than those distant regions, the solar radiation begins to vaporize the
ice in the nucleus.
a) The tail forms, and grows longer as more of the nucleus is vaporized. Even though the tail can be
millions of kilometers long, it is still so tenuous that only 1/500th the mass of the nucleus may be
lost each time it comes closer into the Solar System
b) Therefore a comet may last for many passages around the Sun before dwindling into nothing.
Some comets may hit the Sun and be destroyed.
7) Because new comets come from the places in the Solar System that are farthest from the Sun and thus
the coldest, the probably contain matter that is unchanged since the formation of the Solar System.
a) The study of comets is important for understanding the birth of the Solar System.
b) Some astronomers think that early in Earth’s history, the oceans formed when an onslaught of
water-bearing comets collided with Earth.
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b) He predicted that it would again return in 1758. The reappearance of this bright comet on
Christmas night f that year, 16 years after Halley’s death, was the proof of Halley’s hypothesis
(and Newton’s method)
c) The comet has thereafter been known as Halley’s Comet
d) Since it was the first periodic comet known, it is officially called 1 P, number 1 in the list of the
periodic (P) comets.
3) It seems probable that the bright comets reported every 74 to 79 years since 240 B.C. were earlier
appearances of Halley’s Comet.
a) The fact that it has been observed dozens of times endorses the calculations that show that less
than 1 per cent of a commentary nucleus’s mass is lost at each passage near the Sun
4) Halley’s Comet came especially close to the Earth during its 1910 return, and the Earth actually passed
through it tail.
a) The most recent close approach of Halley’s Comet was in 1986. This time the Earth and Comet
were on opposite sides of the Sun when the comet was brightest.
b) When Halley’s Comet passed through the plane of the Earth’s orbit, it was met by an armada of
spacecraft.
c) The best was the European Space Agency’s spacecraft Giotto which went right up close to
Halley. Giotto’s instruments also studied Halley’s gas, dust, and magnetic field from as close as
600 km from the nucleus.
d) Giotto took a photograph showing the nucleus of Halley’s Comet.
e) Link to Giotto web-site - http://sci.esa.int/science-e/www/area/index.cfm?fareaid=15
5) The nucleus turns out to be potato-shaped.
a) It is about 16 km in its longest dimension, half the size of Manhattan Island.
b) The “dirty-snowball” theory of comets was confirmed in general, but the snowball is darker than
expected – black as velvet with an albedo of only about 3 per cent.
c) Further the evaporating gas and dust is localized into jets that are stronger than expected – they
come out of fissures in the dark crust.
d) It is now realized that comets may shut off not when they have given off all their material, but
when the fissures in their crusts close.
6) About 30 per cent of Halley’s dust particles are made only of hydrogen, carbon, nitrogen, and oxygen.
a) This simple composition resembles that of the oldest type of meteorite. It thus indicates that
these particles may be from the earliest years of the Solar System
b) Radio telescopes were used to determine water vapor as the most prevalent gas, but carbon
monoxide and carbon dioxide were also detected.
7) The next appearance of Halley’s Comet, in 2061, again won’t be spectacular.
a) Not until after that one, in 2134, will the comet show a long tail to Earth-bound observers.
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Comet Shoemaker-Shoemaker-Levy 9
1) In 1993 Eugene Shoemaker, Carolyn Shoemaker, and David Levy had discovered their ninth comet in a
search with a wide-field telescope at the Palomar Observatory.
a) High resolution images taken with telescopes (including Hubble) showed the comet had broken
into bits, forming a chain that resembled beads on a chain.
b) The comet was not orbiting around the Sun, but Jupiter and hit Jupiter a year later.
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Comet Hale-Bopp
1) In 1996 Alan Hale and Thomas Bopp independently found a faint comet, which was soon discovered far
out in the Solar System.
a) Its orbit brought it into the inner Solar System
b) Modern powerful radio telescopes were able to detect many kinds of molecules that had not
before been recorded in a comet. The Hubble Space Telescope took high-resolution images of its
head.
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c) Apparently several decades earlier the comet was captured into a highly eccentric orbit around
Jupiter and in 1992, during its previous close approach, it was torn apart into more than 20
pieces by Jupiter’s tidal forces.
2) Telescopes worldwide and in space were trained on Jupiter when the pieces of Shoemaker-Levy 9 hit the
planet. The first piece actually hit the backside of Jupiter, but the impact site rotated so it could be seen
from Earth every 15 minutes.
a) When Jupiter’s surface could be seen, dark rings were observed.
b) Infrared telescopes detected a tremendous amount of radiation from the heated gas
c) Over the period of a week, one bit of the comet after another hit Jupiter leaving a series of
Earth-sized rings and spots as Jupiter rotated.
d) The dark spots remained visible for a few months and could be seen with backyard telescopes
3) The dark material showed the hydrocarbon and other constituents of the comet
a) Spectra showed sulfur and other elements, presumably dredged up form lower levels of
Jupiter’s atmosphere than we normally see
4) The biggest comet chunk released the equivalent of 6 million megatons of TNT – 100,000 times more
than the largest hydrogen bomb.
a) Had any of the fragments hit Earth, they would have made a crater as larger as Rhode Island
with dust thrown up to much greater distances
b) Had the entire comet (the nucleus was 10 km across) hit Earth at one time, much of life could
have been destroyed.
5) The importance of Shoemaker-Levy 9 is that it raised awareness about Near Earth Objects – objects that
may hit the Earth from space.
a) Hollywood fed off this awareness and released two major movies – Armageddon and Deep
Impact
Spacecraft to Comets
1) In 2004 NASA’s Stardust mission flew through the Comet Wild-2. Its mission was to photograph the
comet and fly through the tail and collect some of its dust in a lightweight material called aerogel.
a) Stardust’s orbit brought it near to Earth in 2006 returned the aerogel to the Utah desert
b) Link to Stardust web-site - http://stardust.jpl.nasa.gov/home/index.html
2) Rosetta is a European Space Agency project which will intercept a comet in 2011 and orbit the Sun with
it for a couple of years, also landing a probe on the comet’s nucleus.
a) Link to Rosetta web-site - http://www.esa.int/esaMI/Rosetta/
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Types and Sizes of Meteorites
1) Tiny meteorites less than a millimeter across, micrometeorites, are the major cause of erosion on the
Moon
2) Micrometeorites also hit the Earth’s upper atmosphere all the time and remnants can be collected for
analysis from balloons or airplanes or from deep-sea sediments
3) They are often slowed down by Earth’s atmosphere to avoid being vaporized before they reach the
ground
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Meteoroids
1) There are small chunks of matter orbiting the Solar System, ranging up to tens of meters across.
a) When these chunks are in space they are called meteoroids.
b) When one hits the Earth’s atmosphere, friction heats it up – usually at a height of about 100 km
– until all or most of it is vaporized.
c) Such events result in streaks of light in the sky which we call meteors (more popularly and
incorrectly called shooting stars)
d) When a fragment of a meteoroid survives its passage through Earth’s atmosphere, the remnant
that we find on Earth is called a meteorite
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4) Space is full of meteoroids of all sizes, with the smallest being the most abundant.
a) Most of the small particles, less than 1 mm across, may come from comets
b) The large particles, more than 1 cm across, may generally come from collisions of asteroids in
the belt around the Sun where most asteroids are found
5) Most of the meteorites are found (as opposed to those that exist) have a very high iron content (about
90 percent) and the rest is nickel
a) Iron meteorites are very dense
6) Most meteorites that hit Earth are stony in nature.
a) Because they resemble ordinary rocks and disintegrate with weathering, they are not easily
discovered unless their fall is observed.
b) That difference explains why most meteorites discovered at random are made of iron.
c) When their fall is observed, most meteorites recovered are made of stone
d) Stone meteorites are rich in carbon, and some of these even have complex molecules like amino
acids
e) A large terrestrial crater that is of meteoric in origin is the Barringer Meteor Crater in Arizona
which resulted from perhaps the most recent meteorite to hit the Earth. The crater was formed
about 50,000 years ago
f) 7)Newly discovered meteorites are rushed to labs in order to find out how long they have been
in space by studying their radioactive elements
a) Many meteorites have recently been found in the Antarctic were they have been well preserved as
they accumulated over the years
b) Some odd Antarctic meteorites are now known to have come from the Moon or even from Mars
c) One of the Antarctic meteorites is the basis for speculation for life on Mars
7) Meteorites that have been examined were formed up to 4.6 million years ago, the beginning of the Solar
System
a) The relative abundances of the elements in meteorites tell us about the solar nebula from which
the Solar System formed
b) Up to the time of the Moon landings, meteorites were the only extraterrestrial source of
material
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Asteroids
1) The eight known planets were not the only bodies to result from the gas and dust cloud that collapsed
to form the Solar System 4.6 billion years ago.
2) Thousands of minor planets, called asteroids, also resulted.
a) They are detected by measuring their small motions in the sky relative to the stars
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Meteor Showers
1) Meteors sometimes occur in showers, when meteors are seen at a rate far above average.
2) The most widely observed – the Perseids – takes place each summer on about August 12th and the
nights on either side of that date.
3) The best winter show is the Geminids which takes place around December 14th.
4) On any clean night a naked-eye observer with a dark sky may see a few sporadic meteors an hour – that
is meteors that are not part of a shower
5) During a meteor shower, one may typically be seen every few minutes
6) Meteor showers result from the Earth’s passing through the orbits of defunct of disintegrating comets
and hitting the meteoroids left behind
7) Though the Perseids and Geminids can be counted on each year, the Leonid meteor shower peaks every
33 years when the Earth crosses the main clump of debris from the Comet Tempel-Tuttle
8) How visible meteors in a shower are depends in large part on how bright the Moon is.
9) Meteors are best seen with the naked eye; using a binocular or telescopes restricts your field of view
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Near-Earth Objects
1) Some asteroids are far from the asteroid belt, their orbits approach or cross that of Earth.
2) We have observed only a small fraction of these types of Near-Earth Objects, bodies that come within
1.3 A.U. of Earth
3) The Near-Earth Asteroid Rendezvous (NEAR Shoemaker) mission passed 253 Mathilde in 1997.
a) NEAR Shoemaker went into orbit around 433 Eros on Valentine’s Day 2000 – Eros was the first
near-Earth object to be discovered.
b) Craters, grooves, layers, and house sized boulders and a 20 km long surface have been
photographed.
c) The existence of the craters and ridge, which indicates that Eros must be a solid body, disagrees
with the previous suggestions that all asteroids are mere rubble piles as Mathilde appears to be
d) Eros’s density, 2.4 g/cm3, is comparable to that of the Earth’s crust, about the same as Ida’s and
Mathilde’s.
e) NEAR Shoemaker’s infrared spectrometer is measuring how the minerals vary from place to
place on Eros’s surface
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3) Most of the asteroids have elliptical orbits between the orbits of Mars and Jupiter, in a zone called the
asteroid belt
a) Most likely, Jupiter’s gravitational tugs perturbed the orbits of asteroids, leading to collisions
among them that were too violent to form a planet.
4) Asteroids are assigned a number in order of discovery and then a name
a) examples: 1 Ceres, 16 Psyche and 433 Eros
b) Often the number is omitted when discussing well known asteroids
5) The concept of the asteroid belt may seem to imply a lot of asteroids close together; asteroids rarely
come within a million kilometers of each other.
a) Occasionally collisions do occur, producing the small chips that make meteoroids
6) Only about 6 asteroids are larger than 300 km in diameter
a) Hundreds are 100 km across, roughly the size of some the moons of the planets
b) Most are small, less than 10 km in diameter
c) All the asteroids together contain less mass than the Moon
7) Spacecraft en route to Jupiter and beyond traveled though the asteroid belt for many months and
showed that the amount of dust among the asteroids is not much greater than the amount of
interplanetary dust in the vicinity of the Earth.
a) Therefore the asteroid belt is not a hazard for space travel to the outer parts of the Solar System
8) Asteroids are made of different materials from each other, and represent the chemical compositions of
different regions of space.
a) The asteroids at the inner edge of the asteroid belt are mostly stony in nature, while the ones at
the outer edge are darker – because they contain more carbon
b) Most of the small asteroids that pass near the Earth belong to the stony group
c) Three of the largest asteroids belong to the high carbon group
d) A third group is mostly composed of iron and nickel
e) The differences may contain information about conditions in the early Solar System as it was
forming and how the conditions varied with distance from the protosun.
9) The path of the Galileo spacecraft to Jupiter sent it near the asteroid Gaspra in 1991.
a) It detected a magnetic field from Gaspra, which means that the asteroid is probably made of
metal and in magnetized.
b) Galileo passed the asteroid Ida in 1993 and discovered that the asteroid has an even smaller
satellite which was then named Dactyl.
c) Other double asteroids have since been discovered and the frequency of such pairs has been
recognized by astronomers
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4) Near-Earth asteroids may well be the source of most meteorites which could be the debris of collisions
that occurred when these asteroids visit the asteroid belt
5) Eventually most Earth-crossing asteroids will probably collide with the Earth
6) Measurements released in 2000 indicate that 1000 to 2000 of them are greater than 1 km in diameter
and none are known to be larger than 10 km across
7) Statistics show that there is a 1 per cent chance of a collision of this tremendous magnitude per
millennium
a) This rate is “pretty often” on a cosmic scale
b) Such collisions would have drastic consequences for life on Earth – even a collision with a small
Near-Earth Object would do substantial damage
8) The question of how much we should worry about Near-Earth Objects hitting us is increasingly
discussed, including a meeting sponsored by the United Nations
a) Increases in budgets for determining the depth of the problem may be the first step in
identifying the number of objects that threaten Earth
NASA JPL Near-Earth Objects Team Video: http://www.jpl.nasa.gov/video/index.cfm?id=561
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NASA Asteroid Watch Web-site: http://www.jpl.nasa.gov/asteroidwatch/