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Titus-Bode Law – During the 18th century, astronomers were intrigued by a
numerical progression called the Titius-Bode law (also referred to as just Bode’s
law). It appeared to predict the distances of the known planets from the Sun, but with
one exception, the law suggested there should be a planet at a distance of 2.8 AU
from the Sun (between the orbits of Mars and Jupiter). When Sir William Herschel
discovered Uranus at a distance that corresponded to the Titius-Bode law, scientific
excitement about the validity of this numerical progression reached an all-time high.
Many astronomers were absolutely convinced that a planet must exist between Mars
and Jupiter.
“The Outlaw Neptune” – The Titus-Bode law was widely accepted until Neptune
was discovered in 1846 and found not to comply.
Ceres – By the end of the 1700s, a group of astronomers had banded together to use
the observatory at Lilienthal, Germany to hunt down the missing planet. They called
themselves the Celestial Police. However, despite their efforts, they were beaten. On
January 1, 1801, Giuseppe Piazzi discovered what he believed to be the missing
planet. The new object was named Ceres, after the Roman goddess of the harvest
(thus the word cereal). However, subsequent observations swiftly established that it
could not be classed a planet, since its diameter is only about 600 miles. The search
for the “real” planet continued.
“Star-Like” – Between 1801 and 1807, members of the Celestial Police tracked down
three other bodies: Pallas, Juno and Vesta, each smaller than Ceres. Because they
were too small to be seen as more than star-like points of light, these objects were
called “asteroids”, which actually means star-like. It became obvious that there was
no single large planet between Mars and Jupiter. Moreover, after eight more years of
unsuccessful searches, most astronomers assumed that there were no more asteroids
and abandoned any further searches. However, astronomer Karl Ludwig Hencke
persisted, and began searching for more asteroids in 1830. In 1845, he found Astraea.
He also found Hebe less than two years later.
Population – As of September 2013, the Minor Planet Center – the official worldwide
organization in charge of collecting observational data for minor planets (asteroids)
and comets – had data on more than one million objects in the inner and outer Solar
System, of which 625,000 had enough information to be given formal designations.
Calculations indicate that the total mass of all the asteroids is still quite small (less
than the Earth’s Moon).
Discovery Methods – The method of discovering asteroids by differentiating their
motion from that of stars has not changed since 1801. However, the technology
available to apply that original method has evolved significantly (from hand-drawn
star charts to astrophotography to automated systems).
Time-Exposure Photographs 1 – In 1891, Max Wolf pioneered the use of
astrophotography to detect asteroids. In time-exposure photographs, asteroids
appear as short streaks due to their planetary motion with respect to fixed stars.
This drastically increased the rate of detection compared with previous visual
methods. Wolf alone discovered 248 asteroids, whereas only slightly more than
300 had been discovered up to that point.
“Vermin of the Skies” – Up until the 1980s, asteroids catalogued were
discovered accidentally on telescopes’ photographic plates, showing up as
small streaks. Most astronomers viewed an asteroid streak as a nuisance,
blemishing an otherwise perfect photographic plate, especially if the asteroid
was streaking across the image of interest to the astronomer. Most streaks
were unreported.
Automated Systems – An increasing interest in identifying asteroids whose orbits
cross Earth’s orbit, and that could, given enough time, collide with Earth, helped
spur the launch of highly efficient automated systems that consist of ChargeCoupled Device (CCD) cameras and computers directly connected to telescopes.
Since 1998, a large majority of the asteroids have been discovered by such
automated systems.
Formal Designation – Once an asteroid’s orbit has been confirmed, it is given a
number, and later may also be given a name (433 Eros). However, because modern
discovery techniques are finding vast numbers of new asteroids, they are increasingly
being left unnamed.
Names – The first few asteroids were named after figures from Greco-Roman
mythology, but as these names started to run out, others were used: famous people,
literary characters, the names of the discoverer’s wives, children, and even television
characters. There was little controversy about this until 1971, when an asteroid was
named Mr. Spock, which was not even named after the Star Trek character, but after
the discoverer’s cat who supposedly bore a resemblance to him. Although the
International Astronomical Union subsequently banned pet names as sources, unusual
asteroid names are still being proposed and accepted, such as Cheshire Cat, James
Bond (which has the number designation of 9007), and Mister Rogers.
Physical Characteristics
General Description – Asteroids are predominantly rocky bodies, ranging in size from
about 600 miles (Ceres) to tens of feet across. The number of asteroids increases
rapidly with decreasing size. This size distribution, with high numbers of small sizes,
is probably produced by collisional fragmentation. The largest asteroids are probably
solid. However, most of the smaller asteroids are thought to be piles of rubble held
together loosely by gravity. Some asteroids have moons or are co-orbiting binaries
(two asteroids orbiting their common center of mass) 2. Asteroid shapes range from
spherical to elongated and irregular (indicating a fragmental origin).
Largest Asteroids – The three largest asteroids, Ceres (≈600 miles), Pallas (≈340
miles), and Vesta (≈325 miles) share many characteristics common to planets, and
are atypical compared to the majority of “potato-shaped” asteroids. NASA’s
Dawn spacecraft was launched in 2007 to study Ceres and Vesta.
Ceres 3 – Ceres is the only asteroid spherical in shape. It is differentiated into
a rocky core, icy mantle, and crust composed of water ice and hydrated
minerals such as carbonates and clays. It has been suggested that a layer of
liquid water within the interior may exist. If the mantle is about 25% of Ceres
mass, it would contain more fresh water than is found on Earth. Ceres appears
to be geologically inactive, with a surface sculpted by impacts. The surface
also displays several bright spots (possibly fresh ice) and a pyramid-shaped
mountain 4 (over 3 miles high) of unknown origin.
Pallas – Pallas is unusual in that, like Uranus, it rotates on its side. Its
composition is similar to that of Ceres, and perhaps partially differentiated.
Vesta 5 – Vesta is the only intact asteroid resurfaced by basaltic lava flows.
This contradicts the conventional idea that asteroids are essentially cold, rocky
fragments left behind from the early days of planetary formation. Vesta is
thought to have accreted from material that included the radioactive isotope
aluminum-26, originating from a nearby supernova explosion. The decay of
this short-lived isotope caused the asteroid to differentiate, followed by the
eruption of molten rock onto its surface. Occurring more than four billion
years ago, Vesta’s surface has remained relatively unchanged, except for the
occasional crater-forming impact.
 Eucrites – Vesta is the probable parent body of a specific group of
meteorites called the Eucrites. They must have originated from a major
impact of the asteroid causing some of its surface material to be ejected
out into space and eventually land on Earth as meteorites.
Life – Asteroids contain traces of organic compounds, and some scientists
speculate that asteroid (and comet) impacts may have seeded the early Earth with
the chemicals necessary to initiate life, or may have even brought life itself to
Asteroid vs Comet – The main difference between an asteroid and a comet is that
a comet shows a coma (discussed later) due to sublimation of surface ice by solar
radiation. A few objects have ended up being dual-listed because they were first
classified as asteroid but later showed evidence of cometary activity. Conversely,
some comets are eventually depleted of their surface volatile ices and become
asteroids. A further distinction is that comets typically have more eccentric orbits
than most asteroids; most asteroids with particularly eccentric orbits are probably
dormant or extinct comets
Classification – The classification of asteroids is based on composition, derived from
measurements of reflectivity (albedo), surface spectrum, density, and similarities to
known meteorite types. Most asteroids are categorized in one of three classes:
1. Carbonaceous or C-Type Asteroids – C-type asteroids are very dark, and
composed of silicates (stony materials), carbon compounds, and hydrated
minerals (chemically combined with water molecules). They are similar to
carbonaceous chondrite meteorites (discussed later). C-type asteroids comprise
75% of the asteroid population.
Mathilde 6 – The Near-Shoemaker spacecraft obtained close-up views of the
C-type asteroid Mathilde. It measures 33 miles across, and the surface
exhibits many large craters, including the deeply shadowed one at the center,
which is estimated to be more than 6 miles deep. The angular shape is
believed to be the result of a violent history of impacts.
2. Silicaceous or S-Type Asteroids – S-type asteroids are moderately bright, and
composed of silicates (stony materials) and small amounts of nickel-iron. They
are similar to a variety of stony meteorites (discussed later). S-type asteroids
comprise 17% of the asteroid population.
Ida 7 – The Galileo spacecraft encountered the S-type asteroid Ida. It is an
irregularly shaped asteroid, about 35 miles across. Its surface is heavily
cratered suggesting that it has existed in its present form for at least a billion
years, perhaps much longer. An unexpected discovery was a small satellite,
named Dactyl, measuring less than 1 mile across.
YORP Effect – Initially, it was speculated that Dactyl is large boulder
ejected from the surface of Ida in a recent impact. However, astronomers
now believe that the YORP effect may be responsible for the formation of
Dactyl and satellites of other smaller asteroids. Specifically, when
sunlight hits one of these smaller asteroids, the material absorbs some of
the radiation and then re-emits it at a slightly different angle, causing the
asteroid to spin up slightly. (The YORP effect is also thought to cause the
rotation rates of some asteroids to slow down, while others to rotate
chaotically.) Subjected to the YORP effect over millions of years, the
asteroid can increase in rotation enough that the force outward (centrifugal
force) will overpower the gravitational pull inward, causing material to
eject along the equator to form a new small satellite.
3. Metallic or M-Type Asteroids – M-type asteroids are relatively bright and
composed of nickel-iron (pure or mixed with stone). They are thought to be the
remains of the cores of differentiated bodies, produced by collisional
fragmentation. This type of asteroid is very similar to iron meteorites (discussed
later). M-type asteroids comprise most of the remaining 8% of the asteroid
Psyche – Measuring over 125 miles across, Psyche is likely the largest M-type
Other Types – In addition to the common types described above, astronomers
have identified more than a dozen other asteroid types. The number of types
continues to grow as more asteroids are studied.
Orbital Groupings
Main Belt Asteroids 8 – The great majority of the asteroids orbit the Sun in the main
asteroid belt from 2 to 4 AU (between the orbits of Mars and Jupiter). They are
believed to be leftover fragments from the formation of the solar system about 4.6
billion years ago. The main asteroid belt is estimated to contain nearly two million
asteroids larger than a half mile in diameter, and millions of smaller ones. Scientists
suspect that asteroids were prevented from accreting into larger objects because of
Jupiter’s massive gravitational pull. Specifically, as Jupiter orbited outside the
asteroid belt, its gravity would tug on the small bodies, perturbing their orbits and
preventing them from coalescing into a single body.
Zoning of Types – The compositional differences among asteroids show a
remarkably distinct distribution with distance from the Sun. While examples of
most asteroid types span a large range of distances from the Sun, there is a clear
progression from S-type in the inner portion of the main asteroid belt, to M-type
within the central portion, and C-type within the outer portion. This gradation
with solar distance likely reflects properties of the primordial solar nebula, with
high-temperature minerals condensing nearer the Sun and mixtures of ices and
organics farther out.
Asteroid Families – An asteroid family is defined as group of asteroids with
similar orbits, suggesting a common origin. Although not clustered together in
space at the present, the members of an asteroid family were all at the same place
at some undetermined time in the past. Members of the same family tend to have
similar reflectivites and spectra. Apparently, the family members are fragments
of broken asteroids shattered in some ancient collision and still following similar
orbital paths. Most asteroid families are found in the main asteroid belt. Family
names include: Hungaria, Flora, Phocaea, Koronos, Eos, Themis, Cybele, and
Main Belt Comets – In 2006, scientists reported that at least three bodies in the
outer part of the main belt demonstrate classical cometary behavior, and defined a
new class of bodies called the main belt comets. This followed the discovery in
2005 that the largest main belt asteroid, Ceres, contains a significant amount of
water-ice. In fact, many main belt asteroids may contain a lot of ice, especially in
the outer part of the belt. The line separating asteroids and comets is growing
increasingly fuzzy.
Kirkwood Gaps – Kirkwood gaps are regions in the main asteroid belt where few
asteroids are found. First observed by Daniel Kirkwood in 1886, these gaps are
cleared of asteroids by the orbital resonance effects with Jupiter.
Centaur Asteroids – Centaur asteroids orbit far from the Sun. For example: Hidalgo,
the first to be discovered, orbits from the inner edge of the main belt out almost as far
as Saturn, Chiron orbits between Saturn and Uranus 9, the orbit of Damocles ranges
from near Mars to beyond Uranus, and Pholus orbits from Saturn to past Neptune.
Their planet-crossing orbits are unstable and are frequently perturbed. Thus,
Centaurs will ultimately collide with the Sun or a planet or else they may be ejected
into interstellar space after a close approach to one of the planets (particularly
Jupiter). The composition of these objects is probably more like that of comets or
Kuiper Belt objects, rather than that of ordinary asteroids. In fact, Chiron has been
reclassified as both an asteroid and a comet. Moreover, any Centaur that is perturbed
close enough to the Sun is expected to become a comet.
Trojan (Langrangian) Asteroids 10 – Trojan asteroids are located in two
gravitationally stable regions of Jupiter’s orbit – 60º preceding and following the
planet. These two regions, referred to as Langrangian points (named for the French
mathematician Joseph Louis Comte Lagrange who demonstrated their existence in
1772), each contain several hundred known asteroids. Several small asteroids have
also been discovered in the Lagrangian points of Mars, Saturn and Neptune.
Near-Earth Asteroids (NEAs) – Asteroids with orbits that bring them within 1.3 AU
of the Sun are referred to as Near-Earth Asteroids (together with the comets whose
orbits take them through near-Earth space form the Near-Earth Objects or NEOs).
Asteroids that actually cross Earth’s orbital path are known as Earth-crossers.
Scientists believe that many NEAs are objects nudged from the main asteroid belt.
The nudging can result from collisions between objects, the gravitational tug of the
Jovian planets, or a subtle force known as the Yarkovsky effect. The Yarkovsky
effect occurs when an asteroid’s Sun-warmed surface re-radiates its heat on its
afternoon side. The photons departing the surface of the asteroid create a tiny change
in the asteroid’s orbital velocity. NEAs are grouped into three categories, named for
famous members of each:
1. Amors 11 – Amors cross Mars’ orbit, but do not quite reach the orbit of the Earth
(perihelion distances between 1.017 and 1.3 AU).
Eros 12 – A member of the Amors, Eros, is the second largest NEA. It is also
one of the most elongated asteroids, with estimated dimensions of 20 by 8 by
8 miles. Eros exhibits a heavily cratered surface with one side dominated by a
huge gouge, and the opposite side by a sharp, raised rimmed crater. On
February 12, 2001, the Near-Shoemaker spacecraft touched down on Eros.
The last image of Eros received from the spacecraft was taken from a range of
394 feet, and measures 20 feet across 13.
2. Atens 14 – Atens orbit largely inside the orbit of the Earth. They cross the Earth’s
orbit when they are near their greatest distance (aphelion) from the Sun.
3. Apollos 15 – Apollos include most Earth-crossing asteroids. They cross the
Earth’s orbit when they are near their closest distance (perihelion) from the Sun.
Toutatis 16 – Measuring nearly 3 miles in length, the Apollo asteroid named
Toutatis is a highly elongated body consisting of two distinct “lobes”. It is
hypothesized that Toutatis formed from two originally separate bodies which
coalesced at some point, with the resultant asteroid being compared to a pile
of rubble. Observations also reveal that Toutatis has an unusual spin state,
consisting of two apparent simultaneous rotations around different axes, with
periods of 5.4 and 7.3 days. So while most asteroids rotate somewhat like a
football thrown in a perfect spiral, Toutatis tumbles like a flubbed pass.
NEAs’ Fate – The orbits of NEAs are unstable as a result of the constantly
varying gravitational influence of Earth, Mars, and Venus. A NEA will
experience one of two fates: (1) it may be gravitationally ejected as the result of
near-miss with a planet or (2) it may terminate its existence dramatically in a
crater-forming impact. Calculations indicate that about one-third of the asteroids
whose orbits currently cross that of the Earth will eventually hit our planet.
NEA Populations –There are about one thousand NEAs larger than a half-mile in
diameter, 4 to 8 thousand larger than 1500 feet in diameter, and 0.5 to 1.5 million
larger than 150 feet in diameter that pose a potential threat to Earth.
NEA Impacts – NEA impacts become a significant hazard when they are larger
than 150 feet in diameter. Asteroids of this size can reach the Earth’s lower
atmosphere or its surface and release a substantial amount of energy, equal to or
greater than the energy released in the detonation of a nuclear weapon. In fact,
like a nuclear explosion, the energy of an impact is measured in megatons of
TNT. However, a significant impact on the Earth is rare. Moreover, large
impacts occur much less frequently than small ones. Nevertheless, it is a random
event. For example, in March 1989 an asteroid (designated 1989 FC)
approximately 1500 feet in diameter, missed the Earth by only a few hours.
The table on the following page summarizes the energy yield, average
frequency, and environmental consequences according to a range of impact
sizes of NEAs.
Impact Effects Of A Near-Earth Asteroid
Energy Yield
(Megatons of TNT)
Average Frequency
150 to 300 feet
10 to 100
600 to 1500 feet
1,000 to 10,000
0.5 to 3 miles
100,000 to
5 miles
100,000,000 to
Environmental Consequences
Referred to as “city-busters”. The
Tunguska blast in 1908 is believed
to have been caused by an airburst
of a 180-foot NEA 17, which killed
herds of Reindeer and leveled trees
out to 12 miles 18.
A land impact could destroy an area
the size of a small to moderate state.
An ocean impact produces tsunamis
capable of devastating all associated
A land impact could destroy an area
the size of a country, approaching
continental scale. Both land and
ocean impacts raise dust, causing
climatic changes. Destruction of
ozone layer.
Referred to as “planet-busters”.
Impact ejecta triggers global-scale
fires. Prolonged “nuclear winter”.
Threatens most species of life. The
K/T Mass Extinction 65 million
years ago was most likely caused by
an impact of a 6-mile NEA. The
probable impact site was identified
as the Chicxulub Crater, Northern
Yucatan, Mexico 19.