The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars.
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The relative masses of the top twelve asteroids known compared to the remaining mass of all the other asteroids in the belt.
The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Pluto's moon Charon (whose diameter is 1200 km).
Ceres, the asteroid belt's only dwarf planet, is about 950 km in diameter, whereas 4 Vesta, 2 Pallas, and 10 Hygiea have mean diameters of less than 600 km.
The remaining bodies range down to the size of a dust particle. The
asteroid material is so thinly distributed that numerous unmanned
spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can produce an asteroid family
whose members have similar orbital characteristics and compositions.
Individual asteroids within the asteroid belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
The asteroid belt formed from the primordial solar nebula as a group of planetesimals. Planetesimals are the smaller precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much orbital energy for them to accrete into a planet.
Collisions became too violent, and instead of fusing together, the
planetesimals and most of the protoplanets shattered. As a result, 99.9%
of the asteroid belt's original mass was lost in the first 100 million
years of the Solar System's history. Some fragments eventually found their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits.
Classes of small Solar System bodies in other regions are the near-Earth objects, the centaurs, the Kuiper belt objects, the scattered disc objects, the sednoids, and the Oort cloud objects.
On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding was unexpected because comets,
not asteroids, are typically considered to "sprout jets and plumes".
According to one of the scientists, "The lines are becoming more and
more blurred between comets and asteroids."
History of observation
Johannes Kepler was the person who first noticed in 1596 that there was something strange about the orbits of Mars and Jupiter.
In 1596, Johannes Kepler predicted “Between Mars and Jupiter, I place a planet” in his Mysterium Cosmographicum. While analyzing Tycho Brahe's data, Kepler thought that there was too large a gap between the orbits of Mars and Jupiter.
In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature, the astronomer Johann Daniel Titius of Wittenberg
noted an apparent pattern in the layout of the planets. If one began a
numerical sequence at 0, then included 3, 6, 12, 24, 48, etc., doubling
each time, and added four to each number and divided by 10, this
produced a remarkably close approximation to the radii of the orbits of
the known planets as measured in astronomical units provided
one allowed for a "missing planet" (equivalent to 24 in the sequence)
between the orbits of Mars (12) and Jupiter (48). In his footnote,
Titius declared "But should the Lord Architect have left that space
empty? Not at all."
When William Herschel discovered Uranus
in 1781, the planet's orbit matched the law almost perfectly, leading
astronomers to conclude that there had to be a planet between the orbits
of Mars and Jupiter.
Giuseppe Piazzi, discoverer of Ceres,
the largest object in the asteroid belt. For several decades after its
discovery, Ceres was known as a planet, after which it was reclassified
as an asteroid. In 2006, it was designated as a dwarf planet.
On January 1, 1801, Giuseppe Piazzi, chair of astronomy at the University of Palermo,
Sicily, found a tiny moving object in an orbit with exactly the radius
predicted by this pattern. He dubbed it "Ceres", after the Roman goddess of the harvest and patron of Sicily. Piazzi initially believed it to be a comet, but its lack of a coma suggested it was a planet.
Thus, the aforementioned pattern, now known as the Titius–Bode law, predicted the semi-major axes of all eight planets of the time (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn and Uranus).
Fifteen months later, Heinrich Olbers discovered a second object in the same region, Pallas.
Unlike the other known planets, Ceres and Pallas remained points of
light even under the highest telescope magnifications instead of
resolving into discs. Apart from their rapid movement, they appeared
indistinguishable from stars.
Accordingly, in 1802, William Herschel suggested they be placed into a separate category, named "asteroids", after the Greek asteroeides, meaning "star-like". Upon completing a series of observations of Ceres and Pallas, he concluded,
Neither the appellation of planets nor that of comets, can with any propriety of language be given to these two stars ... They resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself, however, the liberty of changing that name, if another, more expressive of their nature, should occur.
By 1807, further investigation revealed two new objects in the region: Juno and Vesta. The burning of Lilienthal in the Napoleonic wars, where the main body of work had been done, brought this first period of discovery to a close.
Despite Herschel's coinage, for several decades it remained common practice to refer to these objects as planets
and to prefix their names with numbers representing their date of
discovery: 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta. However, in 1845
astronomers detected a fifth object (5 Astraea)
and, shortly thereafter, new objects were found at an accelerating
rate. Counting them among the planets became increasingly cumbersome.
Eventually, they were dropped from the planet list (as first suggested
by Alexander von Humboldt in the early 1850s) and Herschel's choice of nomenclature, "asteroids", gradually came into common use.
The discovery of Neptune
in 1846 led to the discrediting of the Titius–Bode law in the eyes of
scientists because its orbit was nowhere near the predicted position. To
date, there is no scientific explanation for the law, and astronomers'
consensus regards it as a coincidence.
The expression "asteroid belt" came into use in the very early
1850s, although it is hard to pinpoint who coined the term. The first
English use seems to be in the 1850 translation (by E. C. Otté) of Alexander von Humboldt's Cosmos:
"[...] and the regular appearance, about the 13th of November and the
11th of August, of shooting stars, which probably form part of a belt of
asteroids intersecting the Earth's orbit and moving with planetary
velocity". Another early appearance occurred in Robert James Mann's A Guide to the Knowledge of the Heavens:
"The orbits of the asteroids are placed in a wide belt of space,
extending between the extremes of [...]". The American astronomer Benjamin Peirce seems to have adopted that terminology and to have been one of its promoters.
One hundred asteroids had been located by mid-1868, and in 1891 the introduction of astrophotography by Max Wolf accelerated the rate of discovery still further. A total of 1,000 asteroids had been found by 1921, 10,000 by 1981, and 100,000 by 2000. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.
Origin
The
asteroid belt showing the orbital inclinations versus distances from
the Sun, with asteroids in the core region of the asteroid belt in red
and other asteroids in blue
Formation
In 1802, shortly after discovering Pallas, Olbers suggested to Herschel that Ceres and Pallas were fragments of a much larger planet
that once occupied the Mars–Jupiter region, this planet having suffered
an internal explosion or a cometary impact many million years before.
The large amount of energy required to destroy a planet, combined with
the belt's low combined mass, which is only about 4% of the mass of the Moon,
do not support the hypothesis. Further, the significant chemical
differences between the asteroids become difficult to explain if they
come from the same planet.
As of 2018, a study was released from researchers at the University of
Florida that found the asteroid belt was created from the remnants of
several ancient planets instead of a singular planet.
A hypothesis to the asteroid belt creation is that in general, in the Solar System, a planetary formation
is thought to have occurred via a process comparable to the
long-standing nebular hypothesis: a cloud of interstellar dust and gas
collapsed under the influence of gravity to form a rotating disc of
material that then further condensed to form the Sun and planets. During the first few million years of the Solar System's history, an accretion
process of sticky collisions caused the clumping of small particles,
which gradually increased in size. Once the clumps reached sufficient
mass, they could draw in other bodies through gravitational attraction
and become planetesimals. This gravitational accretion led to the formation of the planets.
Planetesimals within the region which would become the asteroid belt were too strongly perturbed by Jupiter's gravity to form a planet. Instead, they continued to orbit the Sun as before, occasionally colliding.
In regions where the average velocity of the collisions was too high,
the shattering of planetesimals tended to dominate over accretion, preventing the formation of planet-sized bodies. Orbital resonances
occurred where the orbital period of an object in the belt formed an
integer fraction of the orbital period of Jupiter, perturbing the object
into a different orbit; the region lying between the orbits of Mars and
Jupiter contains many such orbital resonances. As Jupiter migrated inward
following its formation, these resonances would have swept across the
asteroid belt, dynamically exciting the region's population and
increasing their velocities relative to each other.
During the early history of the Solar System, the asteroids
melted to some degree, allowing elements within them to be partially or
completely differentiated by mass. Some of the progenitor bodies may
even have undergone periods of explosive volcanism and formed magma
oceans. However, because of the relatively small size of the bodies,
the period of melting was necessarily brief (compared to the much larger
planets), and had generally ended about 4.5 billion years ago, in the
first tens of millions of years of formation. In August 2007, a study of zircon
crystals in an Antarctic meteorite believed to have originated from
Vesta suggested that it, and by extension the rest of the asteroid belt,
had formed rather quickly, within 10 million years of the Solar
System's origin.
Evolution
The
asteroids are not samples of the primordial Solar System. They have
undergone considerable evolution since their formation, including
internal heating (in the first few tens of millions of years), surface
melting from impacts, space weathering from radiation, and bombardment by micrometeorites. Although some scientists refer to the asteroids as residual planetesimals, other scientists consider them distinct.
The current asteroid belt is believed to contain only a small
fraction of the mass of the primordial belt. Computer simulations
suggest that the original asteroid belt may have contained the mass
equivalent to the Earth.
Primarily because of gravitational perturbations, most of the material
was ejected from the belt within about 1 million years of formation,
leaving behind less than 0.1% of the original mass.
Since their formation, the size distribution of the asteroid belt has
remained relatively stable: there has been no significant increase or
decrease in the typical dimensions of the main-belt asteroids.
The 4:1 orbital resonance with Jupiter, at a radius 2.06 AU,
can be considered the inner boundary of the asteroid belt.
Perturbations by Jupiter send bodies straying there into unstable
orbits. Most bodies formed within the radius of this gap were swept up
by Mars (which has an aphelion at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System. The Hungaria asteroids lie closer to the Sun than the 4:1 resonance, but are protected from disruption by their high inclination.
When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a "snow line" below the freezing point of water. Planetesimals formed beyond this radius were able to accumulate ice.
In 2006 it was announced that a population of comets
had been discovered within the asteroid belt beyond the snow line,
which may have provided a source of water for Earth's oceans. According
to some models, there was insufficient outgassing of water during the Earth's formative period to form the oceans, requiring an external source such as a cometary bombardment.
Characteristics
951 Gaspra, the first asteroid imaged by a spacecraft, as viewed during Galileo's 1991 flyby; colors are exaggerated
Fragment of the Allende meteorite, a carbonaceous chondrite that fell to Earth in Mexico in 1969
Contrary to popular imagery, the asteroid belt is mostly empty. The
asteroids are spread over such a large volume that it would be
improbable to reach an asteroid without aiming carefully. Nonetheless,
hundreds of thousands of asteroids are currently known, and the total
number ranges in the millions or more, depending on the lower size
cutoff. Over 200 asteroids are known to be larger than 100 km,
and a survey in the infrared wavelengths has shown that the asteroid
belt has between 700,000 and 1.7 million asteroids with a diameter of
1 km or more. The apparent magnitudes of most of the known asteroids are between 11 and 19, with the median at about 16.
The total mass of the asteroid belt is estimated to be between 2.8×1021 and 3.2×1021 kilograms, which is just 4% of the mass of the Moon. The four largest objects, Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea, account for half of the belt's total mass, with almost one-third accounted for by Ceres alone.
Composition
The
current belt consists primarily of three categories of asteroids:
C-type or carbonaceous asteroids, S-type or silicate asteroids, and
M-type or metallic asteroids.
Carbonaceous asteroids, as their name suggests, are carbon-rich. They dominate the asteroid belt's outer regions. Together they comprise over 75% of the visible asteroids. They are redder in hue than the other asteroids and have a very low albedo. Their surface composition is similar to carbonaceous chondrite meteorites. Chemically, their spectra match the primordial composition of the early Solar System, with only the lighter elements and volatiles removed.
S-type (silicate-rich) asteroids are more common toward the inner region of the belt, within 2.5 AU of the Sun.
The spectra of their surfaces reveal the presence of silicates and some
metal, but no significant carbonaceous compounds. This indicates that
their materials have been significantly modified from their primordial
composition, probably through melting and reformation. They have a
relatively high albedo and form about 17% of the total asteroid
population.
M-type
(metal-rich) asteroids form about 10% of the total population; their
spectra resemble that of iron-nickel. Some are believed to have formed
from the metallic cores of differentiated progenitor bodies that were
disrupted through collision. However, there are also some silicate
compounds that can produce a similar appearance. For example, the large
M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. Within the asteroid belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 AU.
It is not yet clear whether all M-types are compositionally similar, or
whether it is a label for several varieties which do not fit neatly
into the main C and S classes.
One mystery of the asteroid belt is the relative rarity of V-type or basaltic asteroids.
Theories of asteroid formation predict that objects the size of Vesta
or larger should form crusts and mantles, which would be composed mainly
of basaltic rock, resulting in more than half of all asteroids being
composed either of basalt or olivine. Observations, however, suggest that 99 percent of the predicted basaltic material is missing.
Until 2001, most basaltic bodies discovered in the asteroid belt were
believed to originate from the asteroid Vesta (hence their name V-type).
However, the discovery of the asteroid 1459 Magnya
revealed a slightly different chemical composition from the other
basaltic asteroids discovered until then, suggesting a different origin. This hypothesis was reinforced by the further discovery in 2007 of two asteroids in the outer belt, 7472 Kumakiri and (10537) 1991 RY16,
with a differing basaltic composition that could not have originated
from Vesta. These latter two are the only V-type asteroids discovered in
the outer belt to date.
The temperature of the asteroid belt varies with the distance
from the Sun. For dust particles within the belt, typical temperatures
range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU
However, due to rotation, the surface temperature of an asteroid can
vary considerably as the sides are alternately exposed to solar
radiation and then to the stellar background.
Main-belt comets
Several otherwise unremarkable bodies in the outer belt show cometary
activity. Because their orbits cannot be explained through the capture
of classical comets, it is thought that many of the outer asteroids may
be icy, with the ice occasionally exposed to sublimation through small
impacts. Main-belt comets may have been a major source of the Earth's
oceans because the deuterium-hydrogen ratio is too low for classical
comets to have been the principal source.
Orbits
The asteroid belt (showing eccentricities), with the asteroid belt in red and blue ("core" region in red)
Most asteroids within the asteroid belt have orbital eccentricities
of less than 0.4, and an inclination of less than 30°. The orbital
distribution of the asteroids reaches a maximum at an eccentricity of
around 0.07 and an inclination below 4°. Thus although a typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic, some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.
Sometimes, the term main belt is used to refer only to the
more compact "core" region where the greatest concentration of bodies
is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 AU, and at orbital eccentricities less than roughly 0.33, along with orbital inclinations below about 20°. As of 2006, this "core" region contained 93% of all discovered and numbered minor planets within the Solar System. The JPL Small-Body Database lists over 670,000 known main belt asteroids.
Kirkwood gaps
Number of asteroids in the asteroid belt as a function of their semi-major axis. The dashed lines indicate the Kirkwood gaps, where orbital resonances with Jupiter destabilize orbits. The color gives a possible division into three zones:
The semi-major axis of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's orbital period. In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies' orbits from the Sun.
They were located in positions where their period of revolution about
the Sun was an integer fraction of Jupiter's orbital period. Kirkwood
proposed that the gravitational perturbations of the planet led to the
removal of asteroids from these orbits.
When the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter, a mean-motion resonance with the gas giant is created that is sufficient to perturb an asteroid to new orbital elements. Asteroids that become located in the gap orbits (either primordially because of the migration of Jupiter's orbit,
or due to prior perturbations or collisions) are gradually nudged into
different, random orbits with a larger or smaller semi-major axis.
The gaps are not seen in a simple snapshot of the locations of
the asteroids at any one time because asteroid orbits are elliptical,
and many asteroids still cross through the radii corresponding to the
gaps. The actual spatial density of asteroids in these gaps does not
differ significantly from the neighboring regions.
The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion
resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit
the Sun three times for each Jovian orbit, for instance. Weaker
resonances occur at other semi-major axis values, with fewer asteroids
found than nearby. (For example, an 8:3 resonance for asteroids with a
semi-major axis of 2.71 AU.)
The main or core population of the asteroid belt is sometimes
divided into three zones, based on the most prominent Kirkwood gaps:
- Zone I lies between the 4:1 resonance (2.06 AU) and 3:1 resonance (2.5 AU) Kirkwood gaps.
- Zone II continues from the end of Zone I out to the 5:2 resonance gap (2.82 AU).
- Zone III extends from the outer edge of Zone II to the 2:1 resonance gap (3.28 AU).
The asteroid belt may also be divided into the inner and outer belts,
with the inner belt formed by asteroids orbiting nearer to Mars than
the 3:1 Kirkwood gap (2.5 AU), and the outer belt formed by those
asteroids closer to Jupiter's orbit.
Collisions
The zodiacal light, a minor part of which is created by dust from collisions in the asteroid belt
The high population of the asteroid belt makes for a very active
environment, where collisions between asteroids occur frequently (on
astronomical time scales). Collisions between main-belt bodies with a
mean radius of 10 km are expected to occur about once every 10 million
years. A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family).
Conversely, collisions that occur at low relative speeds may also join
two asteroids. After more than 4 billion years of such processes, the
members of the asteroid belt now bear little resemblance to the original
population.
Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to a few hundred micrometres.
This fine material is produced, at least in part, from collisions
between asteroids, and by the impact of micrometeorites upon the
asteroids. Due to the Poynting–Robertson effect, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun.
The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic.
Asteroid particles that produce the visible zodiacal light average
about 40 μm in radius. The typical lifetimes of main-belt zodiacal cloud
particles are about 700,000 years. Thus, to maintain the bands of dust,
new particles must be steadily produced within the asteroid belt. It was once thought that collisions of asteroids form a major component of the zodiacal light.
However, computer simulations by Nesvorný and colleagues attributed 85
percent of the zodiacal-light dust to fragmentations of Jupiter-family
comets, rather than to comets and collisions between asteroids in the
asteroid belt. At most 10 percent of the dust is attributed to the
asteroid belt.
Meteorites
Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere. Of the 50,000 meteorites found on Earth to date, 99.8 percent are believed to have originated in the asteroid belt.
Families and groups
This plot of orbital inclination (ip) versus eccentricity (ep) for the numbered main-belt asteroids clearly shows clumpings representing asteroid families.
In 1918, the Japanese astronomer Kiyotsugu Hirayama noticed that the orbits of some of the asteroids had similar parameters, forming families or groups.
Approximately one-third of the asteroids in the asteroid belt are
members of an asteroid family. These share similar orbital elements,
such as semi-major axis, eccentricity, and orbital inclination as well
as similar spectral features, all of which indicate a common origin in
the breakup of a larger body. Graphical displays of these elements, for
members of the asteroid belt, show concentrations indicating the
presence of an asteroid family. There are about 20 to 30 associations
that are almost certainly asteroid families. Additional groupings have
been found that are less certain. Asteroid families can be confirmed
when the members display common spectral features. Smaller associations of asteroids are called groups or clusters.
Some of the most prominent families in the asteroid belt (in order of increasing semi-major axes) are the Flora, Eunoma, Koronis, Eos, and Themis families.
The Flora family, one of the largest with more than 800 known members,
may have formed from a collision less than 1 billion years ago.
The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the Gefion family) is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise, the HED meteorites may also have originated from Vesta as a result of this collision.
Three prominent bands of dust have been found within the asteroid
belt. These have similar orbital inclinations as the Eos, Koronis, and
Themis asteroid families, and so are possibly associated with those
groupings.
The main belt evolution after the Late Heavy Bombardment was very
likely affected by the passages of large Centaurs and trans-Neptunian
objects (TNOs).
Centaurs and TNOs that reach the inner Solar System can modify the
orbits of main belt asteroids, though only if their mass is of the order
of 10−9 M☉
for single encounters or, one order less in case of multiple close
encounters.
However Centaurs and TNOs are unlikely to have significantly dispersed
young asteroid families in the main belt, but they can have perturbed
some old asteroid families. Current main belt asteroids that originated
as Centaurs or trans-Neptunian objects may lie in the outer belt with
short lifetime of less than 4 million years, most likely between 2.8 and
3.2 AU at larger eccentricities than typical of main belt asteroid.
Periphery
Skirting the inner edge of the belt (ranging between 1.78 and 2.0 AU, with a mean semi-major axis of 1.9 AU) is the Hungaria family of minor planets. They are named after the main member, 434 Hungaria;
the group contains at least 52 named asteroids. The Hungaria group is
separated from the main body by the 4:1 Kirkwood gap and their orbits
have a high inclination. Some members belong to the Mars-crossing
category of asteroids, and gravitational perturbations by Mars are
likely a factor in reducing the total population of this group.
Another high-inclination group in the inner part of the asteroid belt is the Phocaea family. These are composed primarily of S-type asteroids, whereas the neighboring Hungaria family includes some E-types. The Phocaea family orbit between 2.25 and 2.5 AU from the Sun.
Skirting the outer edge of the asteroid belt is the Cybele group, orbiting between 3.3 and 3.5 AU. These have a 7:4 orbital resonance with Jupiter. The Hilda family
orbit between 3.5 and 4.2 AU, and have relatively circular orbits and a
stable 3:2 orbital resonance with Jupiter. There are few asteroids
beyond 4.2 AU, until Jupiter's orbit. Here the two families of Trojan asteroids
can be found, which, at least for objects larger than 1 km, are
approximately as numerous as the asteroids of the asteroid belt.
New families
Some asteroid families have formed recently, in astronomical terms. The Karin Cluster apparently formed about 5.7 million years ago from a collision with a progenitor asteroid 33 km in radius. The Veritas family formed about 8.3 million years ago; evidence includes interplanetary dust recovered from ocean sediment.
More recently, the Datura cluster
appears to have formed about 530,000 years ago from a collision with a
main-belt asteroid. The age estimate is based on the probability of the
members having their current orbits, rather than from any physical
evidence. However, this cluster may have been a source for some zodiacal
dust material. Other recent cluster formations, such as the Iannini cluster (c. 1–5 million years ago), may have provided additional sources of this asteroid dust.
Exploration
The first spacecraft to traverse the asteroid belt was Pioneer 10,
which entered the region on 16 July 1972. At the time there was some
concern that the debris in the belt would pose a hazard to the
spacecraft, but it has since been safely traversed by 12 spacecraft
without incident. Pioneer 11, Voyagers 1 and 2 and Ulysses passed through the belt without imaging any asteroids. Galileo imaged 951 Gaspra in 1991 and 243 Ida in 1993, NEAR imaged 253 Mathilde in 1997 and landed on 433 Eros in February of 2001, Cassini imaged 2685 Masursky in 2000, Stardust imaged 5535 Annefrank in 2002, New Horizons imaged 132524 APL in 2006, Rosetta imaged 2867 Šteins in September 2008 and 21 Lutetia in July 2010, and Dawn orbited Vesta between July 2011 and September 2012 and has orbited Ceres since March 2015. On its way to Jupiter, Juno traversed the asteroid belt without collecting science data.[92]
Due to the low density of materials within the belt, the odds of a
probe running into an asteroid are now estimated at less than 1 in
1 billion.
