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Types of NEOs
Near-Earth Objects (NEOs) are divided into several different groups depending on the shape of their orbits. Amor asteroids orbit further from the Sun than Earth, and while they come close to our planet they do not cross its orbit. Atiras or Apopheles, in contrast, have orbits closer to the Sun than Earth. The Aten and Apollo groups, however, are ‘Earth-crossers’. Aten asteroids spend most of their time at less than 1 AU from the Sun, but some of it beyond Earth’s orbit, while the reverse is true for Apollos. Not all of these asteroids present a risk to Earth, of course – some (like Cruithne, see page 187) follow orbits that keep them away from our planet, while others have inclined orbits that do not intersect with Earth’s. 1862 Apollo itself, after which this category is named, is a roughly 1.5-km (0.9-mile) asteroid first spotted in 1932, but subsequently lost for more than 40 years. It follows a highly elliptical trajectory and crosses the orbits of Venus and Mars as well as that of Earth – the potential gravitational influence of these three bodies makes its path hard to predict beyond a few hundred years in the future.
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1221 Amor
1862 Apollo
Orbit of Mars
2062 Aten
Sun
Orbit of Earth
Orbits of prototype NEO asteroids
1 Ceres
Ceres was the first asteroid to be discovered (in 1801, by Italian astronomer Giuseppe Piazzi). Although now technically classified as a dwarf planet on account of its size (a near-perfect sphere some 945 km or 587 miles across), it still bears a ‘minor planet number’ – 1 – assigned according to a scheme invented in the mid-19th century. Ceres follows an elliptical orbit between 2.6 and 3.0 AU from the Sun, putting it beyond the ‘frost line’ of the solar system where ice is able to persist on the surface. In 2014, the infrared Herschel Space Observatory also discovered a thin atmosphere of water vapour that must be constantly replenished by sublimation from surface ice reservoirs. NASA’s Dawn mission entered orbit around this small world in 2015, and sent back the first detailed images of its surface. Although heavily cratered, the landscape has surprisingly low relief – further evidence that the crust is rich in water ice with a tendency to ‘relax’ over time. Dawn also discovered distinctive bright spots inside some craters that appear to be salt deposits. One theory is that these mark locations where briny water from a subterranean ocean seeps to the surface.
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4 Vesta
Despite being only the third-largest object in the asteroid belt (with a diameter of 560 km/348 miles), asteroid 4 Vesta
is the only one bright enough to be seen from Earth with the naked eye. With an orbit averaging 2.4 AU from the Sun, Vesta is very different from the more distant icy Ceres. It might be large enough for gravity to pull it into a sphere (meriting the designation dwarf planet) were it not for a huge impact crater, called Rheasilvia, that has gouged a huge depression at the south pole and given the asteroid a mushroom-like shape.
NASA’s Dawn mission spent a year orbiting Vesta in 2011/12, and confirmed earlier suspicions that it has a complex geological history. A covering of bright volcanic rock shows that Vesta was once hot enough to develop a layered, planet-like structure of crust, mantle and iron-rich core. Many similar asteroids may once have existed, before being ejected from the solar system or shattered in collisions. Their fragments are linked to distinct families of iron and ‘stony’ meteorites found on Earth.
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951 Gaspra
In 1991, the Jupiter-bound Galileo probe provided the first-ever close-up views of an asteroid during its brief flyby of 951 Gaspra.
This misshapen lump of rock, some 20 km (12.4 miles) long, provided a number of surprises that make sense in the light of more recent thinking about the history of the asteroid belt. Where most astronomers had expected to see an ancient, heavily cratered surface, they found instead a surprisingly smooth one with several flat planes and only a light peppering of impacts. These suggest that Gaspra’s current surface has only been exposed for between 20 and 300 million years. In fact, it’s now thought that Gaspra is a member of a large asteroid grouping, known as the Flora family. Distinguished by similar orbits and silicate-rich mineralogy, all these asteroids are thought to have originated from the shattering of a relatively large object about 200 million years ago. The largest surviving fragment is the 140-km (87-mile) asteroid 8 Flora. About one third of all main-belt asteroids are now understood to form distinct families, and such occasional collisions are thought to have played a key role in the belt’s evolution.
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243 Ida and Dactyl
Following its brief encounter with 951 Gaspra (see page 172), NASA’s Galileo probe moved on to fly past a significantly larger asteroid, 243 Ida. This irregularly shaped lump of rock, some 54 km
(33.6 miles) long, spins on its axis every 4.6 hours, allowing Galileo’s cameras to capture images covering most of its surface.
Ida’s surface shows heavier cratering (and larger individual craters) than Gaspra, indicating that its surface is significantly older. It’s now thought to be a member of the Koronis family,
an asteroid group that originated in the breakup of a parent body about one billion years ago. Like Gaspra, Ida is classed as an S-type asteroid (a classification based on its colour and brightness as observed from Earth). Galileo’s instruments confirmed for the first time that such asteroids are the primary source of stony ‘ordinary chondrite’ meteorites found on Earth. Flyby images also revealed that Ida is orbited by a small moon,
1.4-km (0.9-mile) Dactyl, which probably originated as a fragment chipped off during an impact on the larger asteroid.
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253 Mathilde
Asteroid 253 Mathilde is a member of the inner asteroid belt, far from spherical but with an average diameter of 53 km (33 miles). It follows an orbit that takes it between 1.95 and 3.35 AU from the Sun and, in 1997, was targeted by the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) space probe on its way to Eros (see page 190). Mathilde proved to be a rough-hewn lump of rock, with several sharp-edged facets and a very dark surface (hence its primary features are named after Earth’s major coal fields). It rotates only very slowly – once every 418 hours, while most other asteroids rotate in just a few hours unless their spin has been slowed by the influence of a satellite. Mathilde’s dark rocks are similar to carbonaceous chondrite meteorites (see page 114), but it has much weaker gravity than might be expected for its size, suggesting that its interior must contain large voids. Mathilde’s low gravity may explain its appearance – the major impacts it has endured would have thrown off debris with enough energy to escape the asteroid entirely, rather than settling in an ejecta blanket and smoothing the surface.
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25143 Itokawa
Discovered in 2008 by the Lincoln Near-Earth Asteroid Research (LINEAR) project, 25143 Itokawa is a small asteroid in an Apollo-type orbit around the Sun (see page 166). Although not likely to threaten Earth in the foreseeable future, it is nevertheless classed as a potentially hazardous object. Itokawa’s main claim to fame, however, is as the target of the Japanese
Space Agency’s Hayabusa space probe. Launched
in 2003, Hayabusa reached Itokawa in 2005, collecting dust grains from the surface that were returned to Earth five years later. Hayabusa’s cameras revealed an elongated asteroid with dimensions of 535 × 294 × 209 m (1755 × 965 × 686 ft) and a surface strewn with boulders and surprisingly lacking in impact craters. Itokawa’s low density suggests a cavity-filled interior, and the dust retrieved by Hayabusa proved surprisingly young, having been exposed on the surface for about eight million years. All this suggests that Itokawa is a so-called ‘rubble pile’ asteroid, formed from fragments of earlier asteroids loosely bound together by their weak gravity.
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99942 Apophis
Despite a diameter of a mere 370 m (1,214 ft), 99942 Apophis
is among the most intensively studied asteroids, thanks
in large part to its potential to threaten Earth. A few months after its discovery in June 2004, the asteroid made a relatively close approach to Earth, which allowed its orbit to be properly calculated. This confirmed that Apophis will pass much closer to Earth (around 31,200 km/19,400 miles) on 13 April 2029, and raised the possibility of the asteroid hitting a small gravitational sweet spot or ‘keyhole’ that would divert its orbit and put it on course to strike Earth in 2036.
Fortunately, further observations in 2006 and 2013 confirmed that Apophis will miss the keyhole and instead have its orbit disrupted from that of an Aten-type NEO to an Apollo-type (see page 166). Nevertheless, the 2029 close approach will still be a salutary reminder of our planet’s vulnerability to impact hazards, with the asteroid clearly visible to the naked eye from Europe, Africa and western Asia as it passes over Earth’s night side.
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Radar images of Apophis, captured during its close approach to Earth in 2012
3200 Phaethon
Discovered in 1983, 3200 Phaethon is a roughly spherical asteroid with a diameter of around 5.8 km (3.6 miles), a dark surface and hints of an intriguing past. Phaethon’s highly elliptical Apollo-type orbit (see page 166) carries it between 2.40 AU at aphelion and 0.14 AU at perihelion –
well inside the orbit of Mercury. At the time of discovery, Phaethon’s approach to the Sun was the closest of any known asteroid, and its orbit seemed more like that of a comet. The resemblance deepened when astronomers realized Phaethon’s orbit matches that of the Geminid meteors – a stream of shooting stars that cross Earth’s orbit every December.
Phaethon, therefore, is almost certainly an exhausted comet – albeit one with a significant rock component that has outlasted the loss of its ice. In 2009 and 2012, NASA’s STEREO solar observatory satellites imaged Phaethon near the perihelion of its 524-day orbit and found signs of a ‘dust tail’ still being ejected from its dark surface.
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A sequence of radar images of Phaethon, captured during its 2007 close approach to Earth
592 Scheila
Although the main-belt asteroids are relatively well separated in space, their sheer numbers make collisions inevitable – and in 2010, astronomers apparently captured images of just such an event. 592 Scheila has an average diameter of 57 km (35 miles), and follows a markedly elliptical orbit between 2.45 and 3.41 AU from the Sun. It is thought to be a ‘T-type’ asteroid – one of a group of poorly understood dark-red worlds found in the inner asteroid belt and thought to lack water in their crusts.
When 2010 survey images revealed that Scheila had apparently developed a fuzzy halo or ‘coma’, therefore, it came as something of a surprise. Such comet-like behaviour is not unknown in asteroids from the icy outer belt, but would be unexpected from a dry asteroid like Scheila. Further studies confirmed this, showing that the coma was not composed of gas particles as a comet’s would be. Instead, astronomers concluded that Scheila had been struck by a small object about 35 m (115 ft) across, flinging a substantial cloud of dust into a loose orbit around it.
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3753 Cruithne
small object designated as minor planet 3753, Cruithne is
best known for its sometime reputation as Earth’s ‘other’ moon. In fact, while this 5-km-diameter (3 mile) asteroid does not orbit directly around the Earth, it does have an intimate relationship to our planet. Cruithne’s average distance from the Sun is more or less identical to Earth’s, but its path is slightly more elliptical, so that it goes from just inside Earth’s orbit to just outside it. For some of the time, its year is a little shorter than Earth’s, so that it slowly spirals through space ahead of our planet, moving further ahead with each loop. Eventually,
the asteroid starts to ‘catch up’ with Earth from behind, until
it comes within 15 million km (9.3 million miles). At this distance, tidal forces from Earth rob Cruithne of a little momentum,
so that it starts to orbit more slowly and retreat from Earth. Finally, Earth ‘catches up’ with the asteroid, and this time tidal forces boost Cruithne’s momentum, causing it to pick up speed. This perpetual back-and-forth through space around Earth is known as a ‘horseshoe orbit’.
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1 Cruithne orbits closer to the Sun, faster than Earth.
2 Cruithne slowly spirals ahead of Earth.
3 As Cruithne approaches Earth from behind, it is pulled into a wider orbit.
4 Now Cruithne orbits further out than Earth, and more slowly.
5 Overall ‘horseshoe’ path taken by Cruithne relative to Earth
2002 AA29
In 2002, a systematic hunt for Near-Earth Asteroids discovered a 60-m-wide (200 ft), quarter-million-tonne asteroid lurking close to Earth’s orbit around the Sun. Designated 2002 AA29, this small rock follows a horseshoe orbit similar to that of Cruithne (see page 186). What’s more, computer modelling suggests that every couple of thousand years it enters a brief temporary orbit around our own planet, to become a true ‘second moon’.
Some astronomers have argued that this object’s orbit is so similar to Earth’s that it cannot be a mere captured stray from the asteroid belt. Instead, they suspect 2002 AA29 might have started its journey much closer to home, at one of the ‘Lagrangian points’ of the Earth–Moon system (gravitational sweet spots where an object can follow a stable orbit more or less indefinitely without either of the larger bodies disturbing it). If that’s the case, it raises an intriguing possibility – could 2002 AA29 be a surviving chunk of debris thrown out from the ‘Big Splash’ impact thought to have created the Moon itself?
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Changing relative location of 2002 AA29
Location of Earth
433 Eros
Discovered by Gustav Witt in 1898, 433 Eros is one of many
Near-Earth Asteroids that orbit inside the main belt. Circling the Sun once every 1.76 years, it is currently a Mars-crosser, though there is a strong chance that its orbit will eventually evolve to intercept Earth’s.
As the ultimate destination of the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) space probe, Eros is among the most intensively studied asteroids in the solar system. The mission orbited the asteroid for 12 months, appropriately arriving on Valentine’s Day 2000 and ‘kissing’ the surface with a final touchdown a year later. Eros’s most distinctive feature is a saddle-shaped hollow in its
convex side. The depression, known as Himeros, is almost certainly an ancient impact crater, and boulders up to 50 m (164 ft) across can be seen scattered around it. However, one of the biggest surprises about Eros was just how ‘soft’ its landscape appeared – presumably a result of erosion caused by endless bombardment from micrometeorites no bigger than dust grains.
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Trojan asteroids
Since the discovery of 617 Patroclus and 624 Hektor (see page 194) in the early 20th century, astronomers have learned that Jupiter shares its orbit with at least 6,500 asteroids.
These small objects avoid the disruptive influence of Jupiter’s powerful gravity by orbiting near two of its ‘Lagrangian points’. These are regions some 60 degrees behind and ahead of Jupiter, where the Sun’s influence balances that of the giant planet and a stable orbit is possible. The asteroids in the two groups, named after rival Greeks and Trojans from the mythological Trojan war, probably reached their present orbits after being dispersed from their original locations by the migration of the giant planets proposed in the Nice model (see page 42). For some unknown reason, they contain a high number of binary asteroid pairs.
Today, any object that follows a similar orbit is also known as a Trojan – Earth, Mars, Uranus and Neptune have known Trojans of their own, while Saturn’s satellites Tethys and Dione share their orbits with ‘Trojan moons’.
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Solar System in Minutes Page 7