Asteroid

• 1. Introduction

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  An asteroid is a minor planet, an object larger than a meteoroid that is neither a planet nor an identified comet, orbiting within the inner Solar System. Some are rocky. Some are metallic. Some are icy. The smallest are rubble piles under a kilometer across. The largest is Ceres, a dwarf planet almost 1000 km in diameter. Roughly one million of these objects are known, yet combined they hold only 3% the mass of Earth's Moon.

  The first one was identified in 1801. The story since then runs through a Sicilian priest who thought he had found a comet, a 24-year-old mathematician who rescued a lost world with arithmetic, and a generation of astronomers who dismissed these objects as vermin of the skies. It runs through spacecraft that crashed into one on purpose, and through the dark suspicion that one of these bodies ended the age of the dinosaurs. How did a faint star-like point spotted on the first night of a new century become a million-strong census? What separates an asteroid from a comet when the line between them keeps blurring? And why would NASA deliberately smash a probe into one?

• 2. A Priest And A Lost World

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  On the 1st of January 1801, Giuseppe Piazzi, a Catholic priest at the Academy of Palermo in Sicily, was searching for the 87th star in a catalogue of zodiacal stars compiled by La Caille. He found instead that it was preceded by another. The object was faint, the colour of Jupiter, and looked like a fixed star of the eighth magnitude. By the evening of the third, his suspicion turned to certainty. The thing was moving.

  Piazzi first reported it as a comet, though he wrote that its motion was so slow and uniform that it might be something better than a comet. He observed it 24 times, the final time on the 11th of February 1801, when illness stopped his work. He had announced the discovery on the 24th of January in letters to just two fellow astronomers: his compatriot Barnaba Oriani of Milan, and Johann Elert Bode in Berlin. By the time his full observations were published in September, the object had drifted too close to the Sun's glare for anyone to confirm it.

  To recover the lost object, the mathematician Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. Within a few weeks he predicted its path and sent the results to Franz Xaver von Zach. On the 31st of December 1801, von Zach and his fellow celestial policeman Heinrich Olbers found the object near the predicted position. Piazzi named it Ceres Ferdinandea, in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon.

• 3. The Missing Planet

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  In 1772 Johann Elert Bode, citing Johann Daniel Titius, published a numerical progression now known as the Titius-Bode law. Except for an unexplained gap between Mars and Jupiter, the formula seemed to predict the orbits of the known planets. Bode could not believe the Founder of the universe had left that space empty. His formula predicted a planet near 2.8 astronomical units, about 420 million km from the Sun.

  William Herschel's discovery of Uranus near the predicted distance for a world beyond Saturn gave the law a boost. In 1800 a group headed by von Zach, editor of the German journal Monatliche Correspondenz, sent requests to 24 experienced astronomers he dubbed the celestial police. They were asked to combine their efforts in a methodical search for the expected planet. They did not find Ceres first, but they would later turn up 2 Pallas, 3 Juno, and 4 Vesta.

  At 2.8 AU from the Sun, Ceres seemed to fit the Titius-Bode law almost perfectly. The fit did not hold. When Neptune was discovered in 1846, it sat 8 AU closer than the formula predicted, leading most astronomers to conclude the law was a coincidence. Ceres itself was at first considered a new planet, and it was Herschel who proposed the term asteroid, coined in Greek as asteroeidēs, meaning star-like or star-shaped.

• 4. Vermin Of The Skies

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  By 1807, von Zach's group had found three more bodies, with Vesta discovered that year. Then no new asteroids turned up until 1845. Karl Ludwig Hencke, an amateur, had begun searching in 1830, and fifteen years later, while looking for Vesta, he found 5 Astraea, the first new discovery in 38 years. Gauss was given the honor of naming it. By the end of 1851, fifteen asteroids had been found.

  In 1868, when James Craig Watson discovered the 100th asteroid, the French Academy of Sciences engraved a commemorative medallion. It bore the faces of the three most successful asteroid-hunters of the day: Karl Theodor Robert Luther, John Russell Hind, and Hermann Goldschmidt. By then the bodies were arriving faster than astronomers could keep up, and many did not bother. Some called them vermin of the skies, a phrase attributed variously to Eduard Suess and Edmund Weiss.

  In 1891 Max Wolf pioneered astrophotography to detect asteroids, which appeared as short streaks on long-exposure plates. The method dramatically raised the rate of detection. Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been found up to that point. Even a century later, only a few thousand had been identified, numbered, and named. Today the Minor Planet Center holds data on 1,460,356 minor planets, of which about 826,864 have enough information for numbered designations.

• 5. How A Number Becomes A Name

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  By 1851 the Royal Astronomical Society judged that asteroids were arriving so fast that a new naming system was needed. In 1852, when de Gasparis found the twentieth asteroid, Benjamin Valz gave it a name and a number marking its rank, 20 Massalia. From 1892, new asteroids were listed by year and a capital letter. There were not enough letters for 1893, so 1893Z was followed by 1893AA. A simple chronological numbering system was finally established in 1925.

  A discovery does not begin with a name. First a region of sky was photographed by a wide-field telescope, typically in pairs an hour apart. The plates were then viewed under a stereoscope, where a body orbiting the Sun would seem to float slightly above the background stars. Its location was measured with a digitizing microscope against known star positions. Even then the observer had found only an apparition, given a provisional designation built from the year, a letter for the half-month, and a sequential code.

  The last step is sending the observations to the Minor Planet Center, where programs determine whether the apparition ties earlier ones into a single orbit. If so, the object gets a catalogue number, and the observer of the first apparition with a calculated orbit is declared the discoverer. They earn the honor of naming it, subject to approval by the International Astronomical Union. Early on, discoverers leaned on iconic symbols like those used for the planets. By 1852 there were two dozen, until Johann Franz Encke introduced a numbered disk as the generic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.

• 6. The Big Four

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  Ceres is by far the largest asteroid, with a diameter of 940 km. The next largest, 4 Vesta and 2 Pallas, both run just over 500 km across. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, make up about five-eighths of the asteroid belt's mass. Ceres and Vesta alone account for half. The belt's total mass is estimated at 2.39 times ten to the twenty-first kg, just 3% of the Moon's, and Ceres comprises about 40% of that.

  Pallas is unusual in that, like Uranus, it rotates on its side, its axis tilted at high angles to its orbital plane. Hygiea is the largest carbonaceous asteroid and lies relatively close to the plane of the ecliptic. Because no crater large enough to be the source of its family sits on its surface, Hygiea may have been completely disrupted in the collision that formed the Hygiean family, then recoalesced after losing a little under 2% of its mass. Observations with the Very Large Telescope's SPHERE imager in 2017 and 2018 revealed Hygiea has a nearly spherical shape.

• 7. Rubble, Color, And Water

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  Very few asteroids larger than 100 meters spin with a rotation period under 2.2 hours. Below that threshold, the inertial force at the surface would exceed gravity, flinging loose material away. A solid object could spin much faster. The implication is that most asteroids over 100 meters across are rubble piles, loose accumulations of debris gathered after collisions, perhaps half empty space by volume. Every asteroid found to have a moon has turned out to be one of these piles.

  Asteroids darken and redden with age through space weathering, though most of that color change happens within the first hundred thousand years. The three main spectral groups are C-type for carbon-rich bodies, M-type for metallic, and S-type for stony. The original 1975 system by Chapman, Morrison, and Zellner had just three categories, with C-types making up 75% of known asteroids and S-types 17%. It has since branched into the Tholen and SMASS taxonomies, the latter yielding 24 different types in 2002.

  Water keeps blurring the line between asteroid and comet. In 2009, water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility, alongside organic compounds. On the 22nd of January 2014, ESA scientists reported water vapor on Ceres using the Herschel Space Observatory, a finding unexpected because jets and plumes belong to comets. One scientist remarked that the lines between comets and asteroids are becoming more and more blurred. Meteorites tell a similar story: in November 2019, scientists reported sugar molecules, including ribose, found in meteorites for the first time.

• 8. Crashing Into The Dark

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  In September 2022, NASA's Double Asteroid Redirection Test deliberately crashed into Dimorphos, the minor-planet moon of the double asteroid Didymos. In October, NASA declared it a success, confirming the impact had shortened Dimorphos's orbital period by about 32 minutes. The collision made Dimorphos an active asteroid: it lost roughly 1 million kilograms and grew a dust tail 10000 km long that persisted for several months. It was the first time anyone observed an asteroid activated under precisely known conditions.

  The interest in striking these bodies grows from the fear that one might strike us. The near-Earth asteroid 433 Eros was found as long ago as 1898, and the 1930s brought a flurry of others: 1221 Amor, 1862 Apollo, 2101 Adonis, and 69230 Hermes, which approached within 0.005 AU of Earth in 1937. The alarm sharpened with the Alvarez hypothesis tying an impact to the Cretaceous-Paleogene extinction, and with the 1994 crash of Comet Shoemaker-Levy 9 into Jupiter. The Chicxulub impact is widely thought to have induced that mass extinction.

  Long before DART, Project Icarus was drawn up in 1967 as a contingency plan against 1566 Icarus. It relied on the new Saturn V rocket, which had not yet flown. Six rockets would launch at intervals from months to hours before impact, each fitted with a single 100-megaton nuclear warhead, the warheads detonated 30 meters from the surface to deflect or partly destroy the asteroid. The last-ditch sixth launch would come 18 hours before impact. The thread that began with one priest mistaking a moving light for a comet now reaches forward to ESA's Hera, launched in October 2024, sent to measure the crater DART left behind.