433 Eros

• 1. Introduction

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  433 Eros is a peanut-shaped rock tumbling through space about 70 times farther from us than the Moon, and it has rewritten several chapters of astronomical history. On the 13th of August 1898, two astronomers in two different cities spotted it on the same night, entirely by accident. What they had found was not just another rock in the belt between Mars and Jupiter. It was the first asteroid ever confirmed to cross inside the orbit of Mars, and the first given a male name from mythology. That naming decision sounds trivial today, but it broke a long-standing convention and signaled that something genuinely new had arrived in the catalog of the solar system. How did a single crater blast a billion years ago shape almost half of Eros's surface? What did a spacecraft find when it finally landed there? And why do planetary scientists quietly track a roughly fifty percent chance that Eros could one day cross Earth's path?

• 2. A Chance Discovery

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  Carl Gustav Witt was not looking for Eros when he found it. On the night of the 13th of August 1898, Witt was working at the Berlin Urania Observatory, taking a two-hour photographic exposure of the star beta Aquarii to pin down the position of asteroid 185 Eunike. Eros turned up as an unintended visitor on that plate. The same night, Auguste Charlois at the Nice Observatory independently caught it as well. Both observations together confirmed the discovery.

  The object was temporarily labeled D.Q. before it received a proper name. When astronomers studied its orbit, they realized Eros moved in a path that dipped inside Mars's orbit, making it fundamentally different from the thousands of main-belt asteroids already cataloged. That distinction earned it the first near-Earth asteroid designation ever recorded.

  Because the asteroid crossed into territory that other known minor planets did not, the International Astronomical Union allowed a departure from tradition. Every minor planet up to that point had carried a female name. Eros, named for the Greek god of love and the son of Aphrodite, became the first minor planet in history assigned a male name. The break in naming convention was made precisely because Eros was the first near-Earth asteroid discovered.

• 3. Measuring the Solar System

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  Before spacecraft could fly between planets, astronomers had a much harder problem: how far away is the Sun? One approach was to watch a nearby asteroid closely from multiple points on Earth and use the geometry of that viewing angle, called parallax, to compute the Earth-Sun distance. Eros, swinging closer to Earth than any known asteroid at the time, was the ideal target.

  During the opposition of 1900-1901, a worldwide observing program was organized specifically around Eros to nail down the solar parallax. Charles D. Perrine at the Lick Observatory, University of California, took 965 photographs using the Crossley Reflector and selected 525 of them for measurement. He published progress reports in 1906 and 1908, and the final results appeared in 1910, co-authored by Arthur Hinks of Cambridge.

  When Eros came even closer during the 1930-1931 opposition, Harold Spencer Jones ran a similar program to refine the measurement. The value of the Astronomical Unit that came out of that effort was treated as definitive for nearly four decades. It held until 1968, when radar and dynamical parallax methods began producing sharper numbers. Eros had essentially served as the solar system's measuring stick for most of the twentieth century's first half.

• 4. The NEAR Shoemaker Mission

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  NASA's NEAR Shoemaker spacecraft made a brief flyby of Eros in 1998, collecting early data on its surface. Then in 2000 it returned, eased into orbit, and spent roughly a year photographing the asteroid in systematic detail. On the 12th of February 2001, controllers guided it down using its maneuvering jets, and NEAR Shoemaker became the first spacecraft to land softly on an asteroid.

  Eros had already been detected years earlier by the Arecibo Observatory's radar system, making it the first asteroid that instrument ever identified. The NEAR mission added visual and chemical data at close range. Measurements gathered in December 1998 during the flyby suggested that Eros could hold around 20 billion tonnes of aluminum, along with comparable quantities of metals rare on Earth, including gold and platinum.

  Eros was also the first asteroid the Arecibo system ever detected, so by the time NEAR Shoemaker touched down, a single small asteroid had accumulated three historical firsts with spacecraft and radar alone. The crater that scientists believe caused the largest single reshaping event on Eros's surface was proposed to be named Shoemaker in honor of the mission, but the International Astronomical Union did not formally adopt that name. The area was instead designated Charlois Regio, after Auguste Charlois, one of the two astronomers who discovered Eros on the same night in 1898.

• 5. A Scarred and Sculpted Surface

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  An impact roughly 1 billion years ago appears to have reorganized a substantial portion of Eros's surface in a single violent moment. NEAR scientists concluded that most of the larger boulders scattered across Eros were ejected from one crater during that event. The crater's blast also left its mark on approximately 40 percent of the Erotian surface, which is now almost free of craters smaller than 0.5 kilometers across.

  Earlier explanations suggested the missing small craters had simply been filled in by fallback debris. Later analysis of crater density patterns told a more complicated story. The low-density areas all sit within 9 kilometers of the impact point, measured in a straight line through the body of the asteroid. Because Eros is not a sphere but an elongated shape, parts of the surface that seem to be on the opposite side can still be within 9 kilometers along a straight line through the interior, even when the surface path between them is much longer. Seismic shockwaves from the collision are thought to have traveled through the asteroid and shaken smaller craters into rubble. The same impact compression is believed to have formed the thrust fault called Hinks Dorsum.

  The surface also holds 334 identified dust ponds, each about 10 meters in diameter, with 255 of them measuring larger than 30 meters. These smooth deposits collect in crater floors and other depressions, sitting at slopes of less than 10 degrees. They appear slightly bluer in color than the rocky terrain around them. Ninety-one percent of the identified ponds cluster within 30 degrees of the asteroid's equator.

• 6. Orbit, Danger, and Deep Time

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  Eros orbits with its path inclined about 10.8 degrees to the plane of the solar system. That tilt means Eros rides above the ecliptic when it crosses Mars's orbit, so the two paths never actually intersect, even though Eros is classified as a Mars-crosser. An object in that kind of orbit cannot stay there indefinitely. Gravitational nudges from the planets will eventually shift it, and dynamical modeling suggests the window for Eros's current orbit is only a few hundred million years.

  Within as little as two million years, the models suggest Eros may drift into an Earth-crossing orbit. Over a longer timescale, it carries roughly a 50 percent chance of making that transition. If it ever did strike Earth, the consequences would be severe: Eros is about five times larger than the object that struck what is now the Chicxulub crater site in Mexico and drove the extinction of the non-avian dinosaurs.

  Daytime temperatures on Eros reach about 100 degrees Celsius at perihelion, the closest point in its orbit to the Sun. Nights drop to near negative 150 degrees Celsius. The asteroid's density is 2.67 grams per cubic centimeter, essentially the same as Earth's continental crust. Its surface gravity shifts considerably from place to place because the body is elongated and peanut-shaped, so the distance from any given surface point to the center of mass varies widely.

• 7. Visibility and Cultural Life

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  From Earth, Eros is usually faint, but it has moments of unusual brightness. On the 31st of January 2012, it passed at 0.17867 AU from Earth, roughly 70 times the lunar distance, reaching a visual magnitude of +8.1. During the rare oppositions that occur every 81 years, in years such as 1975 and 2056, it can brighten to magnitude +7.0. At that level it outshines Neptune and every main-belt asteroid except 1 Ceres, 4 Vesta, and occasionally 2 Pallas and 7 Iris. In January and February 2137, Eros is predicted to move retrograde by only 34 minutes in right ascension, an unusually small amount.

  Writers and storytellers have been drawn to Eros since at least 1931, when the comic strip Buck Rogers ran a sequence titled "On the Planetoid Eros." Orson Scott Card used it in Ender's Game as a captured alien base repurposed as humanity's Command School. In The Expanse, both the novel and the television series, Eros is home to a civilian population living in carved-out tunnels, and the so-called Eros Incident ends with the asteroid veering from its orbit and striking Venus. The novel Captive Universe by Harry Harrison is set entirely on Eros. Grant Morrison's Justice League relaunch used the asteroid as a prison for a villain called the General.

  In the Space Angel episode "Visitors from Outer Space," the crew of Scott McCloud destroys Eros by steering it into the Sun to prevent it from becoming a navigation hazard, which is, given the asteroid's real orbital profile, perhaps the most prescient fictional treatment of all.