The first near-Earth asteroid
Eros
Eros holds two records: it was the first asteroid discovered on an orbit that brings it close to Earth, in 1898, and the first small body a spacecraft orbited and landed on, a century later. It is 34 km long and 11 km wide, and looks like a saddle. And it is thanks to Eros that astronomers first measured accurately the distance between Earth and the Sun.
- Size
- 34.4 × 11.2 × 11.2 km
- Distance from Sun
- 218 M km
- Orbital period
- 643 days
- Rotation
- 5.27 hours
- Discovered
- 1898
- Mission
- NEAR Shoemaker (2000–2001)
Physical characteristics
Eros measures 34.4 × 11.2 × 11.2 km: it is one of the largest near-Earth asteroids and has a very elongated shape, curved in the middle. It is an S-type, rocky asteroid, made of silicates and metal like the most common meteorites, the ordinary chondrites. Its density, 2.67 grams per cubic centimetre, points to a body that is solid but riddled with fractures, not a rubble pile.
The NEAR spacecraft photographed its entire surface: more than a hundred thousand craters, countless boulders and a blanket of fine dust, the regolith, which in places has filled depressions to form perfectly flat "ponds". The largest features are Himeros, the great saddle about ten kilometres wide, and the 5 km crater Psyche. Gravity is extremely weak: escape velocity ranges from 3 to 17 metres per second depending on the spot, and in some places a jump would be enough never to come down.
Orbit and rotation
Eros takes 643 days to travel an orbit lying between 1.13 and 1.78 AU from the Sun, 218 million km on average: it crosses the orbit of Mars but does not reach Earth's. It belongs to the Amor group, the near-Earth asteroids that approach our planet from outside without crossing its path. Its inclination is 10.8°.
It spins once every 5.27 hours about its shortest axis, like a propeller. On its most favourable passes it comes within about 22 million km of Earth, as in 1975; the next approach that close will be in 2056. It was these passes that made it valuable: by observing it from observatories far apart, in 1900–1901 and 1930–1931, astronomers measured its parallax and derived the Earth–Sun distance with a precision never achieved before.
Discovery and exploration
Gustav Witt discovered it on the evening of 13 August 1898 from the Urania Observatory in Berlin; that same night Auguste Charlois also photographed it in Nice. It was clear at once that this was no ordinary asteroid: its orbit dipped inside that of Mars. For that reason it was the first asteroid to be given a male name, that of the Greek god of love.
NASA's NEAR Shoemaker spacecraft entered orbit around Eros on 14 February 2000, Valentine's Day, and studied it for a year. On 12 February 2001, at the end of the mission, controllers brought it slowly down until it touched the surface: it was not designed to land, but it survived and kept sending data for another two weeks. It was the first landing on an asteroid.
Frequently asked questions
Can Eros hit Earth?
Not today: its orbit does not cross Earth's, and its closest point to the Sun stays at 1.13 AU. For the coming centuries there is no risk at all. Over the very long term, millions of years, perturbations from Mars could alter its orbit until it crosses ours, but that is a statistical scenario, not a prediction.
How did NEAR manage to land on Eros?
By descending very slowly. Eros's gravity is thousands of times weaker than Earth's, and the spacecraft touched the ground at about 6 km/h, the pace of a brisk walk. NEAR had no legs and was not designed to touch down: the landing was an experiment decided on at the end of the mission, and it worked.
Why is Eros so elongated?
Because it is too small for gravity to pull it round: below a few hundred kilometres in diameter rock holds its own, and a body keeps the shape that collisions gave it. Eros is in all likelihood a fragment of a larger asteroid destroyed in a collision, and its central saddle, Himeros, is the scar of a later impact.
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