First near-Earth asteroid discovered

The first near-Earth asteroid to be found was 433 Eros, which was discovered on 13 August 1898 by Carl Gustav Witt & Felix Linke (both DEU) at the Urania Observatory in Berlin and independently by Auguste Charlois (FRA) at the Nice Observatory. Eros is an irregularly shaped stony (S-type) asteroid that measures 33 x 13 x 13 km (20.5 x 8 x 8 mi) and whose orbit brings it to within around 22 million km (14 million mi) of Earth.

Carl Gustav Witt was the first to publish the discovery of the new body, and is often given sole credit as it is unclear whether Charlois' discovery actually happened at the same time (Charlois took photographs of the same area of the sky on the same night, but it has been suggested that he did not develop the plate until after he heard news of Witt's discovery).

Witt and his assistant Felix Linke were working with an improvised astrophotography set-up that involved components of several near-defunct telescopes and a camera made from nailed-together bits of old cigar boxes. Some components were missing at the time of the discovery because they had recently caught fire. The star-tracking mechanism did not work reliably, so taking the pictures involved sitting at the viewfinder for several hours, constantly moving the adjustment wheels to track the target star.

On the night they spotted 433 Eros, they were trying to spot the main-belt asteroid 185 Eunike. This involved focusing the telescope on a fixed point (in this case the star Beta Aquarii) and taking a long-exposure photograph. When developed, the stars would appear as fixed points of light, while any asteroids or comets would appear as tiny streaks, blurred by their relative movement. The photographic plate they developed showed 185 Eunike (as well as another known asteroid, 259 Aletheia), but it also showed a long streak left by an unknown by fast-moving object. This is what turned out to be 433 Eros.

In 1998, the asteroid was visited by NASA's NEAR-Shoemaker probe, which returned and entered orbit around Eros on 14 Feb 2000. It remained in orbit, mapping the surface and composition of the asteroid, until 12 Feb 2001, when it was de-orbited and soft-landed on the surface. Contact with the probe was lost on 1 March 2001.

A near-Earth object (which can be an asteroid or a comet) is defined by the International Astronomical Union as a small solar system body that makes its closest approach to the Sun (Perihelion) at a distance of less than 1.3 AU (194.5 million‬ km; 120.8 million mi, or about halfway between Earth and Mars). Most asteroids remain between the orbits of Mars and Jupiter, in the area of the Solar System known as the main asteroid belt. Objects that pass within Mars' orbit are relatively rare, and of particular interest because they have the potential to impact Earth.

Near-Earth objects are subdivided into groups based on their semi-major axis (average orbital distance) and relationship to Earth's orbit. The groups are Atiras, which orbit entirely within Earth's orbit; Atens and Apollos, which cross Earth's orbit but have a semi-major axis shorter than and greater than Earth's, respectively; and Amors, which orbit closer to Mars.

Of the roughly 20,000 near-Earth objects, there are around 1,743 near-Earth objects currently classified as Potentially Hazardous Objects (meaning that there is a possibility that they might impact Earth one day). These bodies are closely monitored by a global network of observatories, which aim to give enough time for governments to respond to significant threats. Eros is in the Amor group, and so does not fall into this category, though it has been suggested that its orbit might shift closer to Earth's orbit a few million years from now.