Eros is a Near-Earth Asteroid (NEA) whose orbit brings it into proximity with Earth’s path around the Sun. Discovered in 1898, Eros was the first NEA found, immediately capturing attention due to its trajectory outside the main asteroid belt. Its existence confirmed that not all asteroids reside solely between Mars and Jupiter. This large, irregularly shaped object offers scientists a unique opportunity to examine primordial material left over from the Solar System’s formation, providing insight into the early history of our planetary neighborhood.
Defining the Orbital Path and Classification
The trajectory of 433 Eros classifies it as an Amor-group asteroid, a category of Near-Earth Objects (NEOs). This designation means Eros’s perihelion (closest point to the Sun) is greater than Earth’s aphelion, ensuring it does not cross our planet’s path. Its closest approach to the Sun is approximately 1.13 Astronomical Units (AU).
Its aphelion, or farthest point from the Sun, is about 1.78 AU, positioning it beyond the orbit of Mars. Eros is an outer Earth-grazer, orbiting outside Earth’s trajectory but frequently crossing Mars’s orbit. This wide-ranging path made it the first NEA discovered and an easier target for early telescopic observation.
Eros completes one revolution around the Sun in approximately 1.76 Earth years (about 643 days). Its orbit is tilted relative to the plane of Earth’s orbit, with an inclination of about 10.8 degrees. Although the current path is stable, its NEA classification means gravitational perturbations could potentially alter its trajectory over time.
The orbital mechanics of Eros made it a scientific target of high interest, as it represented a relatively accessible body for a spacecraft to reach and study. Its perihelion distance keeps it close enough to the inner Solar System to be considered a Near-Earth Object, which was crucial for mission planning.
Physical Dimensions and Composition
The physical form of 433 Eros is distinctly elongated and irregular, often described as having a “peanut” or “potato” shape. Its dimensions are substantial for an NEA, measuring about 34 by 13 by 13 kilometers. This size makes Eros the second-largest known NEA, representing a significant body of ancient Solar System material.
Eros is classified as an S-type asteroid, meaning its composition is primarily stony, composed of silicates mixed with metals. Spectral analysis suggests a makeup similar to H-type ordinary chondrite meteorites, rich in iron- and magnesium-bearing silicates, along with metallic nickel and iron. Its bulk density is approximately 2.67 grams per cubic centimeter, comparable to Earth’s crust.
The surface is heavily marked by craters, indicating a long history of impacts. These features include a high density of large craters but a surprising lack of smaller ones. This suggests erasure has occurred, attributed to seismic shaking from impacts causing the loose surface material, or regolith, to shift and fill depressions.
The regolith, a layer of dust and fragmented rock, is estimated to be several tens of meters thick in some areas. The surface also features distinct geological structures, including a prominent central ridge and linear grooves, hinting at a consolidated, fractured internal structure.
Exploration by the NEAR Shoemaker Spacecraft
Detailed knowledge of Eros was gathered by NASA’s Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission. Launched in February 1996, the spacecraft was designed for a long-duration study aiming to understand the link between asteroids and meteorites. The mission was renamed in 2000 in honor of planetary geologist Eugene Shoemaker.
The NEAR Shoemaker spacecraft first entered orbit around 433 Eros on February 14, 2000, becoming the first spacecraft in history to orbit an asteroid. It spent a full year studying the asteroid from various altitudes, collecting extensive data using multispectral imagers and spectrometers. The mission returned over 160,000 images, confirming the S-type composition and irregular structure of Eros.
The mission culminated in a historic, unplanned controlled descent to the surface on February 12, 2001. Although not designed to land, the team successfully maneuvered it to a soft touchdown, making it the first human-made object to land on an asteroid. During the final descent, the spacecraft captured high-resolution images, showing surface features as small as one centimeter across.
Even after landing, the spacecraft continued to function for a period, sending back data from the surface. This provided final confirmation of Eros’s bulk density and composition. The NEAR Shoemaker mission provided the foundational spatial and compositional data that defines our modern understanding of this large Near-Earth Asteroid.