Eros was discovered on Aug. 13, 1898, by Gustav Witt, director of the Urania Observatory in Berlin, and independently observed on the same date by Auguste H.P. Charlois in Nice, France. In a break with tradition at the time, the asteroid was given a male name: Eros, the Greek god of love (in Latin called Cupido or Amor), son of Mercury and Venus. He protected the beautiful Psyche (asteroid 16) from the vengeance of his mother and when he later married her she became immortal.

The target of the NEAR mission is 433 Eros, the first-discovered near-Earth asteroid (NEA) and the second largest. Eros also is one of the most elongated asteroids, a potato-shaped body with dimensions of 33 by 13 by 13 kilometers. Its size qualifies Eros as one of only three NEAs with diameters above 10 kilometers.

Unlike the more abundant main belt asteroids, which orbit the Sun in a vast torus between Mars and Jupiter, NEAs are thought to be dead comets or fragments from main belt asteroid collisions. Approximately 250 NEAs are known, and scientists estimate there are at least 1,000 with diameters of 1 kilometer or more.

As a member of the NEA group known as the Amors, Eros has an orbit that crosses Mars' path but doesn't intersect that of Earth. The asteroid follows a slightly elliptical trajectory, circling the Sun in 1.76 years at an inclination of 10.83 degrees to the ecliptic. Perihelion distance is 1.13 AU (169 million kilometers); aphelion is 1.78 AU (266 million kilometers). Eros' average distance from the Sun is 1.458 AU (218 million kilometers).

The closest approach of Eros to Earth in the 20th century was on January 23, 1975, at approximately 0.15 AU (22 million kilometers). Previous close approaches occurred in 1901 at 0.32 AU (48 million kilometers) and in 1931 at 0.17 AU (26 million kilometers). Because of its repeated close encounters with Earth, Eros has been an important object historically for refining the mass of the Earth-moon system and the value of the astronomical unit. Eros is no threat to actually hit the Earth, however.

More than a century of ground-based study, including a worldwide observation campaign during the 1975 close approach, has made Eros the best observed of the NEAs. Astronomers assign the asteroid a rotation period of 5.27 hours. Geometric albedo is 0.16. Thermal studies indicate a regolith, and radar suggests a rough surface. Eros is known to be compositionally varied: one side appears to have a higher pyroxene content, while the opposite side displays higher olivine content.

Daytime temperature is about 100 deg. C, while at night it plunges to -150 deg. C. Gravity on Eros is very weak but sufficient to hold a spacecraft in orbit. A 45-kilogram object on Earth would weigh about an ounce on Eros, and a rock thrown from the asteroid's surface at 10 meters/sec would escape into space.

Eros is one of the S-type (silicaceous) asteroids, the most common type in the inner asteroid belt and the subject of debate over their relationship to meteorites. Galileo's flyby observations of Gaspra and Ida (both of which are S-types) did not provide the answer, largely because remotely sensed spectral data cannot accurately determine the relative abundance of key elements. This is a major goal of the NEAR mission to Eros.

NEAR first image of Eros shows the target asteroid as a single pixel in a negative image.

This first image of the asteroid 433 Eros was acquired by the multispectral imager on the Near Earth Asteroid Rendezvous (NEAR) spacecraft on November 5, 1998, at a distance of 4 million kilometers from the asteroid. One full rotation of the asteroid.

Eros, located at the center of the image and circled, appears against the star background in a single illuminated pixel. At the time of the image, NEAR was located 321 million kilometers from Earth, and the radio signal, which transmitted the image from the spacecraft, took nearly 18 minutes to reach Earth. The image is a part of a 5.3-hour sequence of images that show Eros over

This montage of images of the asteroid Eros was assembled from images acquired by the Near Earth Asteroid Rendezvous (NEAR) spacecraft on December 23, 1998, as the spacecraft flew by the asteroid at a distance of 3800 kilometers at 1:43 PM EST.

Shown are nine early views out of 29 that were obtained during the flyby. These images were taken between 10:44 AM and 12:44 PM EST as the spacecraft range closed from 11,100 km to 5300 kilometers. During that time, the asteroid completed nearly half of a rotation. The smallest resolved detail is approximately 500 meters across.

This pair of images of the asteroid Eros was acquired by the Near Earth Asteroid Rendezvous (NEAR) spacecraft on December 23, 1998, as the spacecraft flew by the night side of the asteroid at a distance of 3800 kilometers at 1:43 PM EST.

These views, taken at 1:44 PM and 2:05 PM EST as the spacecraft range increased from 2300 miles to 2500 miles (4100 kilometers), show only a tiny portion of the day side of Eros (phase angle ~119°). The smallest resolved detail is approximately 400 meters across.

Just in time for its Valentine's Day date with 433 Eros, the Near Earth Asteroid Rendezvous (NEAR) spacecraft snapped this photo during its approach to the 21-mile-long space rock. Taken Feb.

11, 2000, from 2590 kilometers away, the picture reveals a heart-shaped depression about kilometers long. Scientists at the Johns Hopkins University Applied Physics Laboratory - which manages the NASA mission -processed the image on Feb. 12. Photos taken from closer in during the next few days will help the NEAR team unravel the mystery of this shadowy feature.

This montage shows a selection of images of the asteroid 433 Eros that were acquired from the NEAR spacecraft over three weeks from January 22 through February 12, 2000, as the spacecraft's distance from its target shrank from 29,000 to 2025 kilometers. As the spacecraft closed in on its target, the resolution of the images increased from 2.8 to 0.19 km per pixel.

During the early stages of NEAR's approach, Eros appeared as a small blob only a few pixels across. The apparent size of Eros and the resolution of the pictures increased continuously, at first only slowly and later dramatically day by day until, on February 9, the level of detail visible exceeded that during NEAR's first flyby of Eros on December 23, 1998. In the last images shown here, details of Eros's surface have become visible. Heavy cratering has pockmarked the irregular asteroid's surface. One side is dominated by a scallop-rimmed gouge and the opposite side by a conspicuous, raised-rimmed crater.

On February 12, two days before NEAR's insertion into orbit around Eros, the spacecraft's camera took this image of Eros from a range of 1570 kilometers. This was the last image returned to Earth prior to NEAR's insertion into Eros orbit. Features as small as a 160 meters across can be seen.

This face of Eros is dominated by a huge, hollowed-out gouge, which may also have been caused by an impact. The feature, which from a greater distance appears, heart-shaped (at lower left) is resolved in this view as three impact craters which rims shallow depression.

At February 14. 2000. at 10:33 AM EST the NEAR spacecraft was successfully inserted into orbit around 433 Eros, becoming the first artificial satellite of an asteroid. Just over an hour later NEAR pointed its camera at the asteroid and took this picture from a range of 330 km above the surface. Mission navigators and operators will use this image and others to be taken later to traingulate on landmarks on the asteroid's surface, precisely measuring position of the spacecraft to plot NEAR's course. Features as small as a 30 meters across can be seen.

This view shows the 5-kilometer impact crater, which the spacecraft has spied for over a week during its approach. The two smaller craters superimposed on its rim are each about 2 kilometers across. An enormous boulder a full 50 meters in size sits on the large crater's floor. Other key features of the surface are shallow subsurface layering exposed near the tops of crater walls, and shallow grooves crossing the surface and cutting the crater's rim.

This picture of Eros, the first of an asteroid taken from an orbiting spacecraft, is a mosaic of four images obtained by NEAR on February 14, 2000, immediately after the spacecraft's insertion into orbit. We are looking down over the north pole of Eros at one of the largest craters on the surface, which measures 6 kilometers across. Inside the crater walls are subtle variations in brightness that hint at some layering of the rock in which the crater formed.

Narrow grooves that run parallel to the long axis of Eros cut through the southeastern part of the crater rim. A house-sized boulder is present near the floor of the crater; it appears to have rolled down the bowl-shaped crater wall. A large number of boulders are also present on other parts of the asteroid's surface. The surface of the asteroid is heavily cratered, indicating that Eros is relatively old.

This incredible picture of Eros, taken on February 14, 2000, shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. In this mosaic, constructed from two images taken after the NEAR spacecraft was inserted into orbit, features as small as 35 meters across can be seen. House-sized boulders are present in several places; one lies on the edge of the giant crater separating the two ends of the asteroid.

A bright patch is visible on the asteroid in the top left-hand part of this image, and shallow troughs can be seen just below this patch. The troughs run parallel to the asteroid's long dimension.

This picture was taken from NEAR on February 15, 2000, while the spacecraft was passing directly over the large gouge that creates Eros's characteristic peanut shape. It is a mosaic of individual images showing features as small as 35 meters across. Although most of the asteroid is in shadow, we are able to see inside the gouge. Many narrow parallel troughs closely follow the shape of the gouge.

Although they appear curvilinear from this view, they are most likely oriented parallel to the length of the asteroid. The strong lighting contrast along the terminator (the line separating day from night on Eros) makes it easy to see that most of the surface is saturated with impact craters. Inside the gouge, however, only smaller craters are present, indicating that the area within the gouge is younger than the surface along the terminator. This implies that the event that caused the gouge must have happened more recently than the formation of the rest of the surface of Eros.

This color image of Eros was acquired by NEAR's multispectral imager on February 12, 2000, at a range of 1800 kilometers. It is part of the final approach imaging sequence prior to orbit insertion and is intended to map the color properties of Eros across all of the illuminated surface.

The image shows approximately the color that Eros would appear to the unaided human eye. Its subtle butterscotch hue is typical of a wide variety of minerals thought to be the major components of asteroids like Eros.

Stereo imaging will be an important tool on NEAR for geologic analysis of Eros, because it provides three-dimensional information on the asteroid's landforms and structures. This anaglyph can be viewed using red-blue glasses to show Eros in stereo. It was constructed from images taken on February 14 and 15 that showed the same part of Eros from two slightly different viewing perspectives.

The smallest feature visible is 30 meters across. For this image the spacecraft position was not optimum for stereo, but it will improve over the next few days allowing better 3-D views.

This image mosaic of Eros was taken by the NEAR spacecraft on Feb. 18, 2000 from a range of 361 kilometers. The smallest detail visible on the surface is about 35 meters across. At the time the spacecraft was over the shadowed southern hemisphere, looking north at a crescent Eros. Although this view of Eros is similar to others that have been returned, the coverage at different illumination and viewing geometries provides important information on the shapes of landforms.

On February 23, 2000, the NEAR spacecraft obtained a sequence of image mosaics showing Eros' surface as the asteroid rotated under the spacecraft. At that time the range to the surface was approximately 355 kilometers. These two mosaics, part of that sequence, show the stark beauty of the two opposite hemispheres. The smallest detail visible is 35 meters across.

The top mosaic shows wavy brightness banding exposed in the interior walls of the saddle. In the bottom mosaic, similar banding is visible in one of the craters near the limb at left. To the right, the angle of the illumination accentuates the quasi-linear troughs near the terminator.

NEAR takes several images mosaics of Eros daily for purposes of navigating the spacecraft. The one shown here was taken February 29, 2000, from a range of 289 kilometers. It shows features as small as 30 meters across. All of the mosaics show the same territory over and over, but changes in lighting plus the gradual decrease in the spacecraft's range to the surface are both constantly bringing out new details.

The very oblique illumination in this mosaic is ideal for bringing out small landforms. Many parts of the asteroid have "grooves," linear troughs about 100 meters wide and several kilometers long. Similar features have also been observed on other asteroids such as Gaspra, and they are especially numerous on Mars' moon Phobos. Their origin isn't completely understood, but formation of the grooves probably involves fracturing of the asteroid's subsurface in some way.

This image mosaic of the large crater at Eros' center was taken on March 3, 2000 during an optical navigation imaging sequence from a range of 204 kilometers. This same area was imaged following orbit insertion at a range of approximately 330 kilometers on February 14th. This picture resolves features as small as 20 meters across compared to the resolution of 30 meters in the earlier image. The shadow cast by the boulder near the floor of the crater is now visible.

The walls of the crater display some distinctive variations in their albedo or reflectivity. The upper part of the walls tend to be bright, while the lower portions of the walls and the crater floor show patches of darker or less reflective materials. These albedo patterns are also visible on other crater walls. To the right of the large crater, two sets of closely spaced orthogonal grooves are visible.

This image, showing an oblique view of Eros' large central crater, was taken at a resolution of about 20 meters per pixel. The brightness or albedo patterns on the walls of this crater are clearly visible, with the brighter materials near the tops of the walls and darker materials on the lower walls. Boulders are seen inside this crater and the smaller nearby craters.

The higher density of craters to the left of the large crater implies that this region is older than the smoother area seen associated with the saddle region on the opposite side of the asteroid.

This image looking down the length of Eros was taken from the NEAR Shoemaker spacecraft on March 10, 2000, from a range of 206 kilometers. At once, it shows many of the landforms revealed in the last month to be characteristic of this tiny world. The part of the surface shown here is covered by craters of all sizes, right down to the limit of image resolution. Many of the largest craters, such as the two at upper left, have conspicuous albedo markings on their interior walls.

At the upper left is a portion of the ridge that nearly wraps around Eros' waist. Near the center of the image, oriented from the upper right to lower left, are several broad troughs, or grooves, about 200 meters wide. The three boulders on the far horizon are about 80 meters across - each nearly the size of a football field. The area shown in the image is 10 kilometers high. To put the asteroid's size into human perspective, a famous New York landmark is shown inset at the lower left.

On March 11, 2000, this image of Eros' north polar region was acquired by the imager on the NEAR Shoemaker spacecraft, from a range of 206 kilometers. The area shown in the image is 10 kilometers across. Most of the north polar region is heavily cratered but the region to the left (part of the "saddle") has a lower crater density, indicating that the surface has been modified since it first formed.

Eros' rotational axis lies nearly parallel to its orbital plane, much as with the planet Uranus, giving the asteroid exaggerated "seasons." Now, it is northern summer and the north pole is in continuous sunlight. The Sun will set there this June, at Eros' equivalent of Earth's autumnal equinox. At that time, Eros' south pole will begin 12 months of continuous illumination while the north pole remains in darkness.

As the NEAR Shoemaker spacecraft descends into lower orbits around Eros, it continually returns higher spatial resolution images of the asteroid. The true color image at left was taken February 12, 2000, from a range of 1,748 kilometers, and shows details only as small as 180 meters across.

It was taken two days before orbit insertion, as part of an image sequence designed to provide moderate-resolution color mapping of Eros at a near-constant viewing geometry. The true color image inset at right was taken February 29 from a range of 283 kilometers and shows much smaller details only 27 meters across. The higher spatial resolution (by a factor of six) brings out a whole class of surface details that were either invisible or at the margin of visibility in the earlier images. For example, the bright material on the wall of the large crater in the inset image is barely evident in the lower-resolution image at left, but by virtue of its limited spatial coverage the inset image lacks information on the crater's regional geologic setting. NEAR Shoemaker's imaging strategy makes use of both types of images, with lower-resolution images providing "context" for higher-resolution images that bring specific features into sharper focus.

The NEAR Shoemaker spacecraft has scored two important firsts in the exploration of asteroids: it's the first to orbit an asteroid, Eros, and the first to encounter a denizen of the outer reaches of the asteroid belt, the C-type asteroid Mathilde. In a scheme that reflects how they have historically been a topic for astronomy, not geology, asteroids are classified into groups based on their colors as observed through telescopes.

The two major classes of asteroids are called S-types and C-types. S-types, whose colors are consistent with "stony" or rocky compositions, prevail among asteroids that orbit closer to the Sun than the mid-point of the asteroid belt. Eros and the two asteroids encountered briefly by the Galileo spacecraft on its way to Jupiter -- Gaspra and Ida -- are all S-types. C-types like Mathilde have a dark gray color consistent with a "carbonaceous" composition, rich in carbon compounds and other dark materials. They prevail in the outer part of the asteroid belt. In this montage, Mathilde (at left) and Eros (at right) are shown at the same scale, as they were imaged by NEAR Shoemaker from about 1,800 kilometers on June 27, 1997, and February 12, 2000, respectively. Mathilde is 56 kilometers across, and Eros is 33 kilometers long and 13 kilometers wide. However, Mathilde's brightness is greatly exaggerated for viewing purposes -- it's actually six times darker than Eros, with about the same reflectivity as soot!

Lighting and viewing geometries make a huge difference in the appearance of Eros' surface features. One of the most striking examples is the 2.7-kilometer diameter crater shown in these two images. The image at left, looking at the crater nearly edge-on, was taken February 16, 2000, from a range of 341 kilometers.

The image at right was taken high over the crater on March 2, 2000, from a range of 226 kilometers . In the first image the only visible part of the crater's interior is the far, bright wall, which at the time was well-lit. The lighting, in combination with the particular viewing angle, make the crater appear stunningly bright. In the second view, the brighter material occupies only part of the slightly-shaded interior, greatly reducing the overall brightness contrast between the crater and the surrounding terrain.

One of the most striking features in NEAR Shoemaker images of Eros' surface is the abundance of very large boulders. This image of the southwestern part of the saddle region, taken March 6, 2000, from a range of 201 kilometers, shows a particularly boulder-rich part of the surface. Many of the huge rocks are 50 meters or more in diameter.

They are believed to be fragments of Eros' native rock, shattered over the eons by formation of impact craters. The impacting projectiles themselves were pulverized by the impact process and survive only as fine debris mixed into the regolith.

Eros' many craters have a range of ages dating back to the last time the asteroid's surface was "wiped clean" by geologic processes. This NEAR Shoemaker image of the tip of the asteroid, taken March 6, 2000, from a range of 201 kilometers, shows craters with a variety of shapes and sizes.

When small craters first form, they typically have sharp rims and round floors. As they age, progressively smaller craters are superimposed, rounding the rims and pitting the walls and floors until the original underlying crater becomes almost unrecognizable.

As the NEAR Shoemaker spacecraft continues its descent to lower orbits around Eros, smaller and smaller surface details are becoming visible. This image was taken April 29, 2000, from an orbital height of 84 kilometers, just before the engine firing that placed NEAR in a 50-kilometer altitude orbit.

The image shows features as small as 7 meters across and boulders a mere tens of meters in size. The whole scene is 3.2 kilometers across.