NEAR Science Results from Asteroid 433 Eros

[NEAR GRS Results]

Gamma-Ray Readings from Eros

This chart shows the gamma-ray spectrum from the surface of Eros. These scientific data -- the first ever collected on the surface of an asteroid -- result from 7 days of measurements following NEAR Shoemaker's historic landing on February 12.

The gamma-ray instrument has two detectors -- marked above by the red and blue traces -- which picked up clear signatures of key elements in the composition of Eros. These data, which surpass in quality all the data accumulated by this instrument from orbit, will help NEAR scientists relate the composition of Eros to that of meteorites that fall to Earth.

[NEAR cliff image]

Almost Vertical Cliff on Eros

The NEAR Shoemaker Laser Rangefinder measures heights on Eros' surface by determining the time it takes for short pulses of laser light to travel from the spacecraft to the asteroid and back. A surface height profile measured this way (chart, lower right) reveals a nearly vertical cliff more than 100 meters (330 feet) high within the asteroid's 10-kilometer-wide (6-mile-wide) saddle-shaped depression, whose proposed name is Himeros. The location of the laser height profile is marked a' - f' in the upper panel, which is a mosaic, and in the lower left panel, which is a single image obtained at the same time as the laser height profile. The cliff looks very different in the two images because of the different lighting conditions.

[NEAR topography model]

Measuring Topography

NEAR Shoemaker's Laser Rangefinder measures heights on Eros' surface by determining the time short pulses of laser light take to travel from the spacecraft to the asteroid and back. The elevations of points on the surface of Eros are determined by subtracting the laser-derived distance between the spacecraft and the surface from the distance between the spacecraft and Eros' center of mass, as determined from radio tracking.

NEAR team members use this information to build detailed models of Eros' shape, which in turn help them understand the asteroid's collisional evolution and internal structure. The shape of Eros also provides a context for interpreting images and compositional information.

This image is a high-resolution digital terrain model of Eros near 0 degrees longitude, determined from a shape model derived from 5.6 million laser measurements. The black grid values correspond to latitude and longitude; the colors correspond to distances to the asteroid's center of mass, as noted (in kilometers) on the colored bar on the right. This shape model has a spatial resolution of about 500 meters. The model continues to improve in resolution and accuracy as NEAR scientists collect and analyze more laser data.

[NEAR gravity model]

The Ups and Downs of Eros

While NEAR Shoemaker does not directly measure gravity on Eros' surface, the spacecraft gathers other data that allow scientists to infer this measurement. Radio tracking has been analyzed to determine the asteroid's gravitational "pull" on the orbiting spacecraft. The many images of the asteroid, plus range measurements from the laser altimeter, measure the body's shape. Comparisons of the shape with the gravitational pull felt by the spacecraft from different parts of its orbit show that the density of the interior must be nearly uniform.

The asteroid's shape, density and spin combine to create a bizarre pattern of what is "uphill" and "downhill." In this view, a map of "gravitational topography" has been painted onto a shape model. Red areas are "uphill" and blue areas are "downhill." A ball dropped onto one of the red spots would try to roll across the nearest green area to the nearest blue area.

(Image copyright 2000 Science Magazine)

[NEAR latitude longitude model]

A Roadmap for Eros

These four image mosaics of Eros illustrate the reference system the NEAR team uses to locate points on the asteroid's surface. Choosing from a variety of systems, the team members use latitude and longitude as their directional guide.

Latitude is the angle between the equatorial plane and a line connecting the surface point with the center of the body. By convention, the north pole is +90 degrees, the equator is 0 degrees, and the south pole is -90 degrees. Longitude is the angle around the rotational axis separating the point on the surface from the "prime meridian." The prime meridian selected by NEAR scientists is drawn though a large, bright crater at one end of Eros (indicated by an arrow in the lower left mosaic).

Again by convention, "west longitude" is used for most planetary bodies, including Eros, and it increases west from the prime meridian.

(Image copyright 2000 Science Magazine)

[NEAR shape model]

The Shape of Eros

NEAR Shoemaker images provided the first model of Eros' shape, shown in this picture. An accurate model of Eros' shape helps the NEAR team determine the asteroid's key properties. The total space enclosed by the surface of the shape model represents the asteroid's estimated volume. The asteroid's mass (determined using NEAR Shoemaker's radio tracking) is divided by its volume to yield an estimate of its density - which is valuable for understanding what kind of rock makes up the interior. The shape also provides information about the distribution of mass below the surface, which scientists use to calculate surface gravity and thus the directions and steepness of slopes.

The positions of more than 2,000 landmarks on Eros were determined from more than 12,000 stereo measurements that generate a "grid" of points lying on the asteroid's surface. The shape model is a mathematical representation of the surface passing through all those points. The views at upper left, lower left and lower right show the shape model from over the equator (at 270 and 180 degrees west longitude) and the north pole (90 north latitude), respectively. The shape model at upper right is overlain on an image acquired during approach to Eros.

(Image copyright 2000 Science Magazine)

[NEAR NLR range returns]

Topographic Profiles from the NEAR Laser Rangefinder

The NEAR Laser Rangefinder obtained this 'staircase' altimetric profile from within the wall of the 6 km (10 miles) crater shown in the image inset. The profile represents ten minutes of data obtained from a range of 217 km (135 miles) on March 2, 2000. The purple points are a plot of radius from the center of Eros (left axis), and the blue points show the same measurements but with the overall trend removed (right axis). The laser track runs from left to right approximately in a line along the major axis of the red ellipse drawn on the image. The laser profile reveals a series of shallow depressions, about 50 to 80 meters (162 to 260 feet) wide and 5 to 10 meters (16 to 32 feet) deep, that may be a chain of pits or craters. Alternatively, the 'staircase' profile may be the signature of a series of small ridges. The spatial structure and origin of the features producing the staircase will be revealed as the laser rangefinder accumulates elevation measurements during its one-year mapping mission of Eros. (Insert is a mosaic of images 0126723526 and 0126723790)

[NEAR NLR range returns]

The NEAR Laser Rangefinder (NLR) records the first ever range returns from an asteroid.

The observations were obtained during the first turn-on of the instrument while the NEAR spacecraft orbited Eros on February 28-29, 2000. The returns from this calibration pass were obtained from a short segment of the surface, close to the edge of the giant gouge shown in the image-of-the-day 2000 February 17C. The range was about 290 kilometers to the surface of Eros, over 5 times the designed ranging distance. Data from the NLR obtained during the NEAR mapping mission will be used to make a precise topographic model of Eros and to study the asteroid's shape, internal structure and dynamics. (Credit: NLR Science Team)

[NEAR low-phase flyby spectrum of Eros]

A Spectral View of Eros

This image shows a map of the distribution of the 2000 nm band strength measured with the Near Infrared Spectrometer (NIS) on-board the NEAR spacecraft on Feb. 13 and 14, 2000. The typical resolution element (called a "nixel" used for this map is 2x4 km. Blue areas have weaker absorption bands, yellow and red areas have stronger bands. The 2000 nm band is observed in reflected sunlight when the mineral pyroxene is present. Analysis of maps such as this combined with the results from the other instruments on-board NEAR over the coming year will help us understand the composition and history of Eros.

Data Derived from NIS Low Phase Flyby sequences,Feb. 13-14, 2000; Mission Elapsed Times 125878216-125919596

[NEAR low-phase flyby spectrum of Eros]

Eros Spectrum

This plot shows a typical spectrum of Eros taken by the Near Infrared Spectrometer (NIS) on the NEAR spacecraft just before orbit insertion on February 14, 2000. The instrument measures the amount of sunlight reflected from the asteroid's surface over a wavelength range of 800-2400 nm, beyond the sensitivity of the human eye. Absorption bands due to the rock-forming minerals olivine and pyroxene are seen in the 1000 and 2000 nm regions, revealing information about Eros' composition.

Data Derived from NIS Low Phase Flyby sequences,Feb. 13-14, 2000; Mission Elapsed Times 125835667-125920263

[NEAR low-phase flyby spectrum of Eros]

NEAR's spectrometer measures Eros

Throughout NEAR's approach to Eros, both the Multispectral Imager (MSI) and the Near-Infrared Spectrometer (NIS) have acquired measurements of the asteroid's light-reflecting properties. One of the standard ways that astronomers study asteroids is to measure their "light curves", or how their brightness varies as they spin because of shape, orientation, spin axis, and surface markings.

This plots shows light curves of Eros measured both by MSI and by NIS, on February 6 when NEAR was about 4200 miles (6800 kilometers) from Eros. The plot shows the relative brightness of Eros over the course of one Eros "day" (5.27 hours). Eros was brightest near the beginning of the observations, then dimmed by about 20% over the course of the next 2 hours, and by an hour later it brightened to about 5% brighter than at the start. Then it dimmed again by about 40% at 4 hours from the start of observations, and finally returned back to its original brightness at the start. This pattern of two brightness cycles per Eros "day" is characteristic and results from alternating viewing of Eros's long and short dimensions. Observations like this, taken from telescopes, provided the first clues that Eros has an elongated, peanut-like shape.

(MSI images and NIS spectra 125212660-125232784)

[NEAR encounter image of asteroid Eros]

Scientists study Eros's multitudes of craters

NEAR scientists analyze the numerous craters on Eros by measuring their numbers and sizes. A computer routine then computes latitude and longitudes, as well as statistics about the numbers of craters. Shown here is the first step in this process, where craters have been identified in the first mosaic of images taken after NEAR was placed into orbit around Eros on February 14th. Each circle shows the size of the crater and tags it with a number to keep track of it during later analysis. (Mosaic of images 0125957025 and 0125957087)

[NEAR crater count graph]

Asteroids' crater records compared

Impact crater populations on asteroids visited by spacecraft are compared on this plot. The higher locations on this plot indicate more craters; sparse cratering is lower on the graph. Craters can never be as numerous as indicated by the "geometric saturation" line, where they would lie cheek-by-jowl, totally crowding the surface. An important difference between asteroids is whether or not large craters dominate coverage of the surface compared with small craters (indicated by a line sloping to the upper right) or, instead, if smaller craters dominate the surface (a line sloping to the upper left).

As shown by this plot, Eros has similar numbers of smaller craters as Mathilde and Ida (observed by NEAR and Galileo, respectively). But Eros lacks the numerous huge craters present on Mathilde. Eros shares with Ida and Mathilde a trait for which Gaspra, so far, remains the exception: their larger numbers of craters indicate that they are rather old compared with the more sparsely cratered Gaspra.

Captions and images courtesy of the NEAR Project (JHU/APL).
* NEAR Eros approach images and animations

* More information on asteroid 433 Eros
* Asteroid Fact Sheet
* NSSDCA Asteroid Home Page

* Information on the NEAR Instruments
* Detailed Information on the NEAR Spacecraft and Mission

* Detailed information on NEAR from the NSSDCA Master Catalog
* NSSDCA Planetary Home Page

Dr. David R. Williams,
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771

NASA Official: Dr. Ed Grayzeck,
Last Updated: 18 December 2001, DRW