Broad Scale

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Flying by and then later approaching the asteroid, 433 Eros, the NEAR spacecraft’s Multi-Spectral Imager started collecting images of the broad scale or global features of the asteroid. Even before the spacecraft got close to the asteroid it was apparent that Eros had some extreme features.

Overall, Earth and Eros are very similar because they both have a tilted axis. The axis of both Earth and Eros are fixed in inertial space, meaning that as they move around the sun, the sunlight concentrates on different parts of the surface. The only difference between the two is that Eros is extreme in comparison with Earth. While Earth's axis is tilted at 23.5°, Eros' axis is tilted on it side at nearly 90° (Thomas et al. 2002; Veverka et al. 1999, 2000). Therefore, they both experience effects, but the effects on Eros are much more extreme.

Earth and Eros at summer solstice
One example of Eros' extreme conditions can be seen in comparing the solstice on Earth and Eros. On Earth the sunlight concentration is shifted 23.5° North of the equator for the summer solstice. On the other hand on Eros, the sunlight concentrates directly on the North Pole, leaving the southern part of the asteroid in the dark. Therefore instead of days being a few minutes longer leading up to the solstice, like on Earth, a large portion of the Northern Hemisphere of Eros would be in constant sunlight and the Southern Hemisphere would be left in the dark.

Another result of the extreme tilted axis on Eros is seasonal changes in lighting. As Eros orbits the sun, its axis stays fixed and, in doing so, the angle of the line that separates night and day, or terminator, changes. This is apparent in Flyover_14 and FeatureTrack_270b.

In addition to the axis the first looks at Eros also showed its extreme shape. From the images that were acquired, the shape of the asteroid was confirmed. The elongated shape (34 x 13 x 13 km) was made up of a convex and a concave side. The convex side of the asteroid is dominated by the two largest craters, Shoemaker and Himeros, and four semi-flat surfaces. The concave side is more rounded and home to the third largest crater, Psyche (Veverka et al. 2000, 2001; Thomas 2002; Prockter 2002). The shape of the asteroid is well defined in the final approach images acquired on February 12, 2000.

The major process that forms topographic features on asteroids is impact cratering. The impact craters are formed from projectiles hitting the surface at hypervelocity. The impact forms a crater 20 to 50 times larger than the original projectile (Asphaug et al. 1996). Eros is a good example of this. On Eros, there are three large impact craters: Himeros , Shoemaker (not approved by IAU), and Psyche.

Shoemaker Regio and Himeros (128510736-128511418)

Located on the eastern side of Eros is a large depression called Himeros. This crater is believed to be one of the oldest of the large craters that is still visible on the surface. It is also the largest crater on the surface with a diameter of ~10 km. Due to its massive size, in comparison to the asteroid, the crater walls around Himeros have failed making the crater look more like a depression than a traditional crater. The inside of the crater contains many streamers which can be easily seen in higher resolution images on the eastern and northwestern walls. In addition, the crater also contains one end of Rahe Dorsum, the largest ridge on the surface of Eros. This crater can be seen in many images and orbital movies taken over the eastern hemisphere, but Himeros is especially highlighted in FeatureTrack_270b and MSI_FeatureTrack_281.

Superimposed on the southwest rim of Himeros is another large depression called Shoemaker. Like Himeros, Shoemaker's massive size (7.6 km), in comparison to the asteroid, gives the appearance of the crater walls failing. This crater also overlaps a part of Himeros. This crater is believed to be the youngest of the big three (Himeros, Psyche, and Shoemaker). The Shoemaker crater is home to 44% of boulders larger than 15 am across visible on the surface. It is also believed to be the source crater for many of the boulders that are on the surface. After the boulders on the surface were mapped, computer models were created to simulate the boulder ejecta layouts from different craters on the surface. Comparing the simulation to the map showed that it is most likely that Shoemaker was the source for many of the boulders on the surface. Since the boulders overlap other large craters and there is no indication of large amounts of boulders from either Psyche or Himeros, it can be accepted that Shoemaker is the youngest.

Psyche Crater (127684416-127685098)
Located on the opposite side of the asteroid from Himeros and Shoemaker is the crater Psyche. Psyche is a traditional bowl shaped crater that is 5.3 km wide and approximately 1 km deep. Even though the crater appears unmodified, there is no strong evidence that can determine the relative ages of Himeros and Psyche. One reason for this is their locations. Himeros is located on the eastern side while Psyche lies on the western. However, Psyche can't be too relatively young, because there are four large 1 km diameter craters superposed on the southern crater walls. In addition to Psyche's well defined bowl shape, it stands apart from the other large craters because of its varying albedo levels. The albedo changes are believed to be due to the steep slope and possible seismic shaking, causing the regolith to move down the steep crater walls revealing "unweathered" regolith (Veverka 2000; Thomas 2001). This crater is also seen in many movies, especially in FeatureTracks in where the camera focuses on the crater from midday to sunset. MSI_FeatureTrack_117, FeatuerTrack_259b, and FeatureTrack_270e are good examples.