The NEAR Shoemaker mission was wildly successful. The Mathilde flyby was the closest ever and the first close-up look of a C-type asteroid (Koppes, 97). At Eros, the mission helped to compile the first ever detailed global asteroid map. All the information regarding Eros can be very helpful in the future, since it is the second largest Near Earth Asteroid.
When the mission began, there were three questions that were to be answered. First, the age of Eros was to be determined. The theory was that Eros was a primitive asteroid, from the beginnings of the solar system, over 4.5 billion years ago. Next was the makeup of Eros, whether it was a rubble pile, which are rocks loosely held together by gravity, like many asteroids have been theorized to be (Seeds, 212), or a solid chuck of rock. Finally was the nature of the relationship between S-type asteroids, which Eros was one of, and meteorites, which were assumed to come partly from S-type asteroids. As we take a look at everything learned from the NEAR Shoemaker mission, these questions can be answered or at least put on hold for more information.
The asteroid Mathilde can be described in three words, according to Scott Murchie, big, black and battered (30). Mathilde is 38 miles across with one crater alone over 12 miles wide (Koppes, 96). Because Mathilde is a C-type asteroid, which are carbonaceous, made mostly of carbon, it was one of the only asteroids to not emit any color when imaged. The asteroid is pocketed with craters, so saturated that it cannot possibly fit any more craters without damaging old craters. The craters are high density and the surface is low density, leaving scientists to believe that Mathilde is a rubble pile (Murchie, 32). The amount of impacts shown on Mathilde's surface leads to the conclusion that only a rubble pile could withstand the seismic impact energy of a constant barrage of cratering (Koppes, 96). Without a solidarity to the asteroid, a rubble pile is able to take impacts much easier than a solid asteroid and not break into further pieces. An unusual feature of Mathilde is her slow rotational period, which is about 17.3 days, very slow in comparision to her 4.3 hour orbit in the central region of the asteroid belt (NEAR). There is no real theorizing or solution to this aberration but scientists are still combing through data regarding Mathilde.
The primary goal and destination of the NEAR Shoemaker mission, however, was Eros. The NEAR Shoemaker orbited the asteroid about 230 times before finally landing on its surface (NEAR). During that time, the mission was able to gather a staggering amount of information regardng Eros, only a small part of which has been gone through to conclusion. But we have learned a great deal more about Eros than the scientists who launched this mission could have imagined. Eros is about 21 miles long by 8 miles wide and 8 miles thick, which was smaller than originally predicted. The irregular shape of the asteroid is said to be due to a large impact (Verveka, 4). Most likely Eros was once part of a larger asteroid. It is also theorized that Eros spent most of its life in the asteroid belt.
Eros, like Mathilde is also pocketed in craters. The three major craters on Eros are Psyche, Himeros, and Shoemaker (Prockter,38). Psyche is a younger impact crater, about 3.2 miles wide. Landslides within the crater have covered up much of the younger, less-weathered material inside the crater. Himeros is extremely large crater, about 5.8 miles wide, covering a large portion of Eros' midsection. Himeros is unusually saddle shaped, due to its immense size in proportion to Eros. Finally, Shoemaker is judged to be the youngest large crater on Eros. The crater is covered with boulders and loose rock fragments. The Shoemaker crater is actually within Himeros, which is said to have caused much of the debris within Himeros and Shoemaker. Ponds are the relatively flat bottoms of craters, which are covered in regolith, basically cosmic dust (Prockter, 43). Ponds are common in Shoemaker. The problem arises, however, how the regolith got to these ponds. Some ideas include seismic shaking from impacts or the electrostatic motion due to the terminator between day and night. But, like much of the information regarding Eros, the data is still being processed.
One of the more dominant features of Eros is the Rahe Dorsum, a 11.1 mile ridge around the midsection of the asteroid (Prockter, 44). The ridge is tens of meters high in some places, but has a very shallow slope. Rahe Dorsum is probably an area where a large impact occured, with the likely culprit being the Shoemaker impact. Regolith fills the troughs of Rahe Dorsum, another possibility for seismic shaking or eletrostatic motion of the terminator.
This brings us back to the three questions proposed at the beginning of this mission: How old is Eros? Is Eros a rubble pile or a solid rock? and What is the relationship of meteorites and S-type asteroids such as Eros. As with most things, the more information learned, the most questions arise. But, some of these questions have been at least partially determined. The age of Eros has been determined to be around the same age as the solar system, a primordial asteroid. But, without any samples from the surface, this cannot be determined without a shadow of a doubt. Eros is, however, a solid asteroid. The presence of regolith on the surface leads scientists to believe that Eros is not a rubble pile, but a solid hunk of rock. Finally, it is still thought that most meteorites come from S-type asteroids. But this is not conclusive because even if a sample was taken from the surface, it cannot be determined to be from the same material as the interior, due to space weathering from impacts and the solar wind changing the surface (Koppes, 95). In conclusion, the NEAR mission was extremely important to our continuing mission to understand our solar system. Information from Eros and Mathilde can help us to learn more about our solar system and even the universe beyond.
Images from the NEAR Mission of Eros:
