Unit 8: Beyond Earth Chapter 29: Our Solar System     Our Solar System In 2001, NASA scientists accomplished a daring maneuver-the landing of a spacecraft on a distant asteroid. For a few brief seconds, the space probe NEAR Shoemaker transmitted startlingly close-up images of the cratered surface of asteroid 433 Eros. Then, as expected, all contact with Earth was lost. The spacecraft was out of fuel; the mission was over. NEAR Shoemaker had successfully gone where no spacecraft had gone before. For one year, it had orbited 433 Eros, rising and dipping to within kilometers of the pocked surface before its final descent. In that year, scientists were able to gather crucial information about the make-up and orbits of asteroids. The study of space holds many wonders. In recent years, for example, astronomers have discovered strange new planets in faraway solar systems. In comparison, small, rocky asteroids seem like a rather dry topic. There are thousands upon thousands of asteroids in our solar system-they are nearly as common as pebbles on a beach. Why then do scientists study asteroids with such interest? There are two reasons. First, just as pebbles on a beach can hold clues to an area's geologic history, asteroids contain clues to the early history of the planets. Second, an asteroid's orbit can sometimes send it on a collision course with Earth. Although this is a very rare event, it has happened in the past with devastating results, and thus is worth planning for. The Connection to Early Earth Most asteroids are located in a belt between the orbits of Mars and Jupiter. The majority are quite small, but some, like 433 Eros, are dozens of kilometers in length. Scientists hypothesize that the strong gravitational pull of the gaseous giant Jupiter prevented asteroids from forming into planets. Asteroids have been in existence, shaping and reshaping themselves through collisions, since the early days of the solar system. Thus, these scattered pieces of space rock are similar in composition to the material that eventually made up the early planets, Earth included. How do scientists know this? When the solar system began forming some 4.6 billion years ago, particles of dust and gas coalesced into chunks of rock. Over time, these rocks came together to form planetesimals, or fledgling planets, which grew in mass as more rock was added until finally, they became the early planets. This process generated heat that changed the chemical composition of the original rock. Because asteroids never underwent the planet-forming stage, they still retain the chemical composition of planetesimals. Using data from NEAR Shoemaker, scientists have already confirmed that 433 Eros is made up of iron, magnesium, silicon, and aluminum-the same chemical composition of the oldest meteorites found on Earth. Data from Near Shoemaker also indicates that asteroids have uniform densities and are not scattered pieces of debris bound by gravity as was previously hypothesized. It will take years for scientists to digest all the data gathered from the space probe. In the meantime, let's examine how scientists might use the information from Near Shoemaker to stop an asteroid from colliding with Earth. LINK-UP: Find out more information about the mission of NEAR Shoemaker. The Threat of Impact According to NASA, large asteroids capable of global repercussions strike Earth on average once every 100,000 years. The impact of a massive asteroid can send dust and debris into the atmosphere for years, blocking out sunlight and leading, ultimately, to mass extinctions. Many scientists theorize that dinosaurs became extinct for this very reason. Smaller asteroids strike Earth every 1000 to 10,000 years and can cause extensive damage in a concentrated region. Given the seriousness of the threat, scientists continually monitor the orbits of asteroids to pinpoint those that may swing close to Earth. What would we do if an asteroid was on an apparent collision course with Earth? One possible action is to deflect the orbit of the asteroid so that it passed harmlessly into space. To accomplish this, we would need the ability to guide a spacecraft into orbit around the asteroid and perhaps land on its surface. That's one reason why the mission of Near Shoemaker was hailed as a success. Beyond what the spacecraft has taught us about the nature of asteroids, it represents an important first step toward protecting our planet against catastrophic collisions. Activity An asteroid or other interplanetary object that strikes Earth's surface is known as a meteorite. Upon impact, large meteorites leave impressive craters. Use reference materials and the Internet to identify the ten largest impact craters on Earth. Make a table that shows the crater's name, location, estimated age, and diameter.