Some 66m years ago Earth was hit by a space rock reckoned to have been 10km across. The resulting chaos caused the extinction of dinosaurs and many other species, opening the way for the age of mammals－and ultimately humans. It also left a big hole in what is now southern Mexico. That hole is one of only three known of similar dimensions (the other two are Vredefort in South Africa and Sudbury in Canada). And this is odd. For, during the billions of years that Earth has had a solid crust, many more than three big asteroids might have been expected to have hit it. 

That thought led Brandon Johnson of the MIT and Timothy Bowling of Purdue University in Indiana, to wonder how many other craters have vanished, either by erosion or by being swallowed into Earth’s interior as its crust moves around, and therefore whether it is likely that some have survived and been overlooked. They have just published their analysis in Geology. 

Earth’s crust formed more than 4 billion years ago, but the oldest surviving blocks of it large enough to harbor craters date back only 3.5 billion years. What is known of the sizes and orbits of modern asteroids suggests that, if things have not changed over the aeons, about 14 big asteroids (defined as having a diameter of more than 7.4km, which would cause a crater at least 85km in diameter) should hit Earth every billion years. That means 49, give or take seven, over the past 3.5 billion years. Such impacts may have been more common in the past, when more big asteroids were around. Allowing for this, Earth would have been hit by 113, give or take 11, of them. Either way, a lot of craters are missing.

         A suspension bridge is a type of bridge that use overhead cables to support its roadway. The design of a suspension bridge is simple and straightforward, and uses several architectural techniques to distribute the weight of the bridge safely and evenly.

         The construction of a suspension bridge starts with its towers, which are usually located on dry land and anchored to bedrock. Once the towers are built, master cables are strung across them and anchored into the bank at both ends of the bridge. Next, suspension cables are strung from the master cables, and then the deck used as roadway is attached to them. Most of the weight of the bridge is transferred by the cables to the anchorage systems. These are embedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

         One of the oldest engineering forms, suspension bridges were constructed by primitive peoples using thick vines for cables. A much stronger type was introduced in India around 400 A.D. that used cables of braided bamboo. In the early nineteenth century, suspension bridges used iron chains for cables. Today, the cables are made of thousands of individual steel wires bound tightly together. A single steel wire, only 0.1 inch thick, can support over half a ton of weight without breaking.

         Light and strong, a suspension bridge could span distances from 2,000 to 7,000 feet, which is much longer than any other kind of bridge could. Its simple design allows high clearance under the deck, useful when the bridge is built over a major shipping waterway or a very deep gulf. A suspension bridge is also less subjected to collapse than some other bridge types such as those built on support pillars.