On February 15th, a near-Earth asteroid passed within 17,200 miles of our planet's surface (27,600km). The asteroid, formally designated as 2012 DA14, is an estimated 150 feet across (or about 45 meters wide), which is about half the size of a football field. It is composed of approximately 140,000- tons of stone (instead of metal or ice).
Although 17,000 miles is decently close to Earth, the asteroid was not at risk of colliding with Earth. However, it still captured NASA's attention. This was one of the closest encounters between Earth and a near-Earth object since the 1990's, when we began many sky surveys such as NASA' Near-Earth Object Program. These programs are meant to monitor each and every large chunk of rock that could pose some sort of a threat to our little blue marble. Ultimately, this close pass gave NASA and other space agencies a rare opportunity to collect data on a near-Earth asteroid.
Most asteroids and comets originate in the asteroid belt that is between Mars and Jupiter. These objects come in all sorts of shapes, sizes, and compositions. Many of them are a few kilometers wide. Moreover, many of these asteroids were once comets that made one too many trips through our celestial neighborhood and their once brilliant tails were stripped away after one too many close encounters with our parent star... this process left behind the asteroids, long-dead husks of rock with negligible amounts of ice or metals.
For the sake of discussion, if 2012 DA14 were to hit Earth, it wouldn't be catastrophic (despite being capable of producing the equivalent of 2.4 megatons of TNT, if it were to impact Earth's surface). To figure out exactly what would happen, let's look at the size, shape, and mass of objects that have struck Earth over the last 4 billion years and compare those numbers with the estimated measurements of 2012 DA14.
- About 50,000 years ago, a meteorite struck Earth just about 33 miles (or 69 km) east of Flagstaff (in the northern Arizona Desert - USA), leaving behind what we now call the "Barringer Crater" (also known as Meteor Crater or Canyon Diablo Crater). The object was composed mainly of nickel-iron and was about 164 feet (50 meters) across. It hit the Earth at speeds exceeding 45,000 mph (72,000 kmh), releasing an estimated ten megatons of energy. The crater is about 1 mile wide and 570 ft deep (1.6km/ .16km). Since this is a desert, and since it occurred so long ago, we have little information on the impact to local vegetation.
- In 1908, a huge explosion rocked the Earth with the equivalent of about 185 Hiroshima bombs. A 120 foot rock entered Earth's atmosphere traveling at speeds exceeding 33,500 mph (36m/54,000 kmh) leveling out approximately 830 square miles (2,150 square kilometers) of land in Siberia and flattening over 80 million trees. Since this occurred in a forested area in recent times, we have precise information regarding the devastation that occurred.
Using these two examples (one larger object in the desert and one smaller object in a forest), we get a pretty good idea of what 2012 DA14 would look like if it hit the Earth. A lot of land would be leveled, and there would be a massive crater. If we were unlucky, and it hit a city, the death toll would be extreme. It certainly wouldn't eradicate all life on Earth (or even come close), but it could literally flatten an entire city, which still sounds rather unpleasant. However, considering that about 70% of Earth is covered in water, it is more likely that it would hit there (which brings up the possibility of a small-scale tsunami, but this would also only cause limited damage).
So fear not, we are safe (this time), but the asteroid will pass by us again next year. 2012 DA14 has an orbital period of about 366.2 days (that number changes from 366 days to 317 days in 2020), but since it has an orbit similar to Earth's, its path does take it relatively near-by twice per orbit. Still, it's quite spectacular to imagine an object zooming by us at a distance of about 0.000228 AU. Ultimately, this is closer than the satellites that we have placed in geosynchronous orbit around Earth. It will come close to us again in 2046, when it passes by us at a distance of about 0.0004 AU (60,000 km; 37,000 mi).