The solar system is littered with rocky debris, most of which is pea-sized or smaller. When one of these cosmic bullets encounters our atmosphere, it disintegrates harmlessly as a meteor. Much more rare are fist- to boulder-sized rocks that produce spectacular, exceptionally bright meteors called fireballs. Really large space rocks create bolides, which are fireballs that explode in the upper atmosphere (an airburst), or super-bolides that reach the ground as meteorites — in either case causing serious damage. According to NASA, one or two of these car-sized bodies hits Earth’s atmosphere each year. Astronomers estimate those that cross our orbit number in the millions.
While a strike by a 6-mile-wide (10 kilometers) asteroid can cause global extinction (just ask any dinosaur who was around 66 million years ago), a hit by even a small asteroid can wreak havoc. On June 30, 1908, an asteroid or small comet about 130 feet (40 meters) across and traveling at a speed of 33,500 mph (54,000 km/h) streaked into the atmosphere, exploding some 5 miles (8km) over Tunguska in the Russian tundra. An energy release equivalent to as much as a thousand times that generated by the atomic bomb that destroyed Hiroshima leveled 800 square miles (2,000 square km) of forestland. Fortunately, the impact site was largely unpopulated.
More recently, though, Russia was the unlucky recipient of yet another space rock, this time over an inhabited area. At 9:20 a.m. local time February 15, 2013, an asteroid 65 feet (20m) in diameter generated a brilliant fireball that lit up the sky before exploding 18.4 miles (29.7km) above the city of Chelyabinsk. The shock wave shattered windows for miles in all directions and injured nearly 1,500 people. The energy from the blast equaled 20 to 30 Hiroshima bombs. Coincidentally, on the same day as the Chelyabinsk incident, the slightly larger near-Earth asteroid 2012 DA14 sped past our planet at a distance of just 17,200 miles (27,700km). Perhaps I should correct my opening sentence. It isn’t out there. They’re out there!
Asteroid Day advocates aren’t being alarmist; they’re being realistic. As May notes, “We are currently aware of less than 1 percent of objects comparable to the one that impacted at Tunguska, and nobody knows when the next big one will hit.” Nye adds, “Someday humankind will have to prevent an asteroid impact. The first step toward protecting our planet is to find and track the swarm of space rocks that cross orbits with Earth.” For more information on Asteroid Day 2015, log on to www.asteroidday.org. Also, be sure to check out Astronomy Editor David J. Eicher’s Real Reality Show video “Why Asteroids Should Be Taken Seriously” at www.Astronomy.com/realreality.
Much of the work of locating and tracking near-Earth objects (NEOs) will be done by professional astronomers and amateurs with sophisticated equipment. The rest of us can pay homage to Asteroid Day by turning our telescopes toward Ceres, the asteroid (more properly, “dwarf planet”) currently being visited by the Dawn spacecraft.
This month, Ceres flirts with the 4th-magnitude star Omega (ω) Capricorni. At 8th magnitude, the asteroid is bright enough to be visible through the smallest of telescopes. To find Ceres, use a low-power eyepiece and star-hop from Omega to the predicted location of Ceres. Granted, the minute stellar speck you encounter may not be as visually inspiring as the images currently being sent by Dawn, but you might get a feeling of awe and wonder knowing that a craft from Earth is circling it. Whether you actively engage in an NEO search or simply gaze at Ceres from your backyard, have a great Asteroid Day!
Questions, comments, or suggestions? Email me at firstname.lastname@example.org. Next month: A look back in time at the summer’s brightest stars. Clear skies!