From the April 2015 issue

Asteroid awareness

Honor the inaugural Asteroid Day by learning the facts about a potential impact.
By | Published: April 27, 2015 | Last updated on May 18, 2023
It’s out there — an asteroid as big as a battleship whose orbit brings it dangerously close to Earth. At some time in the future — a few thousand years from now or perhaps within your lifetime — it’s destined to collide with our planet. If it strikes land near a large city, millions will perish. A touchdown at sea will produce tsunamis that would devastate coastal cities and communities. What I describe isn’t the fanciful scenario of some sci-fi disaster movie. It’s a fact-based astronomical prediction.

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!

Scientists know the asteroid threat is real.
Scientists know the asteroid threat is real. Even a 165-foot (50 meters) space rock could cause regional devastation.
James Thew/iStock/Thinkstock
Understanding the need to discover and monitor asteroids that threaten Earth, a group of scientists including the UK Astronomer Royal Lord Martin Rees, astrophysicist/Queen guitarist Brian May, CEO of The Planetary Society and “Science Guy” Bill Nye, and Apollo 9 astronaut Rusty Schweickart has promoted the adoption of Asteroid Day. Its date, June 30, 2015, coincides with the anniversary of the Tunguska event. The goal of Asteroid Day is to educate the world about what asteroids are, how frequently they impact Earth, and how we can protect ourselves from potential disasters. Astronomy is proud to be among the partners of Asteroid Day.

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 Also, be sure to check out Astronomy Editor David J. Eicher’s Real Reality Show video “Why Asteroids Should Be Taken Seriously” at

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 Next month: A look back in time at the summer’s brightest stars. Clear skies!