14 ASTRONOMY • JUNE 2015I
t’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 pro-
duce tsunamis that would dev-
astate coastal cities and com-
munities. 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 bul-
lets encounters our atmosphere,
it disintegrates harmlessly as
a meteor. Much more rare are
fist- to boulder-sized rocks that
produce spectacular, excep-
tionally 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 seriousOBSERVINGBASICS
BY GLENN CHAPLEAsteroid
awareness
Honor the inaugural Asteroid Day by learning the facts
about a potential impact.damage. According to NASA,
one or two of these car-sized
bodies hits Earth’s atmosphere
each year. Astronomers esti-
mate 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 gener-
ated by the atomic bomb
that destroyed Hiroshima
leveled 800 square miles
(2,000 square km) of forest-
land. Fortunately, the impact
site was largely unpopulated.
More recently, though,
Russia was the unlucky recipi-
ent of yet another space rock,
this time over an inhabited
area. At 9:20 a.m. local timeFebruary 15, 2013, an asteroid
65 feet (20m) in diameter gen-
erated a brilliant fireball that
lit up the sky before explod-
ing 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 DA 14 sped past
our planet at a distance of
just 17,200 miles (27,700k m).
Perhaps I should correct my
opening sentence. It isn’t out
there. They’re out there!
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 gui-
tarist 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 anni-
versary 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 part-
ners of Asteroid Day.
Asteroid Day advocates
aren’t being alarmist; they’rebeing 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 http://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 http://www.Astronomy.
com/realreality.
Much of the work of locat-
ing 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 prop-
erly, “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 vis-
ible 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
[email protected]. Next
month: A look back in time at
the summer’s brightest stars.
Clear skies!FROM OUR INBOX
Amazing issue
I’ve been subscribing to Astronomy magazine for many years
and have always considered it the best magazine ever published
because it has such useful and interesting articles and infor-
mation. But your March issue with the “500 Coolest Things
about Space” is the best ever! Thanks to your staff for putting it
together. It’s an amazing presentation of all the wonders of the
universe. — Keith Gunnar, Langley, WashingtonBROWSE THE “OBSERVING BASICS” ARCHIVE AT http://www.Astronomy.com/Chaple.Scientists know the asteroid threat is
real. Even a 165-foot (50 meters) space
rock could cause regional devastation.
JAMES THEW/ISTOCK/THINKSTOCK