Open Flat Closed44 ASTRONOMY • JUNE 2015A: If the Barringer Meteor
Crater impact event in Arizona
occurred in a modern city, it
would completely destroy it. As
the question implies, distance
from the point of impact is
directly related to one’s sur-
vival. The key to determining a
safe distance lies in the energy
of the impact event. Estimates
of that energy exist, but the
range of uncertainty can have
significant consequences. If the
energy was sufficiently small,
one could have had a spectacu-
lar view of the impact event
from Anderson Mesa, a long
volcanic ridge about 15 miles
(24 kilometers) west of the cra-
ter. However, for some of the
larger energy estimates, that
location may have been
uncomfortably close, if not
deadly. I have often thought
that a very nice — and safe
— vantage point would have
been Mount Elden, a towering
volcanic dome in Flagstaff
nearly 40 miles (60km) north-
west of the crater.
The impact produced a
shock wave and air blast thatradiated across the landscape.
If the impact energy was 20
megatons, it was immediately
lethal for human-sized animals
within 4 miles (6km) of the
impact. A sharp change in
pressure caused by the shock
wave produced severe lung
damage within 6 to 7 miles (10
to 12 km) of the impact. Winds
were also catastrophic, with
speeds in excess of 900 mph
(1,500 km/h) within the inner
4-mile-diameter zone and still
more than 60 mph (100 km/h)
at radial distances of 12 miles
(20km). Those winds would
have picked up debris and
hurled it across the landscape
like a shotgun blast. Mam-
moths, mastodons, and giant
ground sloths were among the
unfortunate victims of the
impact event. Let’s hope we
are able to mitigate future
events of that size and larger
so that we never have an
opportunity to witness them
from any distance.
David Kring
Lunar and Planetary Institute,
HoustonAstronomy’s experts from around the globe answer your cosmic questions.
BLAST RADIUS
Q: HOW DO WE KNOW
SPACE IS FLAT?
Dan Vos
Cary, North CarolinaA: We measure how curved
space is the same way the
Greeks measured the size of
Earth: using trigonometry. In
this case, the method is known
as a “standard ruler test.”
If I take a ruler of known
length and move it away from
me, it subtends a smaller and
smaller angle. How rapidly the
angle decreases depends on
how curved the space is. If I live
at the north pole of a sphere
and do this, the angle decreases
at a different rate than if I do
this on a f lat sheet of paper.
(Sometimes it’s easier to visual-
ize this by holding the angle
fixed and seeing how big a
“ruler” you need, but we do the
measurement the other way.)
By using the physics of
sound waves in the early uni-
verse to define our “standard
ruler,” we are able to do this
test on the largest observable
scales with the European Space
Agency’s Planck spacecraft.
In practice, we need several
distance measurements to
take out confounding effects
of other factors (such as howrapidly space is expanding),
but in the end, we find that
space is flat to the limits of our
capability to measure it.
Martin White
University of California, BerkeleyQ: HOW WERE THE
CONSTELLATION BOUND-
ARIES SET? THEY APPEAR
PRETTY ARBITRARY.
Robert Bobo
Mckenzie, TennesseeA: If you think the constella-
tion boundaries look arbitrary
now, consider the situation a
century ago. No official star
chart existed, and those in
common use not only set dif-
ferent boundaries for the star
patterns but also all had differ-
ing numbers of constellations.
Some had as few as 70, and oth-
ers topped out near 100.
Astronomers first formally
addressed these problems in
1922 at the initial meeting of
the International Astronomical
Union. They formed Commis-
sion No. 3, headed by Belgian-
French astronomer Eugène
Joseph Delporte. His task was
to scientifically define the
constellations and assign their
boundaries.ASKASTR0
Q: HOW CLOSE COULD YOU BE TO
WITNESS ARIZONA’S METEOR CRATER
FORM AND STILL LIVE TO TELL THE TALE?
Matthew Petty, Scotts Valley, CaliforniaBarringer Meteor Crater in Arizona formed when an asteroid hit about
50,000 years ago. The blast sent catastrophic winds out at some 900 mph
(1,500 km/h) across the immediate area, instantly killing animals unfortu-
nate enough to witness the impact close up. METEOR CRATEROur universe is freaky flat. How do astronomers know? The cosmic micro-
wave background, which constitutes the oldest light in the universe,
would appear different depending on curvature. Through decades of
observations, scientists have learned that our cosmos is as flat as a coun-
tertop to the extent spacecraft can measure. NASA/WMAP SCIENCE TEAMHow our universe got its spots