Caught between a rock and
a hard place
Cambridge & Great Eastern Regional Group: Secrets of the Space
Invaders Cambridge University, UK
 The
lecture opened arrestingly, with  newspaper
headlines from Friday 13 March 1998 warning of the collision
of an asteroid with Earth in 2028. Fortunately these apocalyptic
warnings were soon disproved. But could such a prediction
ever come true? What evidence is there that we should be concerned?
Where do these space rocks come from and what are they like?
And if there is a threat, can scientists and governments do
anything about it?
In a beautifully illustrated lecture, including the image
on the left (which is one of 170 meteoric craters identified)
Dr Jacqueline Mitton set out to give some answers to these
questions. Firstly, she took us on a tour of the solar system.
The vast space between the planets is populated by countless
chunks of rocky and icy debris, ranging from small specks
up to mini-planets several hundred kilometres across. The
larger rocky objects are asteroids and they reside mainly
between Mars and Jupiter. We now know of nearly 100 000 asteroids
and the count is still rising. Much farther out, beyond Neptune,
are the Kuiper Belt and the Oort Cloud, home of the icy objects
we know as comets.
Turning our eyes closer to home, Dr Mitton explained how
the pockmarked appearance of the lunar surface was due to
asteroid impact craters. Even closer to home is the 1km-wide
Arizona meteor crater, created about 50 000 years ago by a
100 000 ton iron meteorite 30m across.
 Most
terrestrial impact craters are less well preserved, but about
170 impact structures have now been identified. Growing scientific
understanding of past impacts (such as the Tunguska incident,
and the Chicxulub event which perhaps led to the extinction
of the dinosaurs) and the spectacular impact of Comet Shoemaker-Levy
with Jupiter in 1994, led to increased activity to identify
potential Earth impactors, and to prepare for such an eventuality.
By the mid-1990s, astronomers had succeeded in getting the
world to pay attention to their warnings. The media may not
be covering asteroid scares the way it did a few years ago,
but that does not mean that Earth is any less of a sitting
target for space rocks.
After the lecture the audience had the opportunity to see
and touch a piece of iron meteorite, created 4.5 billion years
ago, and to examine some of Dr Mitton’s books on popular
astronomy. The event proved very popular, particularly with
local astronomers. We now look forward to our outdoor astronomy
event — a tour of the Mullard Radio Astronomy Observatory
near Cambridge on 21 July (free admission but limited numbers,
so please reserve a place soon if you are interested).
By John Wilkins
Hon Sec
Cambridge & Great Eastern Regional Group
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