“The Size of Nine Ocean Liners”: Asteroid 1998 QE2

The sky is falling!  And it’s full of ocean liners!

On 30 May 2013 the press reverberated with the news that the Near-Earth Asteroid 1998 QE2, a monster “the size of nine ocean liners”, was going to sail by Earth (without docking).  This is a terrifying image: picture nine ocean liners falling from the sky!  What a splash that would make!  And some sources say it is long as nine ocean liners, and some say the width of nine ocean liners, and some say as big as nine ocean liners…well, whatever.

Of course the comparison of an asteroid with an ocean liner was motivated by the asteroid’s moniker (QE2), and should be understood as poetic license, or at least as unlicensed poeticism.  But is an asteroid of this size really a big threat?  True, it missed us by 5.8 million kilometers THIS time, but its orbit will bring it back across Earth’s orbit over and over again.  We can’t guarantee we will always be so lucky! 

The realists among us will appreciate that the news media routinely push the envelope of truth in order to generate scary headlines.  This is usually done by means of liberal use of adjectives (huge, gigantic, etc.; strangely I haven’t seen anyone refer to it as Titanic) rather than numerical facts.  So, assuming we know about the existence and usefulness of numbers, does QE2 represent a real threat?  Good question… and thanks for asking!

1998 QE2 is in fact about 1.7 miles in diameter, a pretty decent piece of rock, and has a 2000-foot satellite in orbit around it.  Now, let’s see: 1.7 miles is about 2700 meters in diameter, or a radius of 1350 meters and a volume of 10.3 billion cubic meters.  At an average meteorite or asteroid density of 3 tonnes per cubic meter, this is 31 billion tonnes.  Now let’s compare it to a big ocean liner—for example the Queen Elizabeth II:  displacement 44,000 tonnes.  So, by the difficult mathematical operation known as long division, we see that this asteroid (never mind its satellite) would deliver a mass equivalent to 700,000 ocean liners onto our unsuspecting heads.  WHAT?  The sensationalist media have underplayed the story by a factor of 80,000?

Why the disconnect?  Because of the vague use of the word “size”.  Some people use it to mean length, or area, or volume, or mass.  The length of an asteroid tells little about the size of a threat it presents—unless, of course, we know how to do arithmetic. 

But the real measure of its potential for wreaking havoc is its total kinetic energy.  Let’s take an impact speed of 16 kilometers per second as an example (many NEAs are moving a lot faster than that).  That puts it in the million-megaton  (1 teraton) league.  That would be comparable to 100 World War IIIs. 

Nine ocean liners?  Get real!