I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.
Saturday, April 18, 2015
Star Wars teaser far too tame
The second biggest thing to hit the internet yesterday was
the new Star Wars teaser. Like many, I clicked the link with interest. But as a planetary scientist, I was
disappointed from the first scene (via io9).
This is a wrecked star destroyer, half buried in desert
sand. The obvious implication is that
the spacecraft has left space and crashed.
Is this realistic? luckily,
physics, and the internet, can answer this question.
According to this fan site a star destroyer weighs something on the order of 30-50 million metric tons.
This makes it about 3000-5000 times larger that the meteorite which blew up over Chelyabinsk.
If we assume the slowest possible re-entry, that from low orbit (about 8 km/s
on Earth), then we can calculate what sort of impact this would have. Better yet, we can use the internet to let
the experts calculate it for us.
The Earth Impact Effects Program, by Marcus, Melosh, and
Collins, simulates the effect of impactors of various sizes on Earth (our
trusty stand-in for human-inhabitable worlds around the universe). Simplifying
a star destroyer to a 1 km sphere with a density of 100 kg/m3 gives us the
correct mass and a sensible size.
Falling from low Earth orbit, this object would need to dissipate 1.68 x
1018 Joules into the atmosphere or ground. That’s about 400 megatons, or about 8 times
more energy than the Tsar Bomba, the biggest nuclear weapon ever detonated. Given a shallow impact angle, this object
explodes in the atmosphere, raining small debris down onto the ground.
This, of course, is exactly what happened when real
spacecraft suffered uncontrolled or malfunctioning re-entry: Skylab and the
space shuttle Columbia
(at ~70 tons, almost a million times smaller than a Star Destroyer) both broke
up high in the atmosphere, raining debris down over very wide areas.
Of course, the die-hard fan might claim that the Star
Destroyer is much tougher than a 20th century spacecraft, and would reach the
ground intact. In this case, the kinetic
energy would be adsorbed by the ground, not the atmosphere. We can simulate that as well, by using a solid
iron meteorite of the same mass (only 232 meters across, due to the higher density), with a vertical
descent. It still imparts 400 megatons
of kinetic energy on the planetary surface. But instead of an airblast, we end
up with a crater 4.5 km in diameter, and half a km deep. Nothing of this scale is evident in the Star
As shown in the Chelyabinsk post a few years ago, the speeds- and energy- associated with space travel are
so huge that even the most creative minds of Hollywood are unable to grasp their enormity
and power. This was forgivable 30 years ago, before the internet, but in this
day and age, fantastical videos that are tamer than reality are disappointing.