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 Wars teaser.

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.

Related post: Viewing Imaginary Spacecraft from the Ground"


BrianR said...

Why assume it crashed all the way from space? What if the Star Destroyer entered the atmosphere to land, flew around successfully for a while, and then had some failure only a short distance above the surface?

Anonymous said...

I have to agree with Brian. You're assuming a fall from orbit, when a controlled or semi-controlled crash is just as likely. Note the crashed X-wing fighter in the same shot, which would have completely disintegrated if it were to have fallen from space. And we know that Star Destroyer -sized vessels were designed to operate in atmosphere from the prequel movies.

Chuck Magee said...

In order to "cruise around in the atmosphere" you still need to lose those 400 megatonnes of kinetic energy. Assuming an 8 minute re-entry trajectory, this is about one tsar bomba per minute, on average, imparted to the air around the Star Destroyer.

Needless to say, this would be catastrophic for anyone underneath the reentry path, but the Empire has never been concerned about colateral damage.

Chris Phoenix said...

You're assuming that they don't have gravity control.

You're assuming they couldn't thrust at 1G.

I think it's safe to say that a technology base with light sabers, blasters, the Cloud City, heck even the Probot floating around on Hoth... probably has technology to soft-land a Star Destroyer vertically.

Any sufficiently advanced technology is indistinguishable from magic.

Tom said...

Star Wars Rebels, which is one of the few canonical sources right now, shows a Star Destroyer operating in atmosphere in its first episode. No mention of how long it took to get into atmosphere, I think.

Chuck Magee said...

Is that the same canon that shows Han shooting second?

Guardian said...

Tipping the hat:

Also, there's some corroboration of my density estimate you used, as discussed here:

I'll be following up on this with a further analysis regarding the ship crash in Episode III from non-orbital velocities.

I'll also probably discuss your post about viewing ships from the ground, a topic I have covered previously but without such clarity.