A Fact-Check for the Four-Color World

Monday, March 28, 2005

Kryptonian Astrophysics 101

Before I was a political science major in college, I did a stint with physics and astronomy. It's rare that I can put that to use, but this provides me with a limited opportunity.

In Superman: Birthright, Mark Waid tweaked Superman's origin story. Some of his biggest changes came in his alterations to Krypton. Not only did Waid tweak the look of Krypton, but he also made some changes to its physical nature.

Perhaps the most obvious of those changes was Waid's relocation of Krypton to another galaxy. Both Pre- and Post-Crisis, Krypton was located in our Milky Way Galaxy, about fifty light years from Earth. It was also within Green Lantern Tomar-Re's Sector 2813. This particular distance played a big role in Action Comics #600, when Kryptonite radiation reached the Earth fifty years after the planet's explosion, encouraging Superman to visit Krypton's remains. (Superman Adventures #3 also had a low-key and somewhat touching variation on this.)

Birthright moved Krypton to the Andromeda Galaxy, the Milky Way's nearest galactic neighbor. However, "nearest" is still pretty far in astronomical terms: Andromeda is 2.2 million light years away from the Milky Way (and with very little in-between). The in-story reason for this was to provide an explanation for why Krypton had not made any interstellar contact with other inhabited worlds. Jor-El states that Kryptonians had searched for signals of intelligent life for centuries, with no luck.

(This makes for an interesting question about DCU cosmology. The DCU's Milky Way is covered up in alien races. The DCU's Sol System alone managed to produce at least two inhabited planets. In Legion Lost (a mini-series which drove home the point of how far apart galaxies really are), aliens were found in another galaxy. DCU planets, at least in two galaxies, are a fertile breeding ground for life. So why does the Andromeda Galaxy, with twice as many stars as the Milky Way, have just one?)

But what works in service of one part of a story can create troubles for another. The Andromeda Galaxy is over 2.2 million light years from Earth, but is only 125,000 light years across. Jor-El is concerned about whether his son will survive the journey, but opts to send him to a place almost 20 times as far as any planet in his own galaxy. When Jor-El depressingly tells Lara that Earth is "far," he's not exaggerating.

In fact, it's so far that there's no possible way he could produce a picture of Earth like he does on page 10. Imagine taking a good clear photo of Pluto...if Pluto were 1.6 billion times as far away. It ain't happening. The light bouncing off the planet gets too diffused by that point to produce a visual image. It creates a dichotomy: Krypton is so scientifically advanced that it can take a snapshot of a planet in another galaxy, but is fairly incompetent when it comes to geology and space travel. (The Superman animated series had a novel solution to this seeming inconsistency in the Brainiac computer. The animated Krypton was fully scientifically capable of diagnosing the planet's problem, and of addressing it, but their folly was in trusting a machine that lied to them out of its own self-interest.)

And if Jor-El could produce a photo, one would assume he should've also known that the Milky Way was full of inhabited worlds. That may or may not be the case, because any information Jor-El had about the Milky Way would necessarily be two million years out-of-date. Any light or radio transmissions would have taken that long to reach Krypton. If Jor-El's photo of Earth had been even closer, he would've found the inhabitants to be Homo Habilis, the first human species. In past continuities, Jor-El picked Earth because of its hospitable environment. But in Birthright, Jor-El's only stated reasons are Earth's gravity and sun, both of which apply equally well to Mars or Venus, to name two.

(Continuity sidebar: The DCU's Mars had reached the peak of civilization hundreds of millions of years earlier, and was still going strong while Earthmen were just coming into existence in 2.2 million BC. Jor-El got an unbelievably good picture of Earth, but somehow failed to notice that the next planet over was a highly advanced civilization.)

The distance also means that Kal's ship travelled a long, long, LONG way to get to Earth. Since I'm skeptical about a prototype spaceship holding up for well over two million years in transit, the trip would have to involve some sort of warp speed or wormhole or other sci-fi travel device (as most recent continuities have used). Unfortunately, issue one doesn't directly support either of these. Rather, it shows the ship passing a bunch of planets, as if in normal space.

However, let's assume that Jor-El's ship *did* travel faster that light (through hyperspace or whatever). Let's say that little Kal was in suspended animation, and that the trip took 10 years. That would mean that the little prototype ship that could was travelling at a speed equivalent to 150 trillion mph, or about 220,000 times the speed of light. At that speed, you could travel from Earth to Pluto in about 1/14 of a second. Kryptonians may not know much about making spaceships, but they can sure make 'em fast.

Traditionally, Krypton's sun was a red giant star. Birthright officially established that the Kryptonian sun was a red dwarf star. Red dwarfs are fairly common, and at least one is known to have a planet.

But red dwarfs are also fairly unimpressive as suns go. They're small, only about 1/10 to 1/3 the size of our sun. And they're dim. Really dim. They put out less than 1% of the light that our sun does. Unless Krypton's orbit hugs its sun awfully tight, water on Krypton's surface would freeze.

A closer orbit would mean that when Jor-El refers to Krypton's history spanning "ten thousand orbits," that translates to substantially less than 10,000 Earth years. This page, addressing the red dwarf star I linked to above, puts the orbital period for an Earth-type planet at 24 Earth days. That would make Jor-El's "ten thousand orbits" into a mere 657 Earth years. The great era of Kryptonian civilization seems less impressive.

(Then again, Al Schroeder argued that Siegel's Krypton more likely had a red dwarf sun than a giant, but for evolutionary reasons.)

And while Waid made Krypton's sun smaller, he also tinkered with Krypton's makeup. Post-Crisis, Krypton was roughly Earth-sized, and as seen in Starman #51, it had gravity similar to Earth's. Jor-El states that Earth has gravity equal to 0.03 of Krypton's. This means that an object on Krypton's surface would weigh more than it would on the surface of our sun. We're talking powerful gravity here.

Unfortunately, without knowing Krypton's mass or size, it's impossible to say exactly what this means for the planet. If Krypton is made of the same rocky elements as Earth, and has the same density, then it would mean that Krypton's diameter is 33 times that of Earth. About three times the diameter of Jupiter. Or in other words, that Krypton is the same size as, or bigger than, its own red dwarf sun. Some gas giants may be able to reach that size, but solid planets can't.

The other possibility is that Krypton isn't much bigger than Earth, but is extraordinary dense. As in thirty plus times as dense. Unfortunately, normal matter doesn't get that dense. The Earth is the densest planet in our solar system, with an average density of 5515 km/m3. The densest known elements are iridium and osmium, with densities about four times that of the Earth's. Nowhere near thirty times.

Professor Jim Kakalios had a theory pre-Birthright to explain this quandary, and that was to suggest that Krypton had a neutron star at its core. And he suggested that this could also explain why Krypton exploded. I think that ends up making Krypton's gravity too strong (as in a couple of hundred thousand times Earth's gravity), but he's a physics professor and I'm a physics dropout, so I'll cede to him for now.

Krypton's extreme gravity would also result in a really nasty escape velocity for Kal's spaceship. Earth's escape velocity is 11.2 km/s. Krypton's escape velocity would probably be somewhere in the neighborhood of 600 km/s. Maybe that's why Krypton had such difficulty with spaceflight: they couldn't successfully get anything into space to start with.

And to close with, here are a couple of good articles on Superman and physics (in addition to those linked to above). They deal more in 'how could Superman's powers work' than I did, though.

The Science of Superman
The Science of Superheroes