Death from the Skies! (29 page)

The Sun also becomes more luminous, as we’ve seen, but another critical change with its increased size is that its surface gravity gets lower. The gravity felt on the surface of an object depends on the mass of that object and its radius. When the Sun expands into a red giant, the mass stays the same but the radius increases by 100 times. This means the surface gravity will
drop
by a factor of 10,000 times. Currently, the surface gravity of the Sun is about 28 times that of the Earth (so that, for example, I would weigh well over two tons on the surface of the Sun
⁸¹
).

But when the Sun bloats up into a red giant, its surface gravity will drop to less than 1 percent of the Earth’s gravity. Any particle on the Sun’s swollen surface will only be very tenuously held on by gravity.

At the same time, the Sun’s luminosity increases by 2,400 times. Any square inch of Sun will be blasting out 2,400 times as much radiation as it did before the Sun swelled up. This light has momentum that it can transfer to a particle on the surface, giving it an upward kick. To a random atom of hydrogen on the surface of the red-giant Sun, it will be as if someone had shut off the gravity at the same time he had turned on a huge fan from below: particles on the surface will be literally lifted off and blown away.

This stream of particles, called the
stellar wind,
is similar to the solar wind, but far denser. In fact, the red-giant Sun will shed something like one ten-millionth of its mass every year, far, far more than it does now through the solar wind.
⁸²

This mass loss is so great that during the time it takes to swell out to red-giant status, the Sun will lose a significant fraction of its mass. Since its gravity depends on its mass, its gravity will also drop. The planets, feeling a lower gravity, will migrate outward; their orbits will become bigger as the Sun loses its grip on them.

It’s a race! Will the Sun increase in size quickly enough to engulf the inner planets, or will they migrate away from the Sun in time to escape its fiery maw?

For Mercury, the outcome is clear: doom. At 36 million miles from the Sun now, it is too far behind the other planets even when the starting gun goes off. After a few million years, the Sun will catch up and expand right past the planet. Mercury will literally be inside the Sun.

What happens to it then? Interestingly, the outer envelope of a red giant is almost a vacuum. The mass of the Sun is still roughly the same, but the volume increases hugely; when it becomes a red giant the Sun will have a million times the volume it does now, so its average density will drop by that amount. In reality the density in the outer layers is even less than that, because a lot of the mass of the star is stored in the core. In the end, the density is less than one one-thousandth of the density of the Earth’s atmosphere, almost a laboratory vacuum.

But there is matter there, thin as it may be. Mercury orbits the Sun once every eighty-eight days, so as far as Mercury is concerned it’ll be sweeping through stationary material. As it plows through this matter, what is essentially air resistance will slow its orbital motion in the same way a parachute slows down a skydiver. In just a few years, Mercury will slow so much that it will spiral into the center of the Sun, where the increasing density of matter will accelerate the tiny planet’s orbital decay. If it doesn’t vaporize first, Mercury will fall into the center of the Sun, where it will most certainly meet its doom.

Pfffsssssst!

Of course, if drag on the Sun’s matter slows Mercury down, the reverse is true as well: Mercury will accelerate the particles in the Sun’s outer layers. As Mercury slowly spirals into the Sun, it will speed up the Sun’s spin. It won’t be by much, just a percent or two. By the time the plunge into the heart of the star is over, the only indication that the solar system ever had a planet called Mercury will be a very slight increase in the Sun’s spin.

What of Venus? As it happens, our knowledge of how the Sun will expand into a red giant is still a bit too uncertain to know if Venus will evade getting eaten or not. Some models show it escaping, while others show it suffering the same fate as its little brother. Even if it does manage to stay outside the Sun’s greedily expanding surface, Venus is doomed. From just a few million miles away,
the Sun will fill Venus’s sky.
The surface of Venus is hot to start with—900 degrees Fahrenheit, thanks to its runaway greenhouse effect—but when the Sun looms so terribly over the Venusian surface, the temperature will scream upward to nearly that of the Sun itself. Venus’s crust will melt and its atmosphere will be blown away.

The Earth may fare somewhat better. Some studies show the Earth’s orbit expanding more quickly than the Sun, while others show us being consumed by the ever-growing star. Astronomers are still arguing over the details, which are important in this game of catch-me-if-you-can.
⁸³
Depending on the details of how the Sun expands and how much mass it loses, the Earth will end up being about 1.4 times farther from the Sun than it is now. The Earth is currently 93 million miles from the Sun, so when the Sun stops expanding that distance will increase to 130 million miles.
⁸⁴

Even if we escape being engulfed, don’t breathe a sigh of relief just yet: remember, the red-giant Sun is huge. It will fill a large fraction of the Earth’s sky, radiating down on it at a temperature of over
5,000 degrees Fahrenheit.
The Earth’s surface temperature at that point will be about 2,500 degrees, hot enough to melt nearly every metal and rock on its surface. Even before the Sun swelled up the Earth was quite dead, its oceans having boiled off and the atmosphere ripped away. But during the Sun’s red-giant phase, the crust of the Earth will melt as well, and that, pretty much, will be that. While it’s not literally the end of the world, it’s certainly the end of the world as we know it.

We still have room for one more “however,” however. While the Earth will be totally stewed, it’s not the only usable planet in the solar system. Mars too will move out from the Sun, but, unfortunately, will also be too hot for life. Even Jupiter’s moons will warm up too much to sustain us (the average temperature will be something over 500 degrees, hotter than your kitchen oven when you bake cookies). Jupiter’s moon Europa is an icy body, and thought to have liquid water under the surface. When the Sun expands into a red giant, Europa might entirely vaporize.

It’s possible that no existing place in the solar system will be cool enough to support life as we know it. Even the distant moons of Uranus and Neptune will be too warm. You’d have to be about 4.5 billion miles away from the Sun to get temperatures near where they are on Earth today. Of all the places in the solar system, in six billion years only the (currently) icy bodies slowly orbiting the Sun well past Pluto’s orbit may be cool enough for us. They would melt all the way through, becoming essentially giant drops of water a hundred or so miles across, with a red, swollen Sun glaring down on them. It’s known that currently these objects are loaded with organic chemicals. When those icy bodies warm up, all sorts of interesting things could happen to those chemicals. The bodies will stay liquid for hundreds of millions of years while the Sun remains a red giant, which begs the question:

What life might evolve under those circumstances?

At this point in the life of the solar system, things don’t look so good for the home team. The Sun is a swollen, distended blob, it’s eaten one planet, fried three others, vaporized a retinue of moons, and generally made things uncomfortable for almost everyone else.

But what are you gonna do?

Actually, that’s an excellent question. So far, this story has unfolded in this manner because we’ve let it. That is, if we sit back and watch, this is the way it will play out.

But it doesn’t have to be that way.

For example, it will take several hundred million years for the Sun to go from subgiant to giant. During that time, the temperature on Earth will be unbearable. And once the Sun does go all the way to giant, even Mars won’t look so good. But a billion years is a long, long time, and during that time Mars may be the place to be.

It’s smaller than the Earth, and has very little atmosphere. We can’t do much about its small size, but we can bring it air . . . by dropping bombs on it. Bombs in the form of comets.

Comets are large chunks of rock and ice, and some, in the distant outer solar system, are quite large, hundreds of miles across. They move so slowly in that far realm that it wouldn’t take much of a kick to drop a few into the inner solar system. Attaching a small rocket to one would do the trick. Letting smaller pieces hit Mars, one at a time, could easily bring enough water to fill ponds, lakes, and eventually oceans. Careful manipulation of its atmosphere, using genetically engineered plants and chemical processing, could encourage the development of breathable air. Some people estimate it might only take a century or two.

This type of practice, making a planet more Earthlike, is called
terraforming.
It’s a staple of science fiction, but it’s based on fact; the physics, chemistry, biology, and other fields of science involved are generally well understood. The devil’s in the details of course, but we have plenty of time to work them out. I’m guessing that in that dim future, billions of years down the road, a century or two here and there will hardly matter.

In fact, we’ll have the technology to start work like this on Mars much sooner than six billion years from now; realistically it could start early in the next century. Which raises the question: in six billion years, won’t we have terraformed all the planets? Perhaps. With an ever-burgeoning population, future humans will look across the gulf of space at all that real estate with envious eyes, and slowly and surely, as H. G. Wells once wrote, they will draw their plans against them.

Still—and stop me if you’ve heard this before—there’s no place like home. The Earth is a pretty good place, and we’ve spent a lot of time evolving here to make ourselves fit in. Is there no hope for our home planet?

Actually, yes, there is, and the solution is simple: we just need to move it farther from the Sun.

How hard can
that
be?

Okay, in practice, pretty hard. The main problem is that the Earth is a big, massive object, so moving it takes a
lot
of energy.
⁸⁵
To move the Earth out to where the temperature will be more hospitable (around Saturn’s current orbit) takes roughly the same amount of energy that the entire Sun currently emits in a solid year. That’s the equivalent of exploding 200 quadrillion one-megaton nuclear bombs.

There might be some environmental effects from that.

There are alternatives. We could strap a few million rockets nose-down onto the Earth and fire them off, but it’s hard to know where we’d get enough fuel for them. Plus, the Earth’s rotation and revolution around the Sun would complicate things (we can assume, however, that by the time we need to do this our technology will be up to the task).

But there’s a better way, the environmental impact of which—if we’re careful—is essentially zero.

When we send probes to the outer planets, we can give them a boost in speed by “borrowing” (really, stealing) energy from the orbital motions of other planets they pass on the way. This is the so-called
slingshot effect.
If we want the probe to speed up, we send it on a path so that it comes in from “behind” a planet, catching up to it as the planet moves in its orbit around the Sun. As the probe passes the planet, it picks up some energy from the planet’s orbital motion, which increases the probe’s speed. The planet
loses
the same amount of energy, and slows down a bit in its orbit. Since a planet is typically a lot more massive than a space probe, it slows down very little, an immeasurable amount really, while the probe can gain quite a bit of speed. This means we can send probes to the outer planets without having to carry vast amounts of fuel.

Taking energy away from a planet will drop the planet ever so slightly closer to the Sun. But, if we do this in reverse—send the probe in “ahead” of the planet—then the probe
loses
energy, giving it up to the planet. The probe slows and drops closer to the Sun (useful for getting probes to the inner planets, such as Mercury) while the planet gains energy, moving
outward
from the Sun.

If we want to move the Earth farther out from the ever-increasingly sizzling Sun, this is an excellent way to do it. Instead of space probes, we can use asteroids, which are much more massive. That means the energy exchange is greater, requiring fewer slingshots. Moving asteroids isn’t all that difficult; in that case strapping a rocket onto one would work pretty well. By aiming it just so, the asteroid could give some of its orbital energy to the Earth, moving the Earth just a teeny bit outward from the Sun. Lather, rinse, repeat . . . a million times.

This scenario has been studied by the astronomers Donald Korycansky, Greg Laughlin, and Fred Adams, and they found that by using a large but typical asteroid, such a maneuver could feasibly move the Earth slowly out to a safe distance from the Sun.

Here’s how you do it. Start with a large rock about 60 miles across that is well out in the suburbs of the solar system. Change its orbit using a rocket or some other method so that it drops into the inner solar system. Aim it (here a rocket would be useful for fine-tuning) so that it passes in front of the Earth, missing us by about 6,000 miles.
⁸⁶
The exact amount of energy transfer depends on a lot of factors, such as the angle of the incoming rock, how close it passes, and so on, but in general a single passage of a rock this size would add about ten miles to the Earth’s orbital radius.

That’s not much, of course, but at first it doesn’t need to be much. Small steps are okay; we have plenty of time!