by Laurance Doyle - Astronomer
In a previous article we talked about the possible impact of a comet or asteroid on Earth. This has happened before -- certainly at the Cretaceous-Tertiary boundary when the dinosaurs became extinct along with many other species.
The main problem (besides tidal waves a half-mile high and huge areas of burning forests) was that the dust thrown up in the atmosphere blocked out the Sun's light for an extensive time. Our planet runs on photosynthesis.
However, such impacts may not be a threat to our civilization in a couple of hundred years, as we should then have the technological know-how to detect and deflect such large potentially impacting bodies routinely.
Of greater long-term concern are: the evolution of the Sun, the Moons stabilizing influence and the evolution of nearby giant stars, as well as events on an intergalactic scale.
The Sun
A star is a giant balancing act -- in the case of our Sun, a balance between 80 billion nuclear explosions going off in its center every second and the force of gravity from its huge mass (330,000 times the mass of Earth).
At the moment -- and for the next 7 billion years or so -- the Sun will be happily fusing the element hydrogen into the element helium with the resultant energy heating the solar system.
However, when the hydrogen in its core starts to run low, gravity will collapse the core and the solar atmosphere will rebound, expanding the Sun into a red giant star larger than the orbit of Earth.
The Earth will be swallowed up in the Sun's atmosphere and more or less vaporized.
What can we do about this? As it so happens, the expanding Sun will push the circumstellar habitable zone about five times farther away from the Sun, warming the moons of Jupiter to habitable temperatures.
If we are not already occupying huge space stations drifting throughout the galaxy by then, we should have time to move to these moons (but it will be crowded). This state will only last for a few million years or so, but that should give us enough time to plan the move to Saturn's satellites.
The Moon
Of more immediate concern is the Earths rotational axis inclination.
Mars' magnetic poles likely flip around all the time -- up to a 90 degree tip can occur in less than a couple of million years.
Without the Moon, Earth, too, would probably flip around all the time, creating havoc with weather patterns and such. This is because the Moon stabilizes Earth's rotation orientation (so that the orbital momentum does not transfer energy to the rotational moment, to be precise).
However, the Moon is moving away (at about 1 inch, or 2.54 centimeters per year) from Earth and in about 1 billion to 2 billion years, it will no longer be helpful in stabilizing our axis orientation. How to stop this will be quite a challenge for future technology.
And the stars
The Earth, along with the rest of the solar system, is moving through the Milky Way Galaxy, completing one galactic orbit every 200 or so million years. Evident in galaxies like our own are beautiful spiral arms made of stars.
These are the places where stars -- including very large ones -- form. When massive stars finish burning all their core's nuclear fuel, they too collapse and rebound in a much larger explosion called a "supernova" (type II, for aficionados). A supernova puts out as much energy as a whole galaxy, so you would not want to be too close to one when it goes off.
There are different models for the spiral structure of our galaxy. Some theorize that the solar system must have crossed a few spiral arms at least 30 or 40 times. If so, a supernova should have gone off near Earth at least once every 100 million years over the past several billion years.
Perhaps there is evidence of this in the geologic record. A supernova would have certainly modified the upper atmosphere of Earth, perhaps turning large amounts of nitrogen and oxygen into nitrate compounds.
However, another theory of spiral structure, called the Long Density Wave Model, puts the solar system in a very privileged position.
In this model, the solar system hasnt crossed a spiral arm yet and, consequently, has avoided regular supernova contact. This theory postulates a galactic habitable zone based on a galactic orbit that "straddles" spiral arms -- a region that only constitutes about 6 percent of the entire galaxy.
Could only 6 percent of the stars in the galaxy provide a place for long-term habitability?
Intergalactic forces
Finally, in about 1.3 to 2 billion years, we are going to have to deal with a visitor.
For over 10 billion years the Milky Way Galaxy has been swallowing up other, smaller galaxies. Since galaxies consist mostly of empty space, their collisions will almost surely never cause stars to touch. However, they will feel each others gravitational force.
The beautiful spiral Andromeda Galaxy is thought to be slightly larger than the Milky Way, and it is heading right for us.
Within the next 2 billion years, the Milky Way will merge with Andromeda. While the stars will not likely collide, the tidal shake-up produced may be expected to rain comets from the outer portions of our solar system toward the Sun (and, of course, Earth).
This is likely to be one of the first large-scale events that a galactic-scale civilization (or civilizations) will need to prepare for in our home galaxy.
The future has some challenges to it and may require evolution and adaptation of species on a whole new scale --one that can plan ahead, for example. It's going to be an interesting next billion years for our circumstellar habitable zone. It might not be too early for us to start preparing for the next "gigennium."