In reference to our recent post on generation ships, a commenter addressed an important issue: the amount of resources in interstellar space. Most people will probably know that the Solar System contains enormous amounts of resources. But it’s a common perception that space between stellar systems is very empty.
Emptiness is space is, of course, a very relative concept. A cubic meter of average interstellar space does contain much less matter than the best human made matter. This is mainly due to the very large distances between material objects in outer space.
But an important question is where does interstellar space start, or more accurately what are the boundaries of our Solar System? Unfortunately this is not a question with a definite answer. Basically there are two “popular” ideas of what constitutes the boundary of our Solar System: the heliopause and the Oort cloud.
When last summer it was announced that the Voyager 1 had “left” our Solar System, scientist referred to the fact that this space probe that crossed the heliopause. But what is the heliopause? The heliopause is the outer-boundary of the heliosphere, which turns us to the question what is the heliosphere?
Consider this analogy: if you let water flow from your tap into the sink, you will notice there is a kind of “gap” in the layer of water at the bottom of the sink. A similar thing happens with the Sun. The Sun emits constantly all kind of stuff, the so-called Solar winds. We can compare the Sun with your tap, and the Solar winds with the water flowing out of the tap. But just in case of your tap, the Solar wind will ultimately encounter a “thick” medium, and this medium is called the heliosphere. The inner-boundary, where the Solar wind is slowed down, is known as the termination shock. But after a certain distance this thick medium will end, which creates an outer-boundary for the heliosphere: the heliopause.
Because the Solar winds are effectively terminated at the heliopause, many scientists see this mark as the boundary of our Solar System.
The heliopause is located at approximately 119 astronomical units or AU (1 AU is equal to the average distance between the Sun and the Earth) from the Sun. In comparison the orbit of Pluto is located between 30 and 49 AU. Pluto is now considered to be a member of the so-called Kuiper belt, a collection of Trans-Neptunian Objects orbiting the Sun at between 30 and 50 AU form the Sun. Hence the heliopause seems to be a fair choice for the boundary of the Solar system.
But the gravitational field of the Sun has a reach far beyond the heliopause. The orbit of Sedna around the Sun has an estimated perihelion of 76 AU and a perihelion of 943 AU, and hence is large part of Sedna’s orbit is well beyond the heliopause. In fact the Sun’s gravity is in no way affected by the existence of the heliopause. Actually if there were no other stars in the universe, the Sun’s gravity would reach to infinity, though it would still decrease according to an inverse square law.
Astronomers hypothesize that there is a cloud around the Sun, at around 50,000 to 100,000 AU distance, which referred to as the Oort cloud. The Oort cloud is believed to house trillions of objects large than 1 kilometer. All these objects are orbiting the Sun, but what would be the largest possible distance of an orbit around the Sun?
Since the nearest star is roughly the size of the Sun, and is located at about four light years away, the point at which both stars exert an equal gravity is located at about 2 light-years or 125,000 AU. If other star systems have also Oort clouds, those clouds might overlap each other. Consequently a group of Space Settlers which is moving outwards in small increments might enter another star system even without notice.
But even beyond those Oort clouds interstellar space is far from empty. Astronomers speculate that the number of rogue planets, i.e. planets who do not orbit any start but directly around the center of the galaxy, is twice to 100,000 times greater than the numbers of stars in our galaxy. Though the closest known rogue planet is located at about 100 light-years away, there is no particular reason to assume such planet could not be found closer to our Solar System. Only because rogue planets do not radiate much light, they are hard to detect.
For interstellar colonists such a rogue planet might be an attractive place to settle. Enough resources for hundred or even thousands of years, and while the rogue planet continues its way through the galaxy the might came close the another star system. At that point they could make a jump to that star system, if they would wish so.
Though interstellar space is far from empty, we should nevertheless realize that interstellar travel and colonization is an endeavor for the far future and it will probably not happen in this century, maybe not even during the next thousand years. Instead current plans for space colonization should focus on our own Solar System.