The first posting in this series explained how immense the interstellar distances are, and how, as a consequence, any spacecraft would have to travel at a very high speed to get even to our nearest star system in a reasonable length of time, say a century or less. But at such a high speed, hitting even the tiniest dust particle could be disastrous. Let’s explore that.
Interstellar space is very empty, a more complete vacuum than is typically achieved on earth in a vacuum chamber. But interstellar space is not completely empty – it contains a very thin mixture of gases and small dust particles, and very occasionally something larger. We see those occasional larger items as “shooting stars” at night when they burn up in our atmosphere. So anything traveling at high speed through interstellar space for many decades is bound to occasionally run into something, even if it is only a tiny dust particle.
Now since the speeds involved are a significant fraction of the speed of light, relativistic effects begin to appear. Masses increase as we approach the speed of light, though at only 10% the speed of light the change is less than 1%, so we can ignore it and use the standard Newtonian equation for the kinetic energy of an object, which is K.E. = ½ m v2, or kinetic energy is half the mass times the velocity squared. It’s that last term that is the problem for us, the velocity squared. At, say, 10% the speed of light our velocity is already immense, and then we square it!!
And that assumes the particle we run into is standing still relative to us – if it is coming toward us its speed is added to ours in that last term squared!!
So lets put some actual numbers to this, to see what the implications are.
Estimates are that most interstellar dust particles have a mass somewhere between 10-16 kg (0.1 pg.) and 10-4 kg (100 mg.). A standard-size paper clip weighs about 1 gram, so let’s assume a tiny dust grain 1/100 of the size of a paper clip, or 1 mg. And let’s assume we are travelling at 10% the speed of light, or about 30,000 km/sec. Then the kinetic energy at impact of this little dust particle on our little space probe is about 4,500,000,720 Joules.
How much is that (for those who don’t typically think of the world in terms of Joules)? Well, if we fired a typical 180 grain bullet at our space probe from a .357 magnum handgun, it would deliver about 720 Joules of energy, and no doubt thoroughly wreck the space probe. But at 10% the speed of light, impacting our little 1 mg dust particle will do over 6 million times more damage!
If our little interstellar probe is going to survive a century-long trip to Alpha Centauri, it is going to need a massive shield in front of it to protect it from such impacts, and the mass of that shield will have to be accelerated along with the rest of the probe, at an enormous cost in energy. So that is problem number 2.
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