Friday, 31 January 2014

Astrophysics Corner, Part 6 – Hitting Space Debris at High Speed

In the soon to be published Dodecedron Books novel  provisionally titled “Kati of Terra Book 3 – Showdown on the Planet of the Slavers”, the spaceship salesperson shows Kati and her team a neat trick:

“Like all space ships, this one is equipped with a field which repels and/or destroys space debris that can damage a vessel travelling at high speeds through regular space.  On most ships its operation is automatic, outside the pilot’s control.  A ship this small, however, has a field that can be turned on and off at slow speeds, and concentrated and directed to take out larger objects, should that be necessary.  If you throw this switch here,”—she demonstrated—“these red keys can be used to do that.

“Now, under attack, a desperate pilot can slow the vessel to a crawl, lure the attacking ship within range, and direct a powerful, destructive beam at its engine, or at some other vulnerable part.  It only works on unshielded ships so don’t even dream of using it on a Torrones, or any other kind of a warship.  And you only get one shot, so it really is a last resort.  But, I have heard the stories—sometimes the final, desperate manoeuvre is what saves your neck.  Keep it in reserve, in case you need it; if you want to be good at it, it is possible to simulate the process with the computer, and practise until you can do the finger work half asleep.”

So, just how much of a problem would it be to hit space debris? Plenty actually.  The energy that a mass carries goes up linearly with the mass, but by the square of its velocity, so a fast moving object can have a lot of kinetic energy in a collision, even if the mass is relatively small.  Below is a table showing the relative kinetic energy of some well-known phenomena, compared to space vehicles colliding with dust at various speeds, including some speeds approaching c (the speed of light, often denoted by “c”, is 300,000 km/sec or 186,000 miles/sec).

For space dust, I used the Wiki result for the size of near earth orbit dust, computed the mass of an object that size with a specific gravity of2.0, then divided that by 10 for interplanetary dust, and by 100 for interstellar dust, on the assumption that the particles that have been measured from near earth space are probably larger than those in deep interplanetary and especially deep interstellar space.  The other masses and velocities are also from Wiki, or just common sense (I think we can all make pretty good estimates for baseballs).

Interestingly, the data indicates that when the space shuttle or ISS collides with a dust particle, the collision should have about the same energy as a baseball being thrown in from the outfield.  I wonder if you can hear that in the ISS?  Perhaps if well-known ISS astronaut Chris Hadfield is reading this blog he can let us know.  A larger particle, such as a bit of space junk in low Earth orbit, could obviously pose a significant problem for those space vehicles.  For example, a 1 mm metal particle could have an impact energy roughly the same as a 10 gram bullet with a muzzle velocity of 300 m/s.  Note that assumes a high relative velocity between the dust particle and the space station – if, for example the particle was travelling in the opposite direction of the ISS.

mass (kg)
v (m/s)
v (km/hr)
v (mile/hr)
Energy (Joules)
Energy, relative to bullet
Small Shell
near earth space dust, hitting space shuttle
interplanetary dust, hitting Helios spacecraft
interstellar space dust, hitting 1% speed of light space craft
interstellar space dust, hitting 10% speed of light space craft

When we get to interplanetary speeds within the solar system (such as the Helios satellite, which swung around the sun at a very high velocity), the effect of hitting space dust would be that much worse.  The calculation shows that it would be like being hit by a bullet.  By the time we get to potentially achievable interstellar speeds, an impact with a dust particle would be even more substantial.  At 1% of the speed of light it would be like being hit by a small anti-tank shell, while at 10% of the speed of light it would be more energetic than a high mass, high velocity howitzer shell.

Things get worse if an object is moving at relativistic velocities, which just means that it is moving at some appreciable fraction of the speed of light (300,000 km per second or 186,000 miles per second).  That’s because as a physical object approaches the speed of light, its effective mass goes up.  In fact, as the object gets very close to the speed of light, the mass increases without bound - the function blows up, as would anything colliding with it, in a pretty spectacular fashion.  If a space ship was moving at a constant velocity at near light speed, when it hit a stationary dust particle, it would be the same as if a stationary space ship was hit by a dust particle travelling at near light speed.   A lot of energy would be exchanged in a very brief moment.  Here are a few examples of this:

·         At 10% of the speed of light, a 1kg mass would have a “relativistic mass” of 1.005 kg

·         At 50% of the speed of light, a 1kg mass would have a “relativistic mass” of 1.15 kg

·         At 75% of the speed of light, a 1kg mass would have a “relativistic mass” of 1.51 kg

·         At 90% of the speed of light, a 1kg mass would have a “relativistic mass” of 2.29 kg

·         At 99% of the speed of light, a 1kg mass would have a “relativistic mass” of 7.08 kg

·         At 99.9% of the speed of light, a 1kg mass would have a “relativistic mass” of 22.4 kg

·         At 99.99% of the speed of light, a 1kg mass would have a “relativistic mass” of 70.7 kg

At 99.9% of the speed of light, a collision with a dust particle would release tremendous kinetic energy, and this extra relativistic mass would just make things worse.
So, that pretty well finishes off the hope of interstellar travel, does it?  Well not so fast.

Within the solar system, a one week journey accelerating at 9.8 m/s (the acceleration of gravity on the Earth’s surface) would get your ship travelling at about 2% of the speed of light, and you would have covered nearly 2 billion kilometers, which would pretty nearly get you to Saturn and back.  A month of travel, with the first week accelerating to 2% of c, the next 2 weeks cruising at that speed, then the final week decelerating would allow you to cover 12 billion kilometers, which would get you nicely to Pluto and back.
If you could find an energy source to do that, travel within the solar system would be quite reasonable, rather like trans-Atlantic or trans-Pacific voyages of the late 19th century.  Actually, it would be better, since radio communications would still be near real-time (a day or so at most).  And probably ways could be found to protect the ship from dust collisions – perhaps a multiple-hull structure, like some ocean going ships, so that if one part of the ship  takes a hit, the damage is confined to that area without losing overall ship integrity.  The affected part of the ship could then be repaired in flight.  I don’t doubt that smart engineers could come up with something to handle these contingencies.

As for interstellar travel, most SF assumes something along the lines of warp travel, as in Star Trek.  Einstein’s laws of relativity only say that an object can’t exceed the speed of light relative to “local” space-time.  Various solutions to General Relativity exist, that allow for warping of space, meaning  that effective faster than light travel might be possible.  If so, your ship wouldn’t necessarily be travelling very fast at all within its local bubble of space-time, so all these concerns about hitting objects at high speed wouldn’t come into play.

Other ideas involve worm holes or higher dimensional spaces through which the traveller passes, before dropping back into regular space.  Distances in these higher dimensional spaces are assumed to be much shorter than those in the lower dimensional space, something like how the distance between two antipodes of a sphere (D) is shorter than the great circle distance on its surface (3.14D/2).   With this shortening of distances, the “speed” through these dimensions might not have to be that high, so again, collisions with dust particles might not be a major concern (if there were dust particles in higher dimensional spaces).  Obviously this is all highly speculative, but that’s one of the reasons that SF also stands for speculative fiction.

Anyway, those are some of the escape valves that SF generally uses to get around the problem.  For the foreseeable future, Science Fiction is going to need both science and fiction, to posit long range travel.  But, after all, it’s the combination of the intellect and the imagination that gives Science Fiction its power to inspire and fascinate us.
Kati 3 will be out in a couple of months.  You don't have to read Kati 1 and Kati 2 to enjoy Kati 3, but it's always nice to read an entire series.  You can get books 1 and 2 from Amazon or Kobo.  Kati 1 is also in print form, for those who prefer that.


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