Interstellar space is as close to empty as you can get; however, it is not completely empty. There are billions of comets out there and small worlds larger than the largest Main Belt asteroids. Recently a "tenth planet" larger than Pluto has been discovered. Perhaps more "Plutinos" exist. There are certain to be objects the size of boulders, rocks and pebbles floating around. These are rare and widely separated, but collision with a ten kilogram rock at 66% light speed could result in a 47 megaton explosion. Actually, the rock probably won't be stopped entirely and forced to release all that energy, but it could leave a gaping hole in the hull of the Star Carrier and demolish everything in its path within the ship until it exits by punching another hole through the opposite side of the hull. Airtight doors could slam shut throughout the giant vehicle to minimize loss of life. Even so, I don't want this to happen and I don't think anybody else does either.
Mapping the Treacherous Territory Ahead
Extensive mapping of the Kuiper Belt and Oort Cloud with giant radars, perhaps radars powered by nuclear pulses like Arthur C. Clarke's Excalibur from "Rendesvouz with Rama," and deep space probes could be conducted during the centuries that precede the construction of the NODE. Not only would Sol's Oort Cloud be mapped in great detail, but also the Oort Cloud of the Centauri system and other nearby stars. Interstellar probes by the thousands with powerful radars guided by AI computer brains that operate the spacecraft could be propelled across the interstellar abyss by smaller solar collector-propulsion beam systems built centuries before the NODE that propells Star Carriers through space is realized. Data on the positions of objects as small as a half a kilometer wide in the Oort Clouds of Sol and Centauri 3 will be of value to colonists who choose to live in deep space, away from the centers of civilization, as well as interstellar voyagers. The data will be stored in super computers, perhaps quantum computers, that can store quintillions of bits of data in the molecular structure of a memory module no bigger than a sugar cube, and safe paths through interstellar space will be determined. If a straight line to Centauri 3, launched at a given time, considering acceleration and deceleration, is found to intersect a slowly drifting comet, the time for launch will be changed to a more favorable one. It might be possible to map space in even greater detail when we have the ability to build replicating robots thereby making it possible to create swarms of machines in interstellar space built from cometary materials to study the routes that will be followed by starships in great detail as well as other regions of space. We have to know where all the comets are in order to spot comets that may dive in towards the Sun and damage the NODE years, even centuries, ahead of time. This mapping will be essential for many reasons. We could even spot comets that will impact inhabited worlds and space colonies with lots of lead time to divert the dangerous invaders from deep space.
Originally, I envisioned the NODE to consist of a partial Dyson sphere with shells above and below the solar equator suspended by photon pressure to allow light to reach the planets. The shells would collect solar energy and use it to power propulsion beams. This is to grandiose. I now envision the NODE to consist of mammoth solar powered particle accelerators orbiting within the orbit of Mercury to harvest solar wind gases or use mass from captured comets or the atmosphere of Venus to generate jets of mass in opposing directions to keep the accelerators on station as well as propell interstellar vehicles. Thousands of these devices would not be nearly as vulnerable as a ten million mile radius Dyson sphere but we would not want them to be destroyed by comet impacts nonetheless.
Starships will not slam into comets, asteroids, rogue planets or brown dwarfs; of this I am sure, thanks to this mapping project and long range planning with AI computers. Still, there will be small objects that could devastate a manned starship. Erosion shields made of layers of aerogel sandwhiched between layers of polycarbon 60 composite-the super material of tomorrow that even makes space elevators possible-and shield vehicles could absorb the damage done by stardust and gas during the years of racing through space. Magnetic shields could deflect small objects. In the coldness of interstellar space, most matter will become paramagnetic, thus it will be possible to influence matter with magnetic fields surrounding the ships and deflect rocks and pebbles. Lasers could also ionize any rocks or pebbles in the way and then magnetic fields could deflect them. What if we run into a boulder? Could we generate magnetic deflector fields strong enough to deflect such an object? If we don't, the vessel will be destroyed.
What Are Our Chances?
Maybe we shouldn't even worry about this. We take chances every time we swallow something. Blood vessels in our brains might pop during a fit of anger. Levees can break, aircraft engines can fail and buildings can collapse. However, I envision a world of the future where addictions and mental illness are cured, and criminality is rare. When there is a crime against property we will be living in a world so rich thanks to robotic labor that forgiveness will be easy. Material progress will make better people out of us by making us less posessive about our possessions; yet, we will own things that will be irreplacable. Electronic home and personal (wearable, implants) protection systems that summon the police, robotic (flying robotic devices?) as well as human, will be universal and serve as strong deterrents to crime. Computers will drive monorails and cars, subsequently accidents will be non-existent. Computers will fly airplanes and mid-air collision will be non-existent. By non-existent I actually mean incredibly rare as compared to the rate of traveling disasters that occur with human drivers and pilots today with all their imperfections. The people of tomorrow will live in a world so safe thanks to technology and advances in psychology and medicine that they won't be willing to take preventable risks involving space travel, so we should be concerned with avoiding danger in space.
Interplanetary ships will benefit from intensive mapping of planetary orbital spaces and solar system space as well as computerized autopilots that will fire thrusters and move spaceships out of the way as soon as a threataning meteroid is detected by long range radars and ship radars. A collision with a rock in interplanetary space at 30 kilometers per second can also devastate a spaceship and must be avoided. Collision with an asteroid or piece of space debris would also be unforgivable given forseeable technology for interplanetary travel! The point is that super-rich genetically enhanced humans of the future with lifespans of several centuries, genius level intelligences, artistic talent, the ability to hibernate given to them by their ancestors so that they can enjoy space travel, freedom from deformity and inherited disease, physical beauty, and more will not be willing to take insane chances with their lives!
Laser Protection
I doubt that it will be possible to locate every rock and pebble in interstellar space even with our fantastic technologies of the future. Maybe we will locate and determine the orbits of objects down to a few hundred feet in diameter in the immense volume of space surrounding Sol, Centauri 3, Barnard's Star and more. But what about the smaller objects? I am sure that collisions will be rare, but they will be awesomely destructive. What can we do? The LASER is the only answer. Radars aboard the starships will spot threatening objects and vaporize them with high energy lasers. The vapors will be deflected by magnetic fields or absorbed by physical erosion shields. Shipboard radars will scan space for hundreds even thousands of millions of kilometers around and ahead of the interstellar vessel. Computers will determine whether or not a collision is inevitable. Fusion thrusters might move the starship out of the way of the object, but this will not be practical with large Carriers. High energy lasers, probably big free electron lasers, will be needed to zap the navigational hazards.
Most objects in interstellar space will be moving very slowly as the gravity of nearby stars out there will be weak, so they will be almost stationary. This means a bow laser simply pointing forward could zap things in the way of the vessel like pebbles. If something significant far away will drift into the same space that will be occupied by our ship in a certain amount of time, we can only hope that the vessel's radars will detect the object and lasers will zap it. If we have a one million ton bank of fusion reactors to power our Rama class Carrier (see Rama), we will have quite a bit of power to energize the lasers. A fusion rocket engine might have 3500 kW to 4000 kW per kilogram. Fusion electrical power plants will have lower power densities because of the weight of turbogenerators, compressors, pipes, radiators, MHD, thermionics, etc. We might make the pipes and turbine casings out of titanium and use ceramic blades or make everything out of C60 fullerenes to really reduce mass. About 500 kWe to 1000 kWe per kilogram of system mass seems like a safe estimate for the spaceship fusion electric power plants of the future. A one million ton bank of power plants would generate 500 billion to one trillion kilowatts. If 200 kilowatts is devoted to each of 40,000 passengers in our Rama class vessel, about sixteen times as much as the modern American uses in all forms of energy, we will be using eight million kilowatts (or kilojoules per second). That leaves us with may billions of kilowatts for the lasers. The magnetic deflectors won't use much energy because they will be superconducting and charged up ahead of time to protect the ship throughout the voyage. In one second, three billion kilowatts equals the energy of 700 tons of TNT!!! Certainly, that can bust up boulders with big free electron lasers powered by these huge fusion generators! What we really want to do is vaporize them, and we have enough energy to do it. If we can spot the objects on an intercept course far enough ahead of time we can shine the lasers on the boulders longer and vaporize them. We could even charge up capacitors or electron plasmas with enough energy to discharge intense laser beams in a few milliseconds and vaporize threatening objects so we don't have to run the fusion reactors at such a high power level. It might even be possible to devote some power to a laser shining continuously to vaporize small objects the size of pebbles which could do great damage at high fractions of light speed. The vaporized rocks would become gas that is deflected by the magnetic shield. We won't waste time and laser energy on objects that will come close but not collide with the vessel and its magnetic sails. The computers will think faster than any human ever could and make these determinations.
Danger is Unavoidable?
Since it may be less likely to collide with a meteroid in interstellar space at 66%c than it is to be struck by lightning in the first place, the whole anxiety provoking situation may be nothing to worry about. Add some intense mapping of space, erosion shields, lasers and magnetic deflector shields and things may be even less worrisome. Cosmic mapping, route planning, radars, evasive maneuvers, magnetic and physical shields/deflectors, redundancy (IE back-up mag sail loops), partitioning (air-tight doors, sectional mag-sail loops), automatic in-flight repair (robots that can patch any hole and robots that can cut, mend and splice severed mag-sail loops and cables) seem like plenty. The hulls might even have a layer of nano-bots that can seal up a hole quickly.
Interstellar space is so empty that we could be making much ado about nothing, but the idea of colliding with a one gram pebble at 66%c that results in a 7 kiloton explosion really ups the bet!!! In all likelihood, collision with small objects will not result in atomic blasts. Chances are that small objects will penetrate without giving up all their energy and leave a tunnel of destruction through the vessel. Hopefully, small objects that evade the lasers and are not deflected by magnetic fields will be stopped by the shield vehicle or be refracted by or burn up in the layers of C60 and aerogel in the forward hull sections so that the passenger hull is not penetrated. The ablation material might be blown up but not the pressure hull. Damage to the sectional mag-sails will be repaired by robots in flight that replace damaged sections with spare lengths of cable. During acceleration and deceleration the mag-sail will be protected by its own magnetic field as well as the ship's lasers. During cruise it will be coiled up and stored.
Radar requires time for the microwaves to contact the obstacle and return. Some objects may absorb radar waves. Large telescopes on the CARRIERs that can see in the IR, UV and other parts of the spectrum could detect objects shining by the constant glow of starlight that comes from all directions. An object one light hour away will be seen when it is one light hour away by these electronic eyes. More accurately, we will see where it was an hour ago, but chances are that the object will be drifting slowly and with a few observations its real position and path could be determined quickly by AI computers. That would give a starship traveling at 50% light speed two hours warning. Besides thousands of robotic radar stations in the Oort Cloud mapping it in great detail there could be numerous machines using nuclear pulses to power lasers like Sir Clarke's Excalibur from "Rendesvouz with Rama" illuminating the interstellar chasm with flashes of light or other radiations brigther than ambient starlight for fractions of a second while arrays of telescopes study the reflections and locate innumerable objects. The electronic eyes of the telescopes would feed data to computers. With just a few flashes the orbit of the objects could be determined by the mighty thinking machines. If this goes on continuously objects whose path has deviated due to gravitational perturbations can be located and their positions determined and predicted in advance. If we know the positions of the "Excaliburs" and radar stations in space at any time on their predictable orbits about which data is stored in the ship's computers they can be used as navigational beacons as well as hazard warming systems.
Considering our concern about orbital debris and our ability to track thousands of satellites and burned out boosters today it is only logical that we will extend this ability to regions of space traveled by humans and valuable robotic vessels in the future. The sky seems so vast and empty that one could fly through it blindfolded, yet we watch the skies with radar to prevent mid-air collisions. Who would have imagined today's air traffic control systems at the time of the Wright brothers? Ships have collided at sea and radar is now standard equipment. Remember the Titanic? Mapping space in fine detail, ship radars and telescopes, shield vehicles, erosion shields, in flight repair systems and back up systems like spare mag-sails, etc. don't seem like overkill to me. If we want to reach other star systems alive and behold the beauty of an alien sunrise or extraterrestrial life form why leave everything up to fate?
