Jupiter could destroy us or protect us. The planet’s atmosphere has a layer that is rich in deuterium, and detonating a nuclear weapon in that layer might be enough to ignite all of it, creating a huge explosion that would destroy the side of Earth (and any other planet) that was facing it at that moment. A country, group, or crazed rich person could, with future technology, hold the rest of the world hostage with the threat of sending a nuclear space missile to blow up Jupiter. Moreover, if there were a future war between humans living on different planets (like Earth and Mars), one side might decide to detonate Jupiter when its own planet was directly shielded from the explosion by the Sun while the enemy planet was exposed.
Aliens could also detonate Jupiter to exterminate humanity from afar. However, humans could also blow up Jupiter as a sort of “dead man’s switch” that was meant to exterminate any alien fleet or civilization that was conquering our Solar System. We would kill ourselves as well, but at least we’d have the satisfaction of taking them with us and leaving nothing of value behind. Our mere threat of doing this might be enough to convince hostile aliens to leave the Solar System.
In the far future, when we start “mining” Jupiter, we’ll probably make it a priority to slowly siphon off the deuterium in its atmosphere, both to prevent this cataclysm and to fuel spacecraft.
One solution to the Fermi Paradox is that aliens keep quiet to avoid making themselves invasion targets for stronger, hostile aliens. This makes sense once you remember that “information is power”: The more information you have about someone else and the less they have about you, the stronger you are relative to them. The same holds true for intelligent species. If one species knows another exists, but not vice versa, then the first can choose when and under what conditions to make contact, or whether to make contact at all. Those are enormous advantages, particularly if the oblivious species has the ability to hurt the other one.
In short, it’s rational for intelligent species to keep as low a profile as possible, but to keep watch for aliens. That means reducing transmissions from their own planets and building telescopes and other sensors to search for aliens or signs of them. Sending cloaked probes to other star systems, containing downgraded technology and taking circuitous routes to mask their point of origin on the intelligent species’ homeworld, would also probably be a good idea. The probes could search other star systems for alien life even more thoroughly, and could build other types of space ships once there that could attack those aliens.
Part of keeping a low profile means not even revealing one’s self to weaker alien species. Even if they are too weak to hurt you directly, they can kill you indirectly by announcing your presence to everyone else. For example, if a flying saucer full of friendly gray aliens landed on the White House lawn tomorrow, it would be the news event of the millennium. All of our TV signals, many of which shoot into the depths of space, would broadcast the event and its aftermath. Any malevolent insect aliens who had kept a low profile on a planet within 100 light years of Earth would not only learn about humanity’s existence, they would also discover the gray aliens. Because they talked to us, the gray aliens might actually trigger a chain of events that led to their own planet being invaded by insect aliens decades later.
Another reason to colonize space is to establish a secure second strike capability. While aliens could secretly build up a space fleet to suddenly take over Earth before we could have a chance to attack their own planet, it would get exponentially harder with each additional planet (Mars, Venus, Jupiter moons) we controlled. since the alien attack would have to destroy them all simultaneously to prevent our retaliation. If human civilization were spread out among several star systems, exterminating us without suffering a severe, if delayed, counterstrike would be impossible.
Again, secret probes could be of use. If we smuggled them into multiple star systems, they could be programmed to retaliate against any aliens that attacked Earth. Once receiving the attack signal, the probes would build combat space ships, space guns with interstellar ranges, or other types of weapons, and then send them to attack the alien homeworld. This could turn into a multi-century “whack-a-mole” game where hidden probes activated at random intervals, in various star systems in our part of the galaxy, built weapons, sent them off to attack the alien homeworld, and then went into hiding again.
Secret space probes could also be used to take over the galaxy. Consider Ray Kurzweil’s hypothesized “two-phased attack” with self-replicating nanomachines:
How long would it take an out-of-control replicating nanobot to destroy the Earth’s biomass? The biomass has on the order of 1045 carbon atoms. A reasonable estimate of the number of carbon atoms in a single replicating nanobot is about 106. (Note that this analysis is not very sensitive to the accuracy of these figures, only to the approximate order of magnitude.) This malevolent nanobot would need to create on the order of 1034 copies of itself to replace the biomass, which could be accomplished with 113 replications (each of which would potentially double the destroyed biomass). Rob Freitas has estimated a minimum replication time of approximately 100 seconds, so 113 replication cycles would require about three hours.2 However, the actual rate of destruction would be slower because biomass is not “efficiently” laid out. The limiting factor would be the actual movement of the front of destruction. Nanobots cannot travel very quickly because of their small size. It’s likely to take weeks for such a destructive process to circle the globe.
Based on this observation we can envision a more insidious possibility. In a two-phased attack, the nanobots take several weeks to spread throughout the biomass but use up an insignificant portion of the carbon atoms, say one out of every thousand trillion (1015). At this extremely low level of concentration, the nanobots would be as stealthy as possible. Then, at an “optimal” point, the second phase would begin with the seed nanobots expanding rapidly in place to destroy the biomass. For each seed nanobot to multiply itself a thousand trillionfold would require only about 50 binary replications, or about 90 minutes. With the nanobots having already spread out in position throughout the biomass, movement of the destructive wave front would no longer be a limiting factor.
The point is that without defenses, the available biomass could be destroyed by gray goo very rapidly. Clearly, we will need a nanotechnology immune system3 in place before these scenarios become a possibility. This immune system would have to be capable of contending not just with obvious destruction but any potentially dangerous (stealthy) replication, even at very low concentration.
The Singularity is Near
An alien species could, over a long time and with great secrecy, seed every solar system in the galaxy with its own Von Neumann probes, which would contain self-replicating macro- and nano-machines. Once every solar system had a probe, the aliens would send out a signal, and all of the probes would start self-replicating. They wouldn’t just make “Gray Goo” copies of themselves–they might make soldiers, weapons, and other advanced technology. Any other aliens would be overwhelmed, or at least forced to reveal themselves to fight back.
If we could cheaply make antimatter, then we could make “nuclear bullets.” When matter and antimatter touch, they annihilate each other and convert all their mass into pure energy, described by the familiar equation E = MC2. That means just 1 gram of antimatter could create a 43-kiloton explosion, which is twice as powerful as the bigger of the two atom bombs dropped on Japan. The 5.56mm projectile fired from a standard U.S. military rifle weighs 4 grams.
A very powerful adaptation that our posthuman descendants will have is real-time control over their genes and gut bacteria. They’d have computer brain implants and biomechanical implants throughout their bodies. By simply thinking about it, they could tell their brain implants to alter their gene expression–maybe in a specific body part or organ–to do something like produce more of a certain type of chemical.