Rocket launcher is the key for us to escape from the gravitational pull of our home planet. Unfortunately, from the energy point of view, it is far from being efficient. Take Saturn V, the rocket launcher used in the well-known Apollo 11 mission as example. Chemical fuel alone contributed to 90% of the rocket weight, while the lunar module accounted for the remaining 10%. Worse still, the fuel required by the lunar module to carry out a round-trip to the Moon and back accounted for another 61% of the weight of the module. With space probes becoming increasingly massive and destinations becoming more remote, fuel consumption is escalating exponentially. The Titan 4B rocket employed in the Cassini-Huygens mission to planet Saturn had 99.5% of its weight coming from fuel. We can see how inefficient chemical fuel propulsion actually is.
In terms of efficiency in energy consumption, traveling among planets in straight lines not from desirable. Today, space probes customarily make use of Hohmann Transfer Orbits and gravity assists in their interplanetary endeavours. This method, though still quite costly, does help to save fuel to a certain extent. For instance, fuel occupied only 42% of the weight of Galileo, the probe that visited Jupiter several years ago. Scientists are now developing mathematical models that may allow space probes to travel among planets with minimal, or even nil fuel requirements. This model is called the Interplanetary Transport Network.
Much as water currents helping vessels to move across oceans, there exist pathways among planets that allow probes to glide through without expending much energy. These pathways are actually consequences of universal gravitation. Take the Sun and Earth as example. There are five points in space, called Lagrangnian Points L1 to L5, where the gravity of the Sun and Earth exactly cancels out each other. Objects residing at these points are in some sort of gravitational equilibrium. More intriguingly, probes can orbit about these Lagrangnian points even if no object is present there. With suitable initial speed, space probes can be put to orbit around L1 or L2. The collection of this kind of orbits constitutes the so-called interplanetary highways. Mathematicians found that these highways always exist whenever two celestial bodies are revolving around each other.
Any object in these highways will always be attracted towards or driven away by gravity from L1 and L2, without extra energy input. The Genesis probe recently launched by NASA to collect samples of the solar wind was traveling along such kind of orbits. Fuel accounted for only 4% of the weight of the probe, which revolved around L1 for two years before returning to Earth. Traveling by means of this method is more time-consuming yet saves much fuel.
Highways connecting different planets may even coalesce together, allowing space probes to jump from one highway to another. Mathematically speaking, these highways form a giant network of interplanetary superhighways for probes to speed along. Scientists are now planning to build a transfer station at L1 between the Earth and the Moon as the starting point to reach for more remote planets. If successful, interplanetary travel may no longer be a distant dream.