It is almost impossible to find an accurate illustration of the scale of the solar system because nearly all fail to treat the orbit spacings correctly. A clever approach is to create scaled models that span entire cities and place markers in public places for each of the planets. An example of this is the NASA/JPL Scale Model shown here. Once you understand the distances involved, the magnitude of the challenge is made much clearer.

In my book ‘Interplanetary Travel: An Astronomer’s guide‘ I discuss current and planned technologies for interplanetary travel. The bottom line is that it depends a lot on the particular trajectory that you take. Usually, the trajectories are in the form of a ‘great arc’ that gracefully connects a launch time at Earth with a destination point. These arcs are usually many times longer that the straight-line distance between the two planets at a particular moment in time. For reduced-cost travel, astrodynamicists often rely on gravity assists ‘Slingshot Orbits’ from the inner planets to reach Jupiter, and by Jupiter to reach more distant worlds. These loop-de-loops add years of extra travel time to a mission. Let’s assume for our calculations that we just take the simplest direct approach and use the minimum ‘opposition’ distance between Earth and a planet.

The table below gives you some sense for how long it takes to get to each planet at different speeds.

The Space Shuttle, of course, can’t leave Earth orbit but its speed is typical of manned spacecraft. The Galileo spacecraft, which explored Jupiter traveled twice as fast. These travel times are based upon using a big chemical rocket on Earth to blast the spacecraft into the right trajectory. But there is another technology that has been used for several decades.

Ion rocket motors get their speed by being constantly accelerated 24-hours a day for many months, and two versions of this technology are given for a low-power and high-power ion engine. Ssatellites orbiting earth often use ion engine technology for station-keeping. NASA has also used ion engine technology on two interplanetary spacecraft: Deep Space 1 and Dawn. Both spacecraft used very low thrust engines operating for thousands of days to get the spacecraft to asteroids (Dawn visited Ceres and Vesta; DS1: 9969 Braille) and comets (Borrelly) for study.

Finally, and at least on paper, solar sails can reach speeds of nearly that of the solar wind (500 km/sec),. Engineers are even now beginning to space-test this technology.

As you can see from the table, we are currently stuck in the mode of travel where it takes nearly 10 years to get to Pluto. Perhaps in another hundred years, this travel time will be reduced to a year or less… assuming Humanity feels a compelling economic need to continue this kind of exploration.

Assumptions: Ion Drive using a constant thrust of A) 0.1 pounds B) 1 pound with turnaround deceleration added. Two years acceleration to reach top speed. Solar Sail estimated speed 450 km/sec or one million mph)

Another way to gauge the maximum speed of a spacecraft is by the exhaust speed of its engines. Engine exhaust speed is related to an engineering parameter called the Specific Impulse. SI is the exhaust speed divided by the acceleration of gravity at Earth’s surface. For example, chemical rockets have SI=250 seconds and so their maximum exhaust speeds are 250 x 9.8 m/sec2 = 2.4 km/sec. SInce no rocket payload can travel faster than its exhaust speed, we can compare planetary transit times in terms of the SI of the rocket technology.

In this table, I assume that the rocket continuously accelerates from Earth until it reaches half its destination distance, then turns around and decelerates for the second half of the trip. The relevant equations are

        distance = 1/2 acceleration x Time ^2
        speed = acceleration x time

With: distance in meters, time in seconds, speed in m/sec and acceleration in meters/sec^2

So if we could design a ship, call it System A, that produced a constant acceleration of a = 0.05 meters/sec^2, we could get to Mars in just 24 days. Similarly for System C with an acceleration of 0.2 meters/sec^2 the Mars trip takes only a week! For engineers,we can also estimate for System C that the Mars configuration would need SI=34,000 seconds to get us there in 8 days. If we wanted to get to Pluto with the same acceleration, it turns out that accelerating for half the 68-day trip would get you to a maximum speed of about 2900 km/sec which sets the limit to our exhaust speed and leads to a maximum SI of about 300,000 seconds! These SI estimates are far larger than the chemical rockets provide of 300 seconds, and so we have to look to entirely different technologies to make these travel times possible.

Sadly, it doesn’t matter if we drag along huge fuel tanks to run our chemical rockets. They will never provide the high exhaust speeds we need to carry both our fuel and payload to our destinations. We have to use technologies that enormously increase the exhaust speeds themselves.

So far as an astronomer understands the ‘witchcraft’ of spacecraft design, very light weight and durable alloys of titanium, aluminum and magnesium are commonly used. This picture of the ISS shows many different materials that have to be made space-worthy to survive the extreme conditions of temperature and pressure. (Credit: NASA: STS-132).

Aluminum – Most commonly used conventional material (used for hydrazine and nitrous oxide propellant tanks). Low density, good specific strength. Weldable, easily workable (can be extruded, cast, machined etc). Cheap and widely available. Doesn’t have a high absolute strength and has a low melting point (933 K).

Magnesium – Higher stiffness, good specific strength. Less workable than aluminium. Is chemically active and requires a surface coating (thus making is more expensive to produce).

Titanium – Melting point 1933 F. Light weight with high specific strength. Stiff than aluminium (but not as stiff as steel) Corrosion resistant. High temperature capability. Are more brittle (less ductile) than aluminium/steel. Lower availability, less workable than aluminium (6 times more expensive than stainless steel). Used for pressure tanks, fuels tanks, high speed vehicle skins.

Ferrous Alloys like Stainless Steel – Have high strength. High rigidity and hardness Corrosion resistant. High temperature resistance (1200K). Cheap Many applications in spacecraft despite high density (screws, bolts are all mostly steel).

Austentic Steels – Non-magnetic. No brittle transition temperature. Weldable, easily machined. Cheap and widely available. Susceptible to hydrogen embrittlement (hydrogen adsorbed into the lattice make the alloy brittle). Used in propulsion and cryogenic systems.

Beryllium – Stiffest naturally occurring material (beryllium metal doesn’t occur naturally but its compounds do). Low density, high specific strength High temperature tolerance Expensive and difficult to work Toxic (corrosive to tissue and carcinogenic) Low atomic number and transparent to X-rays Pure metal has been used to make rocket nozzles.

Inconel’ (An alloy of Ni and Co) – High temperature applications such as heat shields and rocket nozzles. High density (>steel, 8200 km m-3).

Aluminium-lithium – Similar strength to aluminium but several percent lighter.

Titanium-aluminide – Brittle, but lightweight and high temperature resistant.

In the near future we may use combinations of plastics and various hybrid materials such as metal-matrix composites that will greatly reduce the weight of a spacecraft, and greatly reduce launch costs. In fact, carbon fiber technology has already been used to replace many spacecraft components, except for the outer spacecraft bulkhead itself. Bulkheads are still made from titanium, aluminum or other conventional metals and alloys because of the tremendous thermal and pressure demands. The B-2 Stealth Bomber used carbon fiber materials in its wings, but the forces it would be required to take and the thermal loading are quite small compared to a spacecraft launched from the ground, or re-entering the atmosphere from space.

Fiber-reinforced materials such as carbon, aramid and glass composites have the highest strength and stiffness-to-weight ratios among engineering materials. For demanding applications such as spacecraft, aerospace and high-speed machinery, such properties make for a very efficient and high-performance system. Carbon fiber composites, for example, are five times stiffer than steel for the same weight allowing for much lighter structures for the same level of performance. In addition, carbon and aramid composites have close to zero coefficients of thermal expansion, making them essential in the design of ultra-precise optical benches and dimensionally stable antennas. Some carbon fibers have the highest thermal conductivities among all materials allowing them to be incorporated as heat dissipating elements in electronic and spacecraft applications.

Even carbon nanotubes and fibers are being developed with spacecraft and rocket technology in mind. So, we have a lot to look forward to in this exciting arena. Every pound saved is a major reduction in launch cost to the tune of $5000 per pound. This means that if you replace a pound of aluminum with a pound of some exotic new alloy, you have almost $5000 per pound in new allow cost to play with because aluminum counts for only one percent of the $5000!