Interstellar Comparisons

By 2025 Elon Musk believes SpaceX can get us to Mars – a journey of about 500 million kilometres, needing a speed of over 100,000 km/h. By comparison travelling to the stars within a human lifetime via the known laws of physics requires energies millions of times more potent than that budget-price trip to Mars. In our energy hungry modern world the prospect seems fanciful, yet we are surrounded by energies and forces of comparable scale. By taming those forces we will be able to launch forth towards the stars, save our civilization and extend the reach of our biosphere.

How so? Consider the sunlight received every second by planet Earth, from the Sun. About 1.4 kilowatts of energy for every square metre directly facing the Sun – all 128 trillion of them – means a total power supply of 175,000 trillion watts (175 petawatts.) That’s 8,750 times more than the mere 20 terawatts human beings presently use. Earth itself receives a tiny fraction of the total available – the Sun radiates about 2.2 billion times more, a colossal 385 trillion trillion watts (385 yottawatts).

Just how much does a starship need?

Project Daedalus proposed a fusion propelled star-probe able to fly to nearby stars in 50 years. To do so it would fuse 50,000 tonnes of deuterium and helium-3, expelling them as a rocket exhaust with an effective jet speed of 10,000 km/s. A total useful energy of 2500 million trillion joules (2.5 zettajoules) – the actual fusion energy available in the fuel was about 10 times this, due to the inefficiency of the fusion rocket motor. However that gives us a useful benchmark. This is dwarfed by the energy from the Sun. A full Daedalus fuel-tank is equivalent to about 4 hours of Sunlight received by planet Earth.

Another design, the laser-sail, masses 2,500 metric tons and requires a laser power of 5 petawatts, which accelerates the laser-sail starship 1 gee for 190 days to achieve a cruise speed of half light-speed or 150,000 km/s. A laser-power equal to what Earth intercepts from the Sun, 175 petawatts, could launch ~67 laser-sail starships per year. Total energy required per sail is 8.24 yottajoules, equal to 5.45 days of Earth-sunlight.

What else could we do with power that can launch starships? Power on the scale of Worlds (i.e. 175 petawatts) allows the remaking of Worlds. Terraforming is the shaping of the dead worlds of the Solar System into more life-friendly environments. Mars, for example, is considered to be the most life-friendly nearby planet other than Earth, yet it lacks an oxygen atmosphere, a significant magnetic field, and is colder than Antarctica. To release Earth-levels of oxygen from its rocks, power an artificial magnetosphere to deflect away the potentially harmful solar-wind, add nitrogen to reduce the fire risk, and keep the planet warm, the energies required are similar to those required to launch starships.

Releasing oxygen from Martian rocks requires melting the rock, usually composed of about 30% oxygen, and breaking the chemical bonds. What results is a melt of mixed metals, like iron, and semi-metals, like silicon, and oxygen gas, plus hardy compounds like aluminum oxide. For every kilogram of oxygen released, about 30 megajoules of energy are needed. Earth-normal oxygen levels require a partial pressure of 20 kilopascals (20 kPa), which means a mass of 5.4 tons of oxygen for every square metre of Martian surface – 775 trillion tons in total. The total energy required is 10 yottajoules. Adding 80 kPa of nitrogen, like Earth’s atmosphere, requires mining the frozen nitrogen of Neptune’s moon Triton, doubling the total energy required. Pluto’s vast plains of convecting nitrogen ice is another possible source, though without the handy proximity of a big planet’s gravity well for getting a boost towards the Sun it might prove uneconomical in energy terms. Shipping it from Saturn’s moon, Titan, as Kim Stanley Robinson imagines in his “Mars Trilogy”, requires 8 times the energy of using Triton as a source, due Saturn’s less favourable gravity conditions. Warming Mars to Earth-like levels, via collecting more solar energy with a vast solar mirror array, means collecting and directing about 50 petawatts of solar energy (equal to about 10 laser-sail starships). Before we use that energy to gently warm Mars, it can be concentrated via a “lens” into a solar-torch able to burn oxygen out of Mars’s rocks. With 50 petawatts of useful energy the lens can liberate sufficient oxygen for breathing in a bit over 6 years.

The final task, creating an artificial magnetosphere, is puny by comparison. A superconducting magnetic loop, wrapped around the Martian equator, can be used, powered up to a magnetic field energy of ~620,000 trillion joules (620 petajoules), by about 12.4 seconds of energy from the solar-mirrors. This is sufficient to create a magnetosphere about 8 times the size of Mars, much like Earth’s.

Total one-time energy budget is 20 yottajoules – 8,000 “Daedalus” starprobes, or 243 laser-sail starships equivalent. The ongoing power-supply of 50 petawatts is enough to propel 10 laser-sail starships at a time.

To terraform the other suitable planets and moons of the Solar System requires similar energy and power levels. For example, if we used a solar-torch to break up the surface ice of Jupiter’s moon, Europa, into hydrogen and oxygen, then used it to ‘encourage’ the excess hydrogen to escape into space, the total energy would be about 8 yottajoules, surprisingly similar to what Mars requires. The nitrogen delivery cost is about 6 yottajoules, again similar to Mars. Ongoing energy supply would be 10 petawatts – two starships worth.

A less exotic location to terraform would be the Moon. One advantage, as well as proximity to Earth, is that it requires no extra input of energy from the Sun to stay warm. However, unlike Europa or Mars, water as well as atmosphere would need to be delivered, multiplying the energy required. If shallow seas are sufficient – an average of 100 metres of water over the whole surface – the energy to deliver ice and nitrogen from Triton, then make oxygen from lunar rocks, is 27 yottajoules.

The only solid planet with close to Earth gravity is Venus. To remake Venus is a vastly more challenging task, as it has three main features that make it un-Earthly: too much atmosphere, too much day-time and not enough water. Take away the atmosphere and the planet would cool rapidly, so while it is often likened to Hell, the comparison is temporary. The energy required to remove 1 kilogram from Venus to infinity is 53.7 megajoules. Venus has over a thousand tons of atmosphere for every square metre of surface – some 467,000 trillion tons of which is carbon dioxide. To remove it all requires 25,600 yottajoules, thus removal is far from being an economical option, even in a future age when yottajoule energy budgets are commonplace.

One option is to freeze the atmosphere by shading the planet totally. To do so would require placing a vast shade in an orbit between Venus and the Sun, about a million kilometres closer. In this position, the gravity of the Sun and Venus are balanced, thus allowing the shade to stay fixed in the sky of Venus. With a diameter about twice Venus’s 12,100 kilometres, the shade would allow Venus to cool down over a period of decades. Eventually the carbon dioxide would rain, then snow, covering the planet in dry-ice. Some form of insulation (foamed rock?) would then be spread over the carbon dioxide to keep it from bursting forth as gas again. Alternatively it might be pumped into natural cavities, once the sub-surface of Venus is better mapped. The energy cost of assembling such a vast shade, which would mass thousands of tonnes at least, would be far less than the cost of removing the carbon dioxide. So close to the Sun, the shade would intercept the equivalent of 8 times what Earth receives from the Sun – 1,400 petawatts in total, sufficient to propel 280 laser-sail starships, or power the terraforming of the other planets. Or both.

The next desirable for Venus is the addition of water. If 100 metres depth is required the total energy to ship it from Triton is 144 yottajoules. Using 50 petawatts of power, the time to export the water is about 122 years, with a 30 year travel time for ice falling Sunwards from Neptune. The total energy of creating an artificial magnetosphere similar in size to Earth’s would be 6 exajoules (6 million trillion joules) – a tiny fraction of the energy budget.

Further afield than the Inner System and the Outer Planets (including IX, X, XI…) is the Oort Cloud, a spherical swarm of comets thousand to ten thousand times the Earth-Sun distance. According to current planet formation theories there were once thousands of objects, ranging in size from Pluto to Earth’s Moon, which formed out of the primordial disk of gas and dust surrounding the infant Sun. Most coalesced via collisions to form the cores of the big planets, but a significant fraction were slung outwards by gravitational interactions with their bigger siblings, into orbits far from the Sun. One estimate by astronomer Louis Strigari and colleagues hints at 100,000 such objects for every star.

The technology to send a laser beam to a starship accelerating to half light-speed over thousands of Earth-Sun distances opens up that vast new territory we’re only just beginning to discover. A laser able to send 5 petawatts to a laser-sail at 1,000 times the Earth-Sun distance, would be able to warm a Pluto-sized planet to Earth-like temperatures at a distance of a light-year. Powering starships will thus anable the spread of the Earth’s biosphere to thousands of worlds which would otherwise remain lifeless. Life on Earth spread out in abundance, aeons ago, once it learnt the trick of harnessing the Sun’s energy via photosynthesis to make food from lifeless chemicals. Humankind can do the same, on a vastly greater scale – it’s the natural thing to do.