Propelling Humanity into the Future

With the current UK and worldwide political and climate breakdown, it’s more tempting than ever before to consider relocating to the distant stars. With this and international space week in mind, let’s check in with some of our current interstellar travel options…

Solar sails have been bouncing around as a concept since the 1980s, but the only space vehicle to successfully use them as a primary propulsion method was the Japanese Aerospace Exploration Agency’s IKAROS in 2010. This successfully completed a mission to Venus, before being allowed to roam free in the solar system to maximize data collection. Contact was intermittent over the following five years, with the last recorded speed captured in 2013 at 400m/s (though the craft was still accelerating). For context, that’s 1.17 times the speed of sound or 58% of Concorde’s top speed – not the most mind-boggling figure, though that’s perhaps to be expected. 

“This would theoretically allow speeds between 15-20% of the speed of light to be reached.”

Solar sails use radiation pressure from the sun to accelerate; when light reflects off a surface it exerts a force due to a change in momentum, and the force correspondingly causes the surface (and anything attached) to accelerate. 

Solar sails are essentially giant mirrors that take advantage of this by reflecting as much of the sun’s light as possible. However, as the momentum that light carries is so tiny, the force generated is miniscule – the IKAROS 196m2 solar sail array produced 0.00112 Newtons of thrust. That’s about the same force 1/10 of a stick of gum exerts on your hand under earth’s gravity!

“They’re thinking about juicing these up with what can only be sensibly described as mega-space lasers.”

In 2016 Yuri Milner, Stephen Hawking, and Mark Zuckerburg (you did read that correctly) founded operation Breakthrough Starshot, which proposes following NASA scientist Philip Lubin’s idea to ditch the puny power of the sun in favor of a multi-kilometer 100 gigawatt (~18x the UK’s peak electricity demand) array of lasers fired directly onto a spacecraft’s reflective sails. This would theoretically allow speeds between 15-20% of the speed of light to be reached, albeit even then only on a spacecraft about the size of a paperclip. Unfortunately, it’s probably unlikely we’ll be solar (or laser) sailing anywhere exciting in the near future.

Ion drives are another form of propulsion and aim to tackle the classic ‘rocket equation’ – i.e. we need more fuel to fly further/faster, but more fuel means more mass and we need more fuel to fly that added mass… 

Ion drives bombard their propellant with electrons, which are typically provided through an effect called thermionic emission, where a heated element releases electrons (this is also used in the process that produces X-rays in a hospital!). The electron bombardment causes propellant particles to ionize (have a negative or positive charge). Some of these charged particles are then accelerated through a series of grids up to massive speeds (~40,000m/s!)  before being ejected from the spacecraft to generate thrust. 

The grids are charged which is why the propellant particles must be ionized in order to be accelerated by them (think opposites attract, like repel). As the most common propellant in ion engines is Xenon, which can be stored in high volumes with minimal difficulty, it begins to address some of the ‘rocket equation’ difficulties we face with other fuel. 

However, the ion drive is still fundamentally limited by its electricity requirement. Both the ionization of particles and the charging of grids requires power, and while solar is appropriate over short range (ion drives are used to keep many of earth’s satellites in position), the further away we get the less we have to work with. 

Luckily for us, scientist at the Jet Propulsion Laboratory in the US are working on a different sort of mega-space laser – one focused on providing electricity to an ion drive solar array long after its left earth. 

Much smaller than its Breakthrough counterpart, the proposed 100 megawatt system is still very much in the theoretical stages, and unlikely to provide us any transport out of the solar system in the near future, though could be appropriate for a martian excursion (per Andy Weir’s novel). 

In conclusion? I hate to say it, but unless you’re a paper clip or a super-villain with a mega-space laser array at their disposal, you’re likely stuck here for the time being. Better make the most of it, eh?

image source: Breakthrough Starshot