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Astronomy: Science of the Universe

ailing on light

As light carries momentum, but no mass, we may also be able to continuously power a focused light beam, such as a laser. The required power would be collected from the sun, and no Earth mass would be consumed. Even using the enormous 100GW laser plant envisaged by the Breakthrough Starshot project, which aims to propel spacecraft out of the solar system to explore neighbouring stars, it would still take three billion billion years of continuous use to achieve the orbital change.

But light can also be reflected directly from the sun to the Earth using a solar sailstationed next to the Earth. Researchers have shown that it would need a reflective disc 19 times bigger than the Earth’s diameter to achieve the orbital change over a timescale of one billion years.

Interplanetary billiard

A well-known technique for two orbiting bodies to exchange momentum and change their velocity is with a close passage, or gravitational slingshot. This type of manoeuvre has been extensively used by interplanetary probes. For example, the Rosetta spacecraft that visited comet 67P in 2014-2016, during its ten-year journey to the comet passed in the vicinity of the Earth twice, in 2005 and 2007.

As a result, the gravity field of the Earth imparted a substantial acceleration to Rosetta, which would have been unachievable solely using thrusters. Consequently, the Earth received an opposite and equal impulse — although this did not have any measurable effect due to Earth’s mass.

But what if we could perform a slingshot, using something much more massive than a spacecraft? Asteroids can certainly be redirected by the Earth, and while the mutual effect on Earth’s orbit will be tiny, this action can be repeated numerous times to ultimately achieve a considerable Earth orbit change.

Some regions of the solar system are dense with small bodies such as asteroids and comets, the mass of many of which is small enough to be moved with realistic technology, but still orders of magnitude larger than what can be realistically launched from Earth.

With accurate trajectory design, it is possible to exploit so-called “Δv leveraging” — a small body can be nudged out of its orbit and as a result swing past the Earth, providing a much larger impulse to our planet. This may seem exciting, but it has been estimated that we would need a million such asteroid close passes, each spaced about a few thousand years apart, to keep up with the sun’s expansion.