Guest Blog: Sailing through space

Posted by Alison Klesman
on Wednesday, December 20, 2017

A solar sail allows a small spacecraft to hitch a ride on the stream of particles released by the Sun, eliminating the need to carry fuel. // Kevin Gill

By William Zhu

A history of space sailing

You cannot exactly describe Johannes Kepler, a German astronomer born in the year 1571, as a handsome man. His premature birth contributed to his sickly childhood and smallpox weakened his vision. Yet he was a visionary. Mathematics set him free from his restrictive physical conditions to model and explain the universe. When his Italian colleague Galileo first published news of his discoveries of jovian moons using his telescope, Kepler responded enthusiastically in Dissertatio cum Nuncio Sidere (Conversation with the Starry Messenger) in 1610. In Dissertatio, Kepler endorsed Galileo’s work, praising its significance to the academic world and conveying his trust in Galileo’s observation. He further commented on the possibilities of traveling to other planets through space, famously stating, “Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse.”

About 400 years ago, Kepler anticipated today’s Breakthrough Starshot, a project that for the first time makes feasible the exploration of our closest neighbor star in the galaxy. Gathering a group of distinguished scientists from all over the world at the conference table, the project seeks to send a light sail to the star Proxima Centauri, propelled with laser beams generated by huge laser arrays on Earth. How did “sailing” through space progress from an idea in the mind of a scientist into an actual project? Imaginative people arrived at the idea of space travel through mathematics and literature.

Jules Verne, a French novelist and one of the founders of the science fiction genre, predicted spacecraft propelled by light in 1865. In his fourth book, From the Earth to the Moon, he wrote about a group of scientists and engineers who built a cannon to fire a projectile capable of reaching the Moon. One of the characters aboard the projectile, a French adventurer, gives a motivational speech that includes speculation for the future: “there will someday appear velocities… of which light… will probably be the mechanical agent... we shall one day travel to the moon, the planets, and the stars.”

Jules Verne only mentioned it briefly, but his idea was later investigated by Russian pioneer of spaceflight Fredrich Zander using detailed calculations. Zander was born in 1887. His enthusiasm and ambition for space exploration shone through his personal motto, “Forward, to Mars!” (Вперед, на Марс!). In his 1925 book addressing problems regarding chemical rockets, Problems of flight by jet propulsion interplanetary flights, he inserted a special chapter researching various aspects on using sunlight-propelled light sails for space travel. He experimented and calculated the effects of different sail thickness, light intensity, sail area, and sail arrangement on such travel. Although his work remained theoretical, it paved the road for later scientists to further develop the idea with better technology and broader vision. 

A feather racing with the light

The ongoing project Breakthrough Starshot, headed by Stephen Hawking and funded by millionaire Yuri Milner, endeavors to greet our closest neighbor star, Proxima Centauri, with a man-made probe. This probe is not any ordinary probe – it is an extraordinarily light piece of equipment carried by gigantic thin “mirror.” This novel spacecraft is called a light sail

Proxima Centauri is visible from the Southern Hemisphere; it is the closest known star system to host an exoplanet. // Y. Beletsky (LCO)/ESO

Current light sail technology is, in simple terms, “using a very powerful laser beam [to push] on a lightweight sail,” according to the Chairman of the Breakthrough Starshot Advisory Committee, Professor Avi Loeb from Harvard University, in the project’s 2017 discussion conference, Breakthrough Discuss. He stresses that this technology is the “only one technology that can [achieve the mission of the project]” due to the scope of the mission. Breakthrough Starshot wants to visit the nearest star within a timeframe of 20 years. This, according to Professor Loeb, would require “the spacecraft … to move at a fifth of the speed of light,” which is about 670 million miles per hour (1 billion kph). This is a tremendous increase from humanity’s last “personal record” of solar system exit velocity: 38,088 miles per hour (61,200 kph), held by Voyager 1 only due to gravitational-assisted acceleration.

According to Newton’s second law of motion, F = ma, the smaller the mass of an object, the greater it is accelerated with the same amount of force. A greater acceleration would mean a higher velocity in the end. Thus, Breakthrough Starshot can reach such high velocity by reducing the mass of the spacecraft.

Three things will specifically help the project to cut the mass down.

The first and the most important one is simply leaving the fuel for acceleration behind on Earth. As you increase the amount of fuel carried by a traditional chemical rocket, you will boost its capacity to accelerate through burning that fuel. However, the more fuel you are already carrying on the ship, the less effect you get for each additional unit of fuel because the ship becomes heavier. This is analogous to drinking coffee: the more cups you have already drunk, the less energizing each additional cup will be. According to Tsiolkovsky’s rocket equation, as shown in the picture below, chemical rockets’ initial to final mass ratio – an indication of how much fuel they carry – reach absurd levels as the final velocity requirement increases beyond 20,000 meters per second (65,600 feet per second). 

Initial to final mass ratio as. a function of the change in velocity. // William Zhu

The second is the improvement in nanotechnology that makes thinner and lighter sails possible. During the third session of Breakthrough Discuss, Professor Loeb said that we can now make sails that “weigh on the order of a gram with the surface area of a few meters squared.”

Don’t have a good idea of what that means? Imagine a piece of shiny metal covering the entire surface of your dining table. Scrunch it all together and place it on a scale – it only takes a single paperclip to balance it out.

The third is the miniaturization of electronics. Moore’s law predicts that the number of transistors in an integrated circuit will double every two years, based on observations of the rate of technological development over the course of four decades. This implies an increased density of computing power on a physical electronic chip, as chips become smaller, lighter, and yet have the same or more computing power. According to Professor Loeb, it is now possible to “[pack] a camera, a navigation device, [and] a communication device into a few grams.” This significantly reduces the mass of the spacecraft, considering past spacecraft weigh easily more than 2,000 pounds (1,000 kilograms).

These three aspects of modern technology development make the project possible. Because the light sail travels so quickly to its target – covering a distance of 4.2 light-years in just 20 years – we may well be able to witness the first detailed image of a planet outside of our solar system in the coming decades.

Will there be green plains and blue oceans? We shall see. 

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