Interstellar travel: Sending tiny spacecrafts to the stars

Exploring what lies in the expanse of our universe has always been a topic of interest for scientists and engineers. This curiosity has shaped the field of space exploration and propelled nations to send astronauts to explore the cosmos. 

Experiments enacted beyond the confines of our planet have yielded a plethora of scientific theories, including Galileo’s law of free fall—which was eventually proven correct by the famed hammer and feather experiment on the moon. 

Although all crewed and uncrewed space expeditions were interplanetary missions that have taken place within the bounds of the solar system, the recent exponential surge in technological developments will allow scientists to explore other stars and planetary systems. One such mission could bring us to Proxima Centauri—the second-nearest star to Earth after the sun and one of three stars in the Alpha Centauri stellar system—located around 4.24 light-years, or 9.5 trillion kilometres, away from Earth.

“Proxima Centauri is only 12 [per cent] the mass of the sun, and much more dense,” Dr. Kirsten Dage, a postdoctoral fellow at the McGill Space Institute, wrote in an email to The McGill Tribune. “It’s also more magnetically active and so even though it is a lot smaller, it can have increased X-ray flaring activity that gets as bright in X-rays as the Sun. The good news is that even though it is smaller than the Sun, it uses up less of its fuel and will have a longer lifetime.” 

In 2015, cosmologist Philip Lubin proposed the idea of using a powerful laser to accelerate a tiny spacecraft, weighing only a few grams, to 20 per cent of the speed of light. A few months later, Israeli-Russian billionaire Yuri Milner donated 100 million USD to fund Lubin’s project, and Stephen Hawking endorsed the idea as well. The project, called Breakthrough Starshot, aims to send a nanocraft to Alpha Centauri. 

Just as boats need a sail for cruising, the proposed nanocraft will be equipped with a lightsail just a few atoms thick. It will also carry a SpaceChip the size of a postage stamp that will bear cameras, photon laser thrusters to propel the craft, communication equipment, and a power supply. Due to the wave-particle duality of light, photons possess the energy and momentum to power the spacecraft’s journey to Alpha Centauri. 

Lubin, a professor in the Department of Physics at UC Santa Barbara and director of the UCSB Experimental Cosmology Laboratory, explained how the nanocraft travels.

“Light (directed energy) carries energy and momentum,” Lubin wrote in an email to the  Tribune. “The light from a large laser array is directed at a reflector that reflects light and thus is pushed forward. It is like using water from a hose to push a ‘beach ball’ forward. The directed energy system is not on the spacecraft but is back home (Earth, Orbit-based or lunar-based, for example).”

Designing a spacecraft that will travel 4.24 light-years during our lifetime is a daunting task, but Lubin explains that new technologies are in the works.

“If we want to get to the nearest star systems in the span of a human lifetime, we have to achieve speeds vastly higher than are currently possible with chemical propulsion,” Lubin wrote. “This requires new propulsion technologies. There are only two technologies capable of achieving the speed required. These are matter-antimatter propulsion [and] directed energy (light) propulsion. Our NASA and Breakthrough programs are focussed on the second approach, namely using large-scale directed energy or light itself to propel spacecrafts at speeds that are above 10 per cent [of] the speed of light.”

Since the nanocraft requires immense energy to travel between stars, an array of lasers on Earth is needed to propel it. The various beams from the Earth-based array will merge to form one highly energetic laser beam that has enough power to accelerate the spacecraft to between 10 and 20 per cent of the speed of light, allowing it to reach Proxima Centauri in under three decades. If a spacecraft travelled at these speeds to Mars, it would arrive in a mere three days. 

In 1977, NASA launched two space probes—Voyagers 1 and 2—to study Jupiter, Saturn, and the largest moons of both planets. After taking photographs of these planets, the probes continued past the boundaries of the heliosphere and became the first two probes to enter interstellar space. 

Avi Loeb, who serves as the chair of the Breakthrough Starshot Advisory Committee, is also a New York Times best-selling author of Extraterrestrial: The First Sign of Intelligent Life Beyond Earth and the Frank B. Baird Jr. Professor of Science at Harvard University.

“It would take conventional rockets (like Voyager 1 and 2 and New Horizons) about 50,000 years to reach the nearest star, Proxima Centauri,”  Loeb wrote in an email to the Tribune. “It should have been sent around the time when humans left Africa in order to get there today.”’

According to Dr. Andrew Higgins, a professor in McGill’s Department of Mechanical Engineering and the principal investigator of the experimental research group ‘McGill Interstellar Flight,’ the nanocraft’s lightsail must be made from durable materials capable of travelling light-years to reach Alpha Centauri.

“A promising material is dielectrics: An example of a dielectric is glass, and the glass used in fiber optics is a very promising candidate,” Higgins wrote in an email to the Tribune. “You don’t normally think of glass as reflective but layers of glass of different types can be combined together to make a very reflective mirror. The sail should have a very low absorption of the laser light. This is even more important than high reflectivity. If the sail were to absorb even a fraction of a percent of the laser light incident upon it, it would vaporize!”

In the 1970s, American astronomer Carl Sagan talked about the prospect of developing a spacecraft known as a Solar Sail that would use energy from sunlight to propel itself. Decades later, in 2010, IKAROS became the first spacecraft to employ this technology.

“Solar sails use the photons—particles of light—from the sun to propel a spacecraft without using propellant or onboard energy,” Higgins wrote. “An extreme solar sail that starts very near the sun might be able to get to one per cent of the speed of light, but at that speed, it would take 400 years to reach Proxima Centauri.” 

In Christopher Nolan’s science fiction movie Interstellar (2014), Earth has undergone a climate crisis, leaving humanity on the brink of extinction. The main characters embark on a quest in search of a habitable planet outside of the solar system. With the Breakthrough Starshot, such expeditions may soon become a reality.

The exoplanet, Proxima Centauri b, takes 11 Earth days to orbit the dwarf star Proxima Centauri. Future interstellar missions will provide scientists with more information on the geographical composition of such planets and allow them to conduct tests to figure out if they could sustain human life. 

Jim Peebles, a Canadian-American astrophysicist and the recipient of the 2019 Nobel Prize in Physics, gave insight into this planet’s conditions.

“[Proxima Centauri b] is closer to [its] star than the Earth is to the sun, but [Proxima Centauri] is fainter,” Peebles wrote in an email to the Tribune. “So, the temperature on the planet is about the same as ours, water neither boils nor freezes.” 

Without a doubt, interstellar travel will decode the enigmas beyond the realm of our solar system and revolutionize the meaning of space exploration. 

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