Interstellar travel

Interstellar travel is the hypothetical movement of spacecraft between different star systems. While humans have sent objects into deep space, we have not yet achieved true interstellar flight. Current propulsion technologies are not practical for these massive distances. To reach nearby stars within a human lifetime, a craft must travel at a significant fraction of the speed of light. This requirement demands enormous amounts of energy.

The primary obstacle to interstellar travel is the sheer scale of the universe. Distances within our own Solar System are measured in astronomical units (AU). One AU is the average distance between the Sun and the Earth. For comparison, Neptune is 29.8 AU from the Sun. However, the nearest star, Proxima Centauri, is approximately 268,332 AU away. This is over 9,000 times farther than Neptune. Because of this, scientists use light-years to measure interstellar distance. A light-year is the distance light travels in one Julian year in a vacuum. Proxima Centauri is about 4.24 light-years from Earth.

To understand these distances, we can look at our current technology. Voyager 1 is the farthest human-made object from Earth. As of January 2023, it was 163 AU away. It travels at a speed of 17 km/s, which is only 0.006% of the speed of light. At this current rate, a journey to Proxima Centauri would take 75,000 years. This demonstrates why new propulsion methods are necessary. To make the trip in decades or centuries, we need much higher velocities.

Providing enough energy for such speeds is a massive technical challenge. The energy required depends on the mass of the ship and its final velocity. If a ship needs to slow down upon arrival using its own engines, the required energy effectively doubles. For example, accelerating just one ton to one-tenth of the speed of light requires a massive amount of power. This amount of energy is comparable to the total world energy consumption in 2008. This energy could be stored as fuel, harvested from the interstellar medium, or projected from a distance.

Scientists have proposed several hypothetical propulsion systems to solve these energy needs. Nuclear pulse propulsion and fusion rockets are two possible ideas. A beamed solar sail would use light to push a spacecraft forward. Another concept is the Bussard ramjet, which would harvest gas from the interstellar medium to use as fuel.

For much smaller missions, researchers suggest using nanoprobes. These are tiny spacecraft that might use a nanoparticle field extraction thruster, or nanoFET. This technology uses nanoparticles as propellant to act like small particle accelerators.

Traveling at such high speeds also introduces physical hazards. Collisions with cosmic dust and gas can be catastrophic for a spacecraft. Even tiny particles carry high kinetic energy at near-light speeds. Ships would likely require advanced shielding to survive these impacts. For human crews, the dangers are even more complex. They would face the psychological effects of long-term isolation. They would also deal with ionizing radiation and the physiological effects of weightlessness. Weightlessness can negatively affect muscles, joints, bones, the immune system, and even the eyes.

Because technology is always improving, researchers use a "wait calculation" to plan missions. Physicist Robert L. Forward suggested that missions taking longer than 50 years might be a waste of resources. He argued that a faster ship sent later might eventually overtake a slower ship sent now. In 2006, Andrew Kennedy used this concept to calculate ideal departure dates. He suggested that a journey to Barnard's Star might be best if we wait until approximately 2641 AD. This allows propulsion technology to grow before we commit our resources.

There are many exciting targets for future exploration. There are 59 known stellar systems within 40 light-years of our Sun. Alpha Centauri is the closest system and contains three stars. We have already discovered an Earth-sized exoplanet, Proxima Centauri b, orbiting there. Other targets include TRAPPIST-1, which has seven Earth-like planets. We also look at Tau Ceti and Wolf 1061 because they may have planets in the "Goldilocks" zone. In these zones, liquid water could potentially exist, making them prime candidates for life.