Traveling to other parts of the world is something many people enjoy. Learning and experiencing other cultures is an experience that you can really only understand by visiting that respective country. But have you ever thought about traveling to another part of our universe? I’m not talking about to another planet within our solar system, but literally another solar system far off in the distance. Fascinating to think about, right?
Interstellar travel is something many people have put a lot of thought and research into. It is a method of exploring other places in our universe so we can understand it more and seek out other forms of possible life in the universe. I’m going to talk about how exactly we would achieve this; how exactly would we go about developing spacecraft that can travel and take humans to another planetary system? Well lucky for you, I got the details you may be interested in.
Interstellar travel would need spacecraft to get up to speeds close to the speed of light, 300 million meters per second, in order to make travel times reasonable. Currently, no spacecraft has had the ability to reach speeds anywhere to the speed of light, the fastest one being the Helios 1 space probe that reached speeds of 240,000 km per hour. The problem is that because of this high amount of velocity we need, we also need a huge amount of energy in order to get spacecraft to travel this fast. This energy needs to come from the storage of fuel or a way to utilize the materials within the vacuum of outer space as fuel. There are many hypothesized propulsion systems that could be used to achieve interstellar travel in the future, but before I get to that, lets quickly talk about units, and our main target.
When reading about interstellar travel, there are different astronomical measurement systems used. First, the Astronomical Unit (AU). 1 AU is defined as the average distance between the Sun and the Earth, which is equal to about 150 million km. To provide some scale, our neighbor Mars is around 0.52 AU from Earth. Another form of measurement is light-time. A light-year (ly) is defined as the distance traveled by light in one year, specifically in a vacuum, and is equal to around 9.5 trillion km. For scale, Mars is around 12 light-minutes (lm) from Earth.
What is the proposed target for current interstellar travel purposes? The Alpha Centauri planetary system. It is currently the closest to our own system, being only 4.3 ly away from our own solar system. It is a three-star system, and it was recently discovered that in the habitable zone of one of its stars, Proxima Centauri, there exists an Earth-like exoplanet named Proxima b. So for now, that’s our main destination.
Now, onto the different spacecraft propulsion systems that will get us there. All of these are hypothesized to be able to get us to reasonable speeds that would get us to Proxima Centauri. So, let’s go down the list.
First up, traditional propulsion systems. I’m talking about what we’re currently using in the present-day aerospace industry. There is one mission that is known to have reached interstellar space, which is defined as crossing the sun’s heliopause, and that is the Voyager missions. Both Voyager 1 and 2 are over 100 AU from Earth right now, but also know that these missions launched over 40 years ago, and for reference, Alpha Centauri is ~260,000 AU away. So it is obvious that current propulsion systems will not cut it. But they do lead to an interesting concept called the wait calculation.
So imagine you have this thing called a Generation Ship. It is a spacecraft on its way to Proxima b that uses traditional propulsion methods and houses around 100 people. It would take thousands of years for the ship to get there, so the people on board would just continue to repopulate until they have reached the star. Let’s say that the trip takes around 20,000 years. The biggest problem is that technology by humans develops quickly, and it’s possible that 1,000 years in, humans would’ve developed a propulsion system that could get them to Proxima b in a tiny fraction of the time. This means that by the time the Generation Ship arrives, Proxima b may have already been inhabited and developed by humans, meaning their trip would’ve been for absolutely nothing and could pose many detrimental effects to their well-being.
This makes one think when is the best time to leave, and how do we know if humans won’t create an even faster propulsion system once we do leave? Many researchers have agreed that we must wait to leave for an interstellar mission once we have a propulsion system that travels near the speed of light. This is the wait calculation, and it is a very important aspect of interstellar travel calculations. So now onto propulsion systems that could get us near the speed of light.
Up next, we have nuclear fusion rockets. These are spacecraft that is powered by a nuclear fusion reaction, which is when you fuse together light atoms, which produces an enormous amount of energy. This also includes the idea of nuclear pulse propulsion, which is basically powering a spacecraft through the use of hydrogen bombs (a treaty has banned this from happening), and also the well-known Project Daedalus. This is a great propulsion system because it can reach speeds close to a tenth of the speed of light. The problems with this are that we have yet to create a self-sustaining nuclear fusion reaction, and also that during a reaction, a lot of neutrons are produced and can escape a confined fusion reactor, and this overall contributes to a huge energy loss.
Next, we have electric propulsion. Advanced electric propulsion could possibly be very useful for deep space exploration. An ion engine is a specific type of electric propulsion where electricity is used to charge particles, a common fuel being Xenon, and it then accelerates the particles fast enough to propel a vehicle forward. It is extremely efficient and travels much faster than a chemical rocket because it takes in a little amount of propellant and it exerts it at high speeds. The max speed a chemical rocket is usually around 5 km per second, whereas an ion engine can reach around 40,000 km per second. The issues of electric propulsion are that it is limited by the amount of electric energy it can carry (mainly from solar panels), and also the low thrust it produces, which means it’ll take a longer time to operate.
The next couple of propulsion methods does not require fuel to initially be brought onto the spacecraft in order for it to operate.
First, we have the solar sail. A regular solar sail would use light from the sun on its mirrors in order to exert pressure and basically an acceleration force. But since sunlight won’t be always available, a proposed method is to instead use lasers to simulate this effect with a light sail, and it would actually lead to a greater force being exerted, leading to faster speeds We’re talking about speeds around 20% the speed of light. The main issue with this sort of light sail is figuring out how to slow it down once it reaches the planetary system. Physicist Robert Forward, a major contributor to the field of interstellar travel, stated that this could be done by utilizing a second, smaller solar sail.
Lastly, we have the Bussard Ramjet. It was first proposed by physicist Robert Bussard, and it is a type of fusion rocket that doesn’t need to initially carry fuel on the spacecraft. How it works is that the ramjet would use an electromagnetic field to collect hydrogen from the surrounding interstellar medium, compress it and use it as fuel in a fusion reaction, and then expel it out the back of the spacecraft in order to accelerate the ship. The ramjet needs to be traveling fast in order to actually scoop the hydrogen, so technically some propellant would be needed to get the spacecraft going initially. The main issue is that the drag produced from scooping the hydrogen may exceed the thrust created by the fusion reaction. Though, this idea is thought of as one of the more promising methods for interstellar travel once it is fully developed.
People have thought of some other obscure ways of getting a spacecraft around the universe as well. This includes wormholes, which was shown in the movie Interstellar. It is a way of connecting one point in spacetime to another, basically a small shortcut. It is theorized to work since the math works out, but we’ve never seen a wormhole, so who knows? Next is the creation of an artificial black hole or a black hole starship, which is using a black hole to reflect Hawking radiation and propel a spacecraft (this is maybe possible, but who knows). Last is a dark matter rocket, and since dark matter accounts for 85% of matter in the universe, it is thought we could use it as a fuel we collect en route. But the problem is that we know barely anything about dark matter, so it is hard to say what a spacecraft like this would look like.
Of course, there are many issues that come along with this idea, the biggest problem being cost. There are also problems with a spacecraft traveling near the speed of light, so many things have to be taken into consideration. First, we need a way to deal with the natural space debris and interstellar dust that will come into constant contact with the spacecraft. There was a NIAC proposal that studied some sort of shielding method, more specifically using magnetic shielding through high-temperature superconducting magnets, to prevent this and also radiation exposure. Some proposals have also included having a dedicated robotic in-flight repairing system that could fix any structural issues.
Another issue is the psychological and physiological effects of a crewed interstellar mission. Since they could possibly be decades-long, isolation for such a long time could really affect the human brain. Also the previously mentioned radiation exposure problem. A solution to all of this could possibly be cryosleep so people wouldn’t be awake for most of the journey, and also a traveling ecosystem that could make humans feel slightly closer to home.
So, are we anywhere near seeing crewed interstellar travel? Definitely not, since not all of the technology we would need has not yet been fully developed. But, we may see more uncrewed interstellar travel in our lifetime. A company called Breakthrough Initiatives is developing a mission called Breakthrough Starshot proposed to cost around $10 billion dollars that would send nanoprobes using the light sail laser technology I mentioned earlier to Proxima b. They could get close enough to take pictures detailed enough to include continents and oceans. So we may possibly be seeing real pictures of another solar system in our lifetime. You have to admit, even though we ourselves won’t be leaving our solar system anytime soon, its awesome to know that things like this are currently in the works!
The future holds exciting prospects for us fans of deep space exploration!