Traveling to Another Star
Austin Morris, Director of Engineering
6 minute read
First and foremost, let’s start with one of my favorite and most frequently referenced quotes from Douglas Adams: “Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.”
As Adams indicates, space really is beyond humongous. For measuring distances in space, we often use units of Astronomical Units (AU) or Light-years (LY). To give some context to this, one AU is the distance between the Earth and the Sun, which is approximately 93,000,000 (that’s 93 million) miles. One LY is the distance that light travels in one Earth year, which is approximately 63,241 AU or 5,881,413,000,000 (5.88 trillion) miles. For comparison, the distance from New York to LA, which most would agree is pretty far, is only about 2,500 miles. Hopefully you can see how when we start talking about distances in space and on a grand cosmological scale, we are talking about nearly incomprehensible distances that are extremely difficult for the human mind to truly fathom. Keeping that in mind, let’s go ahead and stick with AU and LY for the rest of this column.
The biggest steps so far taken by humanity toward probing interstellar space came in the form of the Voyager program, which entailed launching the Voyager-1 and Voyager-2 space probes back in 1977. These probes used the position of the planets in our solar system at the time to get some major gravitational assistance to slingshot themselves way out into space, leaving the solar system forever. Even with this huge acceleration, Voyager-1 was able to look back after 13 years of travel and take a picture of the planet it launched from, in the famous “Pale Blue Dot” image.
Continuing on its journey, Voyager-1 finally left our solar system and entered interstellar space in 2012, 35 years after it began. At the time of this writing in 2022, Voyager-1 is the farthest human-made object from Earth at a distance of 155.5 AU (check here to see the current status of the Voyager probes). This means that after nearly 45 years of travel, Voyager-1 has traveled the remarkable distance of 0.00058% of the way to the nearest star, Proxima Centauri. At this rate, Voyager-1 will have traveled 4.24 LY, far enough to reach Proxima Centauri, in only…78,000 years. So maybe we shouldn’t hold our breath for that.
If you’ll allow me one more ridiculous example of distance, let’s consider that the above example is only so far as the nearest star and traveling to any other stars in our galaxy will entail even greater distance traveled. On top of that there are, in fact, other galaxies in our universe as well. But let’s look even farther than that, to the Milky Way’s nearest neighboring galaxy, the Canis Major Dwarf galaxy. Though its status as a galaxy is currently disputed, for the sake of this column let’s just say that it is the nearest galaxy to our home at a meager 25,000 LY, making it nearly 6,000 times as far as Proxima Centauri. If we wanted to visit any one of the estimated one billion stars within Canis Major Dwarf, assuming that we could travel at Voyager-1 speeds we would currently be looking at a travel time of somewhere around the scale of 500 million years before arrival. If we wanted to instead visit what is probably the most well-known galaxy other than the Milky Way, we could take a spin toward the Andromeda Galaxy for only an additional 2.5 million LY and 50 billion years. Hopefully there’s a few pit stops along the way (and a restaurant at the end of the universe).
The point of presenting all these vast distances is not to give you an existential crisis about the scale of the universe and our relative insignificance within it (although if you’re like me, it’s giving you that crisis anyway). The point of all this is to put things in perspective with our current technological capabilities. There is a common theory that has been presented in numerous works of science fiction (including some by the aforementioned Douglas Adams) in which a civilization launches a ship of some sort to journey to a faraway celestial neighborhood to settle a new planet. But when the passengers of that ship arrive, they find that there is already a society established there, built by people who launched later from the same home planet, but were aboard faster ships with more advanced technology. This concept seems to ring true in the sense that when thinking on the scale of thousands or millions of years, it might be reasonable to assume that humanity will have developed technologies that are currently only available in science fiction, like warp drives and hyperspace or faster-than-light travel. But it’s worth taking note of the fact that we already have developed some technologies that seem straight out of science fiction.
When we consider the universe on a larger, cosmological scale and look at journeying to another solar system or another galaxy, how does our current propulsion technology fare? By combining electric propulsion like a Hall-effect thruster with a power generation system such as a radioisotope thermoelectric generator (RTG), you can build a spacecraft that is crammed full of Xenon gas and able to fire its thruster for potentially decades of constant acceleration toward its goal. On the grand scale, this type of maneuver will perform much better than things like chemical rockets, which provide a much faster acceleration but at much lower efficiency. When thinking on the large scale necessary for interplanetary, interstellar, or intergalactic travel, high efficiency will typically result in a much greater delta-v output. Additionally, as this type of travel will include departing from the solar system, a spacecraft of this nature unfortunately can’t sustain itself nearly as well on solar power as a spacecraft can in Earth’s orbit. This is where the RTG comes into play, by using a self-contained radioactive chain reaction to generate power for a long time to come. There’s also the fun reminder that of all the quirks involved in space travel, the lack of friction is a delight for coasting once you reach your cruising speed, but necessitates that you hit the brakes for as long as your acceleration period or else you will impact your destination at your cruising speed.. As you can see, building these types of long-travel missions entails vastly different operating concerns than building missions for Low Earth Orbit, these are concerns that can largely be addressed by existing technologies.
The final major note that I’ll address before letting our brains rest for a bit is time dilation. As has been previously discussed in a few columns about relativity, as one moves faster, their experience of time changes. Accordingly, if one were to be aboard a spacecraft that is continuously accelerating for decades and approaches the speed of light, their experience of time would change even more drastically. Except, as things tend to get a bit confusing when discussing relativity, their perception of time wouldn’t actually change. For an outside observer, say someone on Earth, it may look like that spacecraft was traveling for thousands of years. For someone aboard that ship however, the journey may only take a few centuries. The obvious issue with this then is that while it might seem like a great hack for humanity to reach interstellar destinations, even at the fastest speeds we can reach with current technology it would still take considerably more than a human lifespan to reach these faraway stars. This means that without an incredibly major shift in available technology, no currently living human will be able to reach the stars in this lifetime, because as I’ll repeat again, “Space is big.”
While some of the technologies necessary to enable interstellar or intergalactic travel seem like far-off sci-fi concepts like warp drives and hyperdrives, there are some nearer-term possibilities like Bussard ramjets, beam-powered propulsion, and generation ships that may help enable humanity to expand into the stars. Stay tuned for a future column where we’ll dive into these possibilities and examine how they compare to the sci-fi technologies that we all know and love.
Recommended column to read next: Ascension to Space