the distance dilemmawhy interstellar travel is still impossible

If you've been following along with my blog for a while then you'll know that this sort of post is a little outside what I typically write about, but I've fallen down a bit of a rabbit hole, and if you'd like, you are welcome to join me on this written expedition.

A few days ago, I was having a perfectly normal day, minding my own business. Then, as if fading in from nowhere, the podcast I have droning on in the background mentions exoplanets in some sort of habitable zone. For whatever reason, I'm hooked. The podcast went on about some of the basics about two exoplanets that have become recent interests in the science community: TOI-715 b, and HD20794 d.

Alright, before we go full sci-fi, let’s break down what an exoplanet is—and what makes a habitable zone special, and while we're at it, wtf is a light-year.

An exoplanet is any planet that exists outside our solar system. AKA, far away shit. A habitable zone is an area around a star where conditions might be just right for liquid water to exist on a planet's surface. Because conditions need to be just right, it's often referred to as the Goldilocks Zone.

The Goldilocks Zone isn't static. As a star ages and gets brighter, the Goldilocks Zone moves outwards. The same is true for our star, the sun, which will eventually push Earth out of the habitable zone.

And, since we'll also need to consider some distances, particularly light-years, remember that 1 light-year is equal to the distance that light travels in 1 year. So, how fast is the speed of light? Absurdly fast. In the vacuum of space, light zips along at 299,792,458 meters per second—roughly 186,000 miles per second for the Americans in the room. Over the course of a year, that adds up to 5.88 trillion miles. In other words, pretty darn far.

Ok, we're wowed at the speed of light, fun names for the habitable zone, and have the lingo for faraway planets down. Now, let’s actually talk about these exoplanets, since ultimately, they aren't actually the star of this show (no pun intended, but I decided to leave it so apologies in advance); antimatter is.

The first of our two exoplanets of interest is HD 20794 d. The planet is only 20 light-years away (that's 117 trillion miles), which, believe it or not, is relatively close for another potentially habitable planet. HD 20794 d is what's called a 'super-Earth', weighing in at around 6.6 times the mass of our planet. HD 20794 d orbits a Sun-like star too. But unfortunately, the planet has an undesirable elliptical orbit that causes the planet to swing in and out of the habitable zone, likely creating a rather unusual climate. Probably a neat study subject, but not a great fit for Earth 2.0.

Our second planet is TOI-715 b, which, at 137 light-years away, is several times farther from Earth than HD 20794 d. For a bit of quick context, 137 light years is around 805 trillion miles.

TOI-715 b is about 1.5 times the width of Earth and orbits a red dwarf star. Unlike most exoplanets we’ve found, this one sits in the conservative habitable zone, and conditions might be just right for liquid water. Interestingly, it seems there might be a second, Earth-sized planet, TOI-715 c (makes me wonder what TOI-715 a is), lurking in the same system

So we've talked about some neat planets, and what I'm hearing from all this is that all these planets are far af. 805 trillion miles? Let's consider how that stacks up with our current technology.

SpaceX's Starship will need to reach around 25,000 mph to escape Earth's gravitational pull and reach Mars. 25,000 mph is faster than any other manned spacecraft has ever travelled before. Apollo 10 holds the record at 24,791 mph. And the exoplanets we're talking about are 805 trillion miles away. Napkin math, 805 trillion divided by 25,000 mph is still 32 billion hours. 1.3 billion days.

More than 3 million years.

That's more than 150,000 human generations. How many civilizations would rise and fall in that time? How many languages created or lost? How different would the landscape of the world, the universe, be?

Even a relatively close planet like TOI-715 b is completely inaccessible with our best propulsion technology. Simply put, we travel far to slowly to reach any meaningful destinations outside our solar system.

We need to go faster. Like, a lot faster.

If you thought that the first bit of this article was the original rabbit hole, you were wrong. That was but a red herring. The true rabbit hole starts here, with a question:

How do we go faster?

In theory, if we are currently limited to 25,000 mph, and there is no (air) resistance in space, then it seems like our biggest limitation may be propulsion; particularly fuel efficiency and acceleration time.

So how do we dial that in? How will humans eventually travel at light-speed?

There are of course many theories on this, but the use of antimatter stood out as a fascinating option.

Antimatter is basically matter’s evil twin, of course it’s not technically evil—just incredibly rare, inconvenient to work with, and is prefixed with 'anti', which is probably why everyone thinks it has a bad rap. Anyways, every particle of matter has a bizarro, opposite version.

• Electrons have positrons
• Protons have antiprotons
• Neutrons have antineutrons

When matter and antimatter meet, they don’t fight. They annihilate each other. Completely. And in doing so, here it is, they convert their entire mass into pure energy.

That's a really big deal, because our current fuel sources are laughably inefficient compared to antimatter. Most of the energy we produce is expelled as a byproduct, heat, and as a result, we're only able to harness a small amount of the energy itself.

Chemical rockets (think SpaceX, Apollo, you know, traditional rockets) are powered by combustion, wherein only about 1% of the fuel’s energy is actually converted into thrust—the rest is lost as heat and exhaust.

Nuclear fission, the infamous splitting of the atom, wastes around 99% of it's potential energy as heat. But, humans still manage to make lemonade with nuclear power; since nuclear power is generated by harnessing the heat generated from fission to boil water, which produces steam and turns a turbine, thus generating electricity. However propulsion can't take advantage of heat in the same way.

But antimatter is different. The energy produced from matter and antimatter meeting and destroying each other, produces nearly 100% energy conversion efficiency.

It's almost unfathomable just how powerful that is.

Antimatter is small. Ten milligrams of antimatter (which a cursory search suggests is around the weight of a grain of sand) could potentially release as much energy as the space shuttle's entire fuel tank; which contains hundreds of thousands of kilos of propellant. If you had a few grams of antimatter, you could propel a spacecraft to a meaningful fraction of the speed of light.

Yes, the speed of light. Which is exactly what we need if we want to turn 420-year travel times into something even remotely reasonable.

Antimatter sounds pretty great. Until you start looking into why we aren't using it today.

Antimatter is hard to come by. There simply isn't as much of it in the universe as there is matter. We can make antimatter (neat!), but it's prohibitively expensive, even for the wealthiest people or nations.

Making even a few nanograms (that's 1 millionth of a milligram, a billionth of a gram) costs billions of dollars, and requires the sort of tooling and tech seen at CERN, which makes it nearly impossible to scale up production quantities.

Some napkin math, if a nanogram of antimatter costs 1 billion dollars to make, and you need 2 million nanograms (2 milligrams) for propulsion, then that's $2M x $1B, which equals $2 quadrillion (the USA has a GDP of around $30 trillion).

But the challenge is greater than cost and scalability alone. Once we have antimatter, we have to store it. If antimatter comes into contact with regular matter, they annihilate each other instantly. And although we have some tools that can store antimatter successfully, even they struggle to hold it beyond a couple minutes.

So we probably won't see antimatter as fuel anytime soon, but you might be surprised to learn that antimatter is already being used in the real world, just not as jet fuel. Most commonly, it's used in medical imaging (PET scans. The "P" stands for positron, a type of antimatter), or in some experimental cancer therapies.

So, will antimatter enable interstellar travel?

Not anytime soon, and maybe never. But in theory, it may be a possible option. But just because antimatter might not be responsible for humankind's first jump to light-speed, it doesn't mean that scientists aren't exploring other options to get us there sooner. You never know what tomorrow's discoveries might reveal to help humans leapfrog a few steps in the path to interstellar travel.