The Space Between

Space captivates me in a way few other things can. I’ll happily fritter away a Sunday learning everything I can about a new cosmic-related subject. Absorbing mind-breaking facts, agreed on by most of Earth’s brightest astrophysicists, that feel like fiction to a mind accustomed to things here on Earth. I always leave wanting to know more.

• Like, how can a teaspoon (5 mg) of neutronium, the hypothetical substance made purely of neutrons that we believe is inside of every neutron star, weight 5.5 billion (5,500,000,000) metric tons?¹
• And how can a red dwarf, the smallest possible star, live for trillions of years² when we believe the universe itself is only 13.8 billion (13,800,000,000) years old?³
• Or how does the Sun convert 600 million (600,000,000) tons of hydrogen into 596 million (596,000,000) tons of helium every second?⁴

See what I mean about mind-warping? The science of space takes the numbers and measurements we are used to in everyday life and blows them up in spectacular fashion.

Now let’s take a look at our closest cosmic neighbor. The Moon’s orbit is elliptical, so the distance between us isn’t always the same, but at its closest it’s about 360,000 km⁵ from us. That’s hundreds of thousands of kilometers from us! The Earth’s circumference at the equator, the longest possible journey around our planet, is just over 40,000 km, 1/9^th the distance. If we had a highway between us and drove it at 120 km/h, it would take about 125 days⁶ of non-stop driving to get there. That’s a hell of a long way and this is the closest thing to us out there.

But hundreds of thousands is still a number we are used to seeing, right? If I told you I had $360,000 (I don’t), you would be happy for me (I assume), but you wouldn’t be in awe. It falls in the realm of numbers we come across in our Earth-shackled lives. So let’s go a little farther out.

When we look at the planets closest to us, things really start to get interesting. Venus, our sister planet,⁷ comes within 41,000,000 km⁸ from us. MILLIONS! We are a bit beyond comparing to car rides here because a drive to Venus would take about 39 years.⁹ I hope you brought a snack! It gets even more intense with Mars. The red planet’s closest approach to Earth is about 56,000,000 km.¹⁰ When NASA delivered its most recent Mars rover, Perseverance, the journey took more than 29 weeks, with the spacecraft hurtling through space at nearly 40,000 km/h.¹¹ The trip isn’t a straight line and is significantly longer than 56,000,000 km (because space) but you get the idea. Getting anywhere off-planet, even our cosmic backyard, is really hard and takes a lot of time.

Voyager 1, launched by NASA in 1977 to explore the outer solar system, reached 167.34 AU in February 2025,²⁶ which is about 25 billion kilometers. It is the most distant human-made object in space yet despite nearly 50 years of travel, it has covered only a tiny fraction of the distance to the nearest star beyond our Sun, which is an astonishing 268,000 AU away.

Science fiction has delivered a ton of fantastic stories about space travel and voyages to alien planets outside of our solar system, but making any of them a reality will be incredibly difficult, and maybe even impossible, simply due to the distances involved. That’s why some of the best stories include time dilation or other cool science quirks. With our current technology, we can’t exit our solar system in one person’s lifetime. Or even 10 people’s lifetimes. The distance is just too great.

Astronomers believe the Scattered Disc to be the spawn point of most short-period comets,²⁷ big dirty snowballs²⁸ zipping around the Sun, with an orbital period under 200 years. Up to and over 100 AU from us, these objects are far but not as far as the Oort Cloud, the hypothetical home of long-range comets.²⁹ Although only theorized, because we can’t actually see it, we believe there is a cloud of icy planetesimals that surrounds the Sun at a distance of 2,000 to 200,000 AU.

200,000 AU is just shy of 30 trillion (30,000,000,000,000) km. That number is so massive, it’s hard to even wrap your head around it. But let’s try anyway. Ever hear the “how many seconds are in a million compared to a billion” fact? I’ve seen it a few times in stories attempting to put billionaires’ money into perspective. Basically, if a person got 1$ every second, they would be a millionaire in under 12 days, but it would take almost 32 years to become a billionaire. Pretty big difference, right? Well, it would take over 950,000 years to hit 30 trillion.³⁰🤯 Astronomical!!

Unless stars are in a system together, they usually have a lot of space between each other. So much so that when the Milky Way collides with the Andromeda galaxy in 4 to 5 billion years, astronomers believe the odds of any two stars actually hitting each other are extremely low.³² A cloud with billions of stars crashing into another cloud with billions of stars and none will actually hit each other? Seems implausible at first but only because our minds can’t actually visualize that much space. Our sun’s diameter is about 1,390,000 km (1.39 × 10⁶ km) but the distance between Proxima Centauri and us is about 4.2 light-years (4.01 × 10¹³ km), which is approximately 29 million times bigger.³³ You can fit a whole lot of other suns in there.

The Parker Solar Probe, a cool little probe NASA launched in 2018 to make observations on the Sun’s corona, is the current title-holder for fastest man-made object in space. It reaches speeds of 690,000 km/hour³⁴ and it would still take over 6,600 years³⁵ to reach Proxima Centauri, the star closest to our sun. A craft travelling almost 700,000 km/hour would still need over six and a half millennia to reach the closest star to us? I love trying to find relative ways like this to consider cosmic distances. It paints a picture in my brain.

Now let’s take a big step back and look at the Milky Way. Our galaxy is over 90,000 ly in length and Sagittarius A*, the black hole at the center, is about 26,000 ly from us.³⁸

90,000 ly is about eight hundred fifty quadrillion (850,000,000,000,000,000) km. I’ve got nothing to compare that to. We aren’t driving that, probes aren’t flying that, and even the douchebags who own everything don’t have that kind of money. It’s hard not to lose all perspective at this point. Might as well say the galaxy is ten million gazzillion km in diameter because that makes about as much sense to me.

With galaxies needing so much space, there must be like what? 7 or 8 of them out there? Well, no. There are somewhere between 200 billion (200,000,000,000) and 2 trillion (2,000,000,000,000) galaxies in the observable universe,³⁹ and they are all absurdly far from one another. In the mid 1990s, NASA pointed the Hubble Space Telescope to a tiny piece of space no bigger than a person’s thumbnail would appear on their outstretched arm, about one 24-millionth of the whole sky. They left the telescope pointed to that one location for days to collect as much light as possible and see as far into space as possible. The image they got revealed a sea of galaxies gradually fading into oblivion.⁴⁰

A parsec is not that much bigger than a light-year, 1 parsec = 3.26 light-years, but it’s handy in defining big distances because, like meters, you can add prefixes to it and multiply its value exponentially.

• 1 parsec (pc) = 3.26 light-years (ly)
• 1 kiloparsec (kpc) = 3,261.56 light-years (ly)
• 1 megaparsec (Mpc) = 3,261,563.78 light-years (ly)

So although the Milky Way is over 90,000 ly in diameter, that is only about 27.5 kpc. Every time the values start to get unmanageable, our old friend science kicks the door in with a new unit of measurement. Huzzah for science! 🎉🔭🎉 Why do we use megaparsecs instead of light-centuries and light-millennia? No idea.

I mentioned earlier how the Milky Way would one day collide with the Andromeda galaxy. That’s because we are both part of a local group of galaxies known as the Local Group (again, brilliant!). So far we’ve counted about 80 galaxies and it covers a distance of roughly 3 Mpc (10,000,000 ly).⁴¹ With gravity’s help, every star in this group will eventually be in one giant galaxy. I’ve heard the name Milkdromeda used to refer to it. Yeesh! At least we’ll all be dead, amiright everyone? *waits for high five*

The Local Group is part of a cluster of galaxy groups known as the Virgo Supercluster, which has an estimated diameter of 33 Mpc (110,000,000 ly) and contains at least 100 galaxy groups and clusters.⁴² It’s like Russian nesting dolls, you keep pulling back and you get bigger and bigger structures made of stars.

The Virgo Supercluster is in an even bigger supercluster named the Laniakea Supercluster. At around 160 Mpc (520 million light-years) in diameter, this giant supercluster is estimated to contain over 100,000 galaxies,⁴³ each of which able to produce billions or even trillions of stars.

Once we go back far enough, we start to see that the universe’s galaxies occupy what looks like giant filaments of matter. Often compared to a spider web or the neurons in a person’s brain, these enormous structures are separated from one another by even bigger voids and supervoids of mostly empty space.

The biggest structure of named galaxies we are part of (that I could find) is the Pisces–Cetus Supercluster Complex. The estimates I found put its diameter at around 300 Mpc, almost 1 billion light-years.⁴⁴ No references left. Just look at the awesome illustration by Richard Powell. Can you see your house?

So how big does it all get? Can the universe just keep going on forever? Well, it might. Truth is, we aren’t all that sure. Our best science says there was a start to the universe (Big Bang⁴⁶) about 13.8 billion years ago and it has been growing bigger ever since. So, with that, we could say there is a border or rim around it all, but again its just too big for us to know for sure.

The observable universe is the part of the Universe that we can actually see, and its nowhere near the whole thing.

We very strongly believe the universe keeps going beyond what is observable but we will never see it. The universe keeps expanding and all the giant clusters of galaxies are moving away from each other so we can see less and less of the far universe everyday. More space is being made right now but less of it is visible to us. Eventually, only the stars in our Local Group will be available to us, everything else will be beyond our reach.