Where can we actually go (now or in the near future)?
While there are certainly some fantastic science fiction stories set on Earth, I personally am always fascinated by stories of people (and aliens) visiting all kinds of interesting planets. In some stories the protagonist visits dozens of planets, seeing all kinds of amazing environments. So cool.
But what would it actually take to travel in space? Can we do it? And if not, could we one day? These are the questions I want to address (at least in brief) in this short blog series.
The Universal Speed Limit
Before we get started, it is key to mention that physics has found a limit on the fastest speed anything, spaceship, person, or bug-eyed alien, can travel. This is known as the speed of light (and yes, this is how fast light travels). This is a key component of Einstein's theory of relativity. The speed of light comes to an amazing 670 miles per hour. So light is incredibly fast.
That said, things in space are incredibly far apart. So even at the speed of light, it takes a while to get places. A lot of distances are reported in light years (or how many years it takes light to travel that far). So something 10 light years away takes at least 10 years to get to.
I'll talk in the next blog about possibly ways to circumvent this speed limit, like wormholes, but for the rest of this post I'll assume that we're limited by this universal speed limit.
One of the big topics in space news lately has been talk of sending people to Mars. Now, if you aren't too invested in conspiracy theories, we've already been able to send people to the moon. Mars is (depending on orbital alignment), roughly 1,000 times farther away than the moon, so this is a huge undertaking. That said, there is obviously a lot of reason to believe this will succeed. We've sent a number of probes to Mars already and we know how to get people in space. Put all that together and a trip to Mars can absolutely happen.
I'm not going to belabor this topic since millions of websites are rich in details about traveling to Mars. I just want to point out a few things before moving on.
'Interstellar' just means 'between stars.' So here I'm talking about traveling between stars. Now this gets much more interesting. Many instruments, most famously NASA's Kepler satellite, have found planets orbiting other stars in the Milky Way. As of now, it really looks like most stars are surrounded by planets. So interstellar travel means visiting planets unlike any we've ever seen, maybe even some with other forms of life. Amazing for human curiosity and great grounds for science fiction.
There's a lot we could say about this, but I'm going to focus now on whether we could make it to another star system.
The short answer... yes (probably), but not any time soon.
Now the long answer:
First, what are the distances involved? The nearest star to us is Alpha Centauri at just over 4 light years. Others are farther still, but we have over 40 stars within 15 light years. Plenty to choose from.
A light year is about 5.9 trillion miles! This is a really, really long way roughly 40,000 times farther than the distance to Mars. So if we say we could get to Mars in 100 days, we'd be looking at something like 4 million days (11,000 years) to get there. Hmmm so...
Okay, that sounds bad, but there's more to the story. If we could produce a sustained acceleration of one gee (9.8 meters per square second), a ship would keep speeding up the farther the trip got. This means we could make it there faster.
If we accelerated for the first half of the trip and decelerated for the second half, we could reach Alpha Centauri somewhere between 4 and 10 years! That doesn't sound too bad at all. (I'm being cagey because I haven't run the full relativistic calculation, which we'd need for a precise answer). Even if we slowed down the acceleration, a trip of 50-100 years seems possible. That's more or less within a human lifetime. (I'll get to how humans could survive such a trip in the third blog post.)
Now actually keeping up one gee of acceleration for several years straight is well beyond what we could do now. I haven't checked numbers, but the amount of chemical fuel required would likely be near to impossible to obtain. Maybe we'd get closer with nuclear fuel, but still, a tough proposition. I'll talk more about the possible technology to actually make such a trip in the next blog post so stay tuned.
One thing worth bringing up is a proposed project called Starshot. This is a probe that could travel to Alpha Centauri in as little as 20 years. Definitely a critical first step to sending people. Now the technology (lasers on Earth pushing a very tiny spaceship), wouldn't scale up easily to a manned spacecraft, but it shows that the nearby stars may not stay beyond our reach for long.
Just as interstellar means between stars, intergalactic means between galaxies. Now a galaxy is a collection of billions of stars, so these things are huge. The Milky Way is our home galaxy and it is a long way across. The stars like Alpha Centauri are our closest neighbors within the Milky Way and those are hard to get to. Crossing the Milky Way galaxy itself is well beyond our reach.
Getting to a different galaxy is, right now, as close to impossible as anything.
The nearest galaxy to us (excluding small dwarf galaxies) is Andromeda (M31) and it is 2.5 million light years away. That is nearly 1 million times farther away than Alpha Centauri, and such a ridiculous number of miles we can't even imagine it. And that is just the closest galaxy.
At this point we don't have a hint of any technology we could use to make the trip. I will mention a few of the more far out travel ideas in the second blog, but suffice to say this kind of trip is in our distant future or beyond.
So what is realistic?
Now, I think there's no need to confine fiction to the realistic. Some of the greatest science fiction stories and movies throw any realism to the wind. However, if we do want to consider what we could actually do, here's the quick summary.
Science fiction draws its inspiration from...you guessed it - SCIENCE!