[music playing] If we want to get to the stars, we don't have to learn how to live on other planets.

Mars and Venus look tempting, but both are capable of killing you in so many interesting ways.

The surface of planet Earth is an unusual place.

It's the only patch of the universe that we know of that won't kill you almost instantly.

Wow, how ridiculously lucky that we found ourselves here, but what happens if we try to leave?

I like to think that humanity is destined to become an interplanetary or even interstellar species.

However, in order to do so, we first have to learn to live on other planets.

The most obvious first steps are to build colonies on Mars or Venus.

Now, we talked a bit about Venus as a colonization option in a previous episode.

Today I want to compare, not their advantages, but rather the very interesting and varied ways that both plants would try to kill you.

Might help us make a decision.

We'll start with Mars, because it sits in popular consciousness as our go-to Earth 2.0.

However, there are some real problems with this planet.

Mass is only 1/10 the mass of the Earth, and 40% of its surface gravity.

This puny size means it's hard for Mars to hold onto its atmosphere, for two reasons.

Point one, due to its small size, it's molten interior has solidified long ago, grinding its once spinning iron core to a halt and essentially turning off its magnetic field.

That lack of a strong magnetosphere has allowed the solar wind, the constant stream of energetic particles from the sun, to whittle away at Mars' atmosphere.

And this is exacerbated by point two, the atmosphere was already only weakly held by Mars' low gravity.

The atmospheric pressure on the surface of Mars is now around 0.5% of that of Earth.

So we're talking close to a vacuum here.

So let's say you take the six-month trip to begin your new life on Mars.

Of course, you immediately decide to leave your colony's habitat and go for a nice stroll on the surface without any gear.

Well, first it would be a bit chilly, but it's sometimes within the range of human survivability, getting as high as 20 Celsius or 68 Fahrenheit on the equator.

A more typical value is minus 55 Celsius, minus 67 Fahrenheit.

But let's say you brought a jacket.

Of course, with that non-atmosphere, you're going to asphyxiate immediately.

So let's also give you an air tank so you can live long enough to enjoy other ways to die on Mars.

See, this extremely low pressure is going to do horrible things to your body.

Perhaps anti-climactically, you're not going to explosively decompress.

Sorry "Total Recall."

See, skin is awesome at keeping people in people shape.

The most dangerous thing about the drop in pressure is that the boiling point of liquids also drops dramatically.

Water immediately boils off your tongue and eyeballs, bubbles from of your skin, resulting in some pretty unsightly bloating.

Your blood stays mostly liquid though, because it's kept under some pressure.

But it does start to form bubbles of nitrogen and oxygen, and this is what will actually kill you.

The effect is called ebulism.

And pretty soon after exposure to a vacuum, your blood will be leached of oxygen, starving your brain, and at some point blocking blood flow all together.

This might take a minute or two to stop your heart, but at which point, you're done for.

Happily for you, the de-oxygenation of your blood means you'll pass out in 5 to 15 seconds, depending on how much you freak out.

See, freaking out rises oxygen consumption, so don't do that.

In 1966, a NASA vacuum chamber space suit test went horribly wrong, leaving Jim LeBlanc exposed to a vacuum.

He reported the sensation of saliva boiling off his tongue right before blacking out at 15 seconds, which is actually pretty impressive.

They got to him quick, and he was completely fine.

In fact, it's understood that you can survive exposure to a complete vacuum for around 90 seconds, as long as someone drags you back into that habitat air lock quickly.

You'll probably experience a quick and complete recovery.

At this point, you're probably thinking you'll just stay in the nice oxygenated pressurized heated habitat and be fine.

Yeah?

Not so fast.

See, space is flooded with extremely energetic solar radiation and cosmic rays.

Both thick atmosphere and strong magnetic field are excellent protection against this stuff.

Mars, not so much.

The steady bombardment by high-speed atomic nuclei smashing through our bodies will certainly damaged cells and DNA.

Now, the 180-day flight to Mars alone gets you over the recommended lifetime limit of radiation.

If you're going to stay on the surface for an extended period, then you will certainly have a notable increased chance of cancer.

It's estimated as something like 5% for a short-ish round-trip visit.

Now, as far as risks for space travel go, this might be considered acceptable for a single trip, but the risk will be a lot higher for a long-term colony.

Then shielding tech will have to improve.

But that low constant solar wind is nothing compared to a coronal mass ejection.

This is when a magnetic storm on the sun's surface sends out a blast of extremely high energy particles, most notably protons and electrons.

If you're caught in one, then you don't just have a slight increase in cancer risk, you get radiation poisoning and die.

This is a serious issue for all space travel, but it's also a concern for Martian colonies.

Now, you can actually shield against a coronal mass ejection with around a 1 meter thickness of water or potentially food, human waste products, whatever.

Now, this isn't feasible for an entire spacecraft or Mars base.

However, you actually only need a smaller bunker with that thickness of shielding.

See, these blasts typically take days to reach Mars from the sun and are preceded by some visible indication like a solar flare, whose light reaches you in minutes.

So we can know when one of these coronal mass ejections are coming and take cover.

OK. Mars is sounding less rosy.

Let's talk about Venus.

Now, the pressure and temperature on the surface, 90 atmospheres and 450 degrees Celsius, will implode and roast you instantly.

But we discussed a much better option in a previous episode-- floating cloud cities at an altitude of 50 kilometers.

So let's say you've taken the much easier three-month flight to Venus, half the time of the Mars trip.

You decide to go for a nice walk outside to catch the views from the cloud city.

To start with, things seem lovely.

It's a pleasant 0.9 times Earth's gravity, and the atmospheric pressure is around that of Earth's surface.

It's warm-ish at around 70 degrees Celsius, but that's at least briefly survivable while it stays clear and dry out.

Unfortunately, it's almost always cloudy, because at 50 kilometers you're right at the base of the Venusian cloud layer.

So you probably have scalding fog or rain.

Oh, and it's sulfuric acid fog or rain, not water.

So there's that.

Your skin will start to blister and dissolve at the same time.

Fortunately, the atmosphere is mostly CO2, so you asphyxiate before you go through any of that.

The good news is that all of this stuff is moderately easy to protect yourself against.

It just means that your pleasant stroll through the cloud city has to be done in an acid-proof, heat-resistant suit with an air tank.

Once you've remembered to put on your walking outfit, your stay on Venus is actually not that.

You have a think atmosphere above to protect you from bad space radiation.

Venus does not generate its own magnetic field.

However, the interaction of the sun's magnetic field with Venus's think atmosphere actually induces something of a protective magnetic sheath around the planet, not as good as Earth, but not as bad as Mars.

OK.

I'm leaning towards the hashtag Occupy Venus movement, despite what masses 0.4 g would do for my basketball game.

One thing is clear though.

We have it pretty sweet here on a Earth.

Guess we should try to keep it that way.

So we'll be around for many more episodes of "Space Time."

In the last episode, we finished our series on the origin of matter and time.

And you guys really brought it with some great questions.

Jona Storm asks, if there's no such thing as universal time, how can we say the universe is 13.8 billion years old?

OK.

So imagine a clock that magically appeared at the moment of the Big Bang and somehow survived all of that high-energy craziness, then somehow found its way to planet Earth without having spent a lot of time traveling close to the speed of light, with respect to us.

That clock would now read around 13.8 billion years.

That's just another way of saying that there have been around 13.8 billion years worth of interaction and change since the Big Bang for things in our reference frame.

The universe is 13.8 billion years old here, and that measurement of age is really just a measurement of how much change has happened in our frame.

Vlad Tchompalov asks whether all world lines converge at the Big Bang.

Well, yes and no.

If you're willing to project the Big Bang model back far enough, then any two world lines in the universe will eventually become arbitrarily close to each other.

But this may not be a reasonable thing to do.

We just don't understand the physics well enough to confidently project the size of the universe to infinitesimal smallness, to a singularity.

For one thing, we don't have a theory of quantum gravity to get us into the Planck era.

We'll address these issues in some episodes coming up soon.

AFastidiousCuba wonders where the people who study fundamental physics sometimes just do a Rick and Morty, as in they say everything is an allusion, free will doesn't exist, nothing has a purpose, now let's go watch some TV.

You know what?

Kind of.

Sometimes you have to tune out.

Living in a state of constant existential awe gives you a migraine.

So let's let's all veg out and watch some interdimensional cable.

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