[pfdietz]

It actually doesn't require THAT much energy to get into orbit. The energy of a kilogram of mass in low earth orbit is about 9 kWh, although rockets will use a bit more than that (not enormously more, though). Fuel cost remains a minor fraction of launch cost, even with Falcon 9.

In comparison, the best aluminum plants in the world use 13 kWh to make a kilogram of aluminum. Yet no one says aluminum is out of reach because it takes too much energy to make it.

The per capita primary energy consumption in the US is about 10 kw; your share of this energy over your lifespan would be enough to accelerate 1 kg to 2% of the speed of light.

[shagie]

An interesting problem is "how much energy would it take to launch from a super-earth?"

https://www.space.com/40375-super-earth-exoplanets-hard-alie...

> To launch the equivalent of an Apollo moon mission, a rocket on a super-Earth would need to have a mass of about 440,000 tons (400,000 metric tons), due to fuel requirements, the study said. That's on the order of the mass of the Great Pyramid of Giza in Egypt.

We may be in a Goldilocks zone not only for distance from the primary star (lots of systems are binary which gets problematic), but also in terms of size of the planet (not to light to loose atmosphere, not to heavy to trap everything), and we've got a nice big moon to keep things sloshing around and stabilize the tilt.

I am of the opinion that life is rare because we live in a surprisingly boring solar system with a terrestrial planet in the right range, not to large and not too small, and with a massive moon.

[pfdietz]

A super earth would require exponentially more energy with chemical rockets, due to that darn rocket equation. Fortunately, Earth isn't that bad.

[foobarian]

> A super earth would require exponentially more

Polynomially, right?

[pfdietz]

No, exponentially. I used that word precisely, because the mass ratio in the rocket equation is exponential in (delta V)/(exhaust velocity).

[foobarian]

Thanks for the reply. Context: there are so few exponential processes in nature, especially around classical physics that it really stuck out here, so I made the comment. I should have searched and read up on it instead, but thought there is still some value in others seeing the conversation.

And lastly: that is really depressing :-(

[nayuki]

This phenomenon is called "The Tyranny of the Rocket Equation": https://www.nasa.gov/mission_pages/station/expeditions/exped...

[pfdietz]

That exponential dependence comes from the use of chemical rockets, which have a pretty firm upper limit on their exhaust velocity. Once in space, lower thrust could be used, which admits the possibility of higher exhaust velocity. If one can vary the exhaust velocity then the problem can be evaded (although the specific power (power/mass) of the vehicle must be allowed to become very large, or else acceleration will decrease.)

Recent example: https://phys.org/news/2022-02-laser-mars.html

Note the very high specific power. High Isp rocketry is all about the heat dissipation.

[saiya-jin]

I agree with rare earth theory too, but even with it, the observable universe is so huge that even those tiny fractions of various probabilities should at the end materialize on a massive scale.

And its not like with higher G or some other singular aspect moving into more challenging territory, everything becomes suddenly impossible. Think about how many obstacles mankind could conquer in a relatively short amount of time, say a million years.

[GoblinSlayer]

We see only super earthes because they are easier to see. Small planets can be more numerous.

[shagie]

They can... but a challenge with a small planet in the habitable zone of a star is that it also has a weaker magnetic field and is more prone to having its atmosphere stripped away.

Consider that the moon had an atmosphere - https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#Ancient...

> In October 2017, NASA scientists at the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced their finding, based on studies of Moon magma samples retrieved by the Apollo missions, that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars.

http://time.com/4974580/nasa-moon-had-atmosphere-volcanoes/

Which brings us to

https://worldbuilding.stackexchange.com/questions/13583/what...

While smaller planets are certainly more numerous - many of them likely lack the mass and active magnetic field to retain an atmosphere in the habitable zone.

[hoseja]

Aluminium has been out of reach for much of history.

[pfdietz]

But not because it needed too much energy.

[zabzonk]

Then for what other reasons?

[pfdietz]

Well, it needs electricity. For most of history, we didn't have electricity. And, we didn't have cheap electricity until electromagnetism was understood and electric power could be made from rotating machinery. After that, there needed to be found a chemistry that would allow aluminum to be made directly, rather than from sodium or potassium reducing aluminum compounds.

The actual physical amount of energy needed to make aluminum would have been available since forever; it's the form of that energy that wasn't there.