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The rocket equation makes this number fairly calculable. https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldnt-

by shagie 4y ago

The rocket equation makes this number fairly calculable. https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldnt-work https://space.stackexchange.com/questions/14383/how-much-big... One doesn't have to go too much larger before it is impractical to get off the planet with even the best theoretical chemical rockets.

elevaet 4y ago
So conversely the best candidate for a space-faring species might be from a very small planet.
- Ekaros 4y ago
  Probably something between Earth and size of Mars. As getting too light makes holding atmosphere harder.
- lampshades 4y ago
  Kind of like how the best candidates for original sea travel came from small islands.
- koheripbal 4y ago
  No, because smaller planets cannot hold an O2/CO2/N2 atmosphere like ours.
  - elevaet 4y ago
    There must be a sweet spot size of planet that is less massive than ours, but that is still big enough to hold an atmosphere. edit: "That critical size, according to Arnscheidt and the other authors of the study, is 2.7 percent the mass of Earth. They say that any smaller than that, and the planet simply won’t be able to hold onto its atmosphere and water long enough for life to appear." shorturl.at/bdgIK So, a mercury sized planet could hold an atmosphere in theory, and require around 1/3 the velocity to escape (vs. earth)
    - shagie 4y ago
      The tangent to this is - the reason that Mars doesn't have much of an atmosphere isn't that its not gravitationally massive enough to hold on to it, but rather that its core cooled and lost its magnetic shielding. The lack of a magnetosphere then allowed the solar wind to strip its atmosphere. https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space https://www.nasa.gov/press-release/nasas-maven-reveals-most-...
  - PeterisP 4y ago
    Is an atmosphere necessary? After all, our life evolved in the oceans as far as I understand, and you can have oceans without atmosphere like Europa in our solar system.
floxy 4y ago
Maybe you don't carry your own fuel? https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1079.9524&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.10... ...not saying this is directly applicable, but maybe food for thought?
- shagie 4y ago
  That's interstellar - which is a different set of problems. The main one is getting out of the gravity well of the planet. https://arxiv.org/pdf/1803.11384.pdf https://arxiv.org/pdf/1803.11384.pdf On a super earth, such as Kepler 20b ( https://en.wikipedia.org/wiki/Kepler-20b https://en.wikipedia.org/wiki/Kepler-20b ) in order to lift 1 ton of mass to orbit, it would need 9000 tons of fuel -- about 3x larger than the Saturn V. To get the JWST off the planet, it would take 55,000t of fuel. To get that 45t of an Apollo mission off the planet, it would take about 400,000t of fuel over 100x more than the fuel needed for Saturn V.

by shagie 4y ago

elevaet 4y ago

So conversely the best candidate for a space-faring species might be from a very small planet.

Ekaros 4y ago
Probably something between Earth and size of Mars. As getting too light makes holding atmosphere harder.
lampshades 4y ago
Kind of like how the best candidates for original sea travel came from small islands.
koheripbal 4y ago
No, because smaller planets cannot hold an O2/CO2/N2 atmosphere like ours.
- elevaet 4y ago
  There must be a sweet spot size of planet that is less massive than ours, but that is still big enough to hold an atmosphere. edit: "That critical size, according to Arnscheidt and the other authors of the study, is 2.7 percent the mass of Earth. They say that any smaller than that, and the planet simply won’t be able to hold onto its atmosphere and water long enough for life to appear." shorturl.at/bdgIK So, a mercury sized planet could hold an atmosphere in theory, and require around 1/3 the velocity to escape (vs. earth)
  - shagie 4y ago
    The tangent to this is - the reason that Mars doesn't have much of an atmosphere isn't that its not gravitationally massive enough to hold on to it, but rather that its core cooled and lost its magnetic shielding. The lack of a magnetosphere then allowed the solar wind to strip its atmosphere. https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space https://www.nasa.gov/press-release/nasas-maven-reveals-most-...
- PeterisP 4y ago
  Is an atmosphere necessary? After all, our life evolved in the oceans as far as I understand, and you can have oceans without atmosphere like Europa in our solar system.

floxy 4y ago

Maybe you don't carry your own fuel? https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1079.9524&rep=rep1&type=pdf https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.10... ...not saying this is directly applicable, but maybe food for thought?

shagie 4y ago
That's interstellar - which is a different set of problems. The main one is getting out of the gravity well of the planet. https://arxiv.org/pdf/1803.11384.pdf https://arxiv.org/pdf/1803.11384.pdf On a super earth, such as Kepler 20b ( https://en.wikipedia.org/wiki/Kepler-20b https://en.wikipedia.org/wiki/Kepler-20b ) in order to lift 1 ton of mass to orbit, it would need 9000 tons of fuel -- about 3x larger than the Saturn V. To get the JWST off the planet, it would take 55,000t of fuel. To get that 45t of an Apollo mission off the planet, it would take about 400,000t of fuel over 100x more than the fuel needed for Saturn V.