To add some more math here (yay math!): the interesting thing that Michael points out is that it would take roughly 4X the mass of fuel to launch a Saturn V type rocket from his Fictional Planet into interplanetary space, compared to the amount of fuel it’d take to do the same thing from Earth. The smaller planet Gliese 581g has a higher surface gravity (1.32 g), but a lower delta-V requirement and therefore a lower fuel requirement – only 2X the mass of fuel required to launch a Saturn V type rocket to interplanetary space. And it’s the amount of fuel that can be linked to the cost of a rocket launch.

The math is Tsiolkovsky’s rocket equation, V = Isp * g * ln(m0 / m1). This just says that your change in velocity, V, is a function of the rocket engine’s performance (Isp) and the ratio of the mass of your rocket with fuel (m0) and without fuel (m1). (“g” is just Earth’s gravity and is thrown in to balance the equation.) Since there’s a natural log function here, we know that the relationship between delta-V and fuel mass isn’t linear.

But then things get a little tricky. All successful interplanetary and orbital launches from this planet are made using multi-stage rockets. That just means that once a particular stage has burned through its fuel allotment, it is jettisoned, getting rid of the “dry mass” of the empty stage and giving a better (m0 / m1) ratio for the rest of the flight. This also means that the engines of each stage can be optimized for their individual portions of the trajectory – for instance, the first-stage engines typically have worse performance than the upper-stage engines, since they have to operate inside Earth’s atmosphere. The result is that the amount of fuel required for a multi-stage rocket is less than that for a single-stage rocket, but the math is a little more complicated (to say nothing of the engineering).

]]>So I guess what I’m saying is I get why “rocket” is in scare quotes. It was a rocket in the same sense that my ass is a doorknob.

]]>Thank you for writing scientifically literate posts that are accessible and interesting to both general public (I assume) and scientists. Geek out time! A hearty “thank you” that is long overdue.

]]>Bigger things are easier to spot, but over the years we’ve been finding smaller and smaller exoplanets and it’s only a matter of time until we find things the same size and smaller than earth.

]]>Something else to chew over:

A lot of exoplanets are turning up in orbits close to their stars. Many of these, or course, will be so close that there’s no chance of their being complex organic life. But some are going to be in the “goldilocks zone” of dim stars. As I recall, luminosity is roughly proportional to mass (in stellar masses) ^ 3.3. Which means that to get the same amount of insolation a planet is going to be more than proportionally closer to its sun and thus proportionally deeper in its gravity well. So the velocity required to leave the world’s vicinity is going to be higher. So, interplanetary travel will be more difficult.

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