Home > Fermi Problems, Your Turn > Your Turn, Round 2-6: How to Escape From a Gas Giant

Your Turn, Round 2-6: How to Escape From a Gas Giant


Between 2 and 3 years ago, I did a series of posts describing a sciencefiction setting of my own invention.  I was exploring different ways one might resolve the Fermi Paradox.  For those unaware, the Fermi Paradox goes like this: We observe many places in the universe where technological life could appear and persist; but we have no evidence of alien visitors to Earth.  Where is everybody?

The default solution to this paradox is to have technological life be rare in the universe.  Other possible solutions include civilizations encountering various problems; barriers that prevent them from traveling or sending robotic emissaries across interstellar distances.  So I call the setting “Fermi Problems” (geeky reference is geeky).

One situation I considered was life on a gas giant; specifically a hypothetical superjovian planet around HIP 66461, a G-type star 150 parsecs from the Sun.  Since HIP 66461 is in the section of sky allocated to the constellation Ursa Major, I called these hypothetical aliens the ursians and the planet Ursa.  I had thought that Ursa was a deep enough gravitational well that the ursians could never escape without outside intervention.  I may have been wrong.

So, How Do You Escape From A Gas Giant?

To go into orbit around a gas giant with 2.8 times the mass of Jupiter, you have to acquire a speed of at least 50 km/s.  Escape velocity requires twice the energy of the minimum orbital speed – i.e. 70 km/s in velocity, plus a bit more to account for the remaining gas drag.  That is very fast.

It’s faster than any chemical rocket can provide, at least with a reasonable ratio of mass of fuel to mass of payload.  Electric propulsion could provide that velocity change, but can’t give the acceleration needed to lift something when the planet’s gravity is about 7 gees.  Nuclear fission and fusion propulsion could do the job, but you won’t find fission fuels in the atmosphere of a gas giant.  And high-thrust fusion rocket designs either require fission bombs as the first stage in a nuclear pulse unit; an abundant supply of tritium – in turn made in part from things the ursians don’t have in their atmosphere; or both.  Directed-energy propulsion is ruled out by the energy density required and Ursa’s atmosphere being in the way.

I had thought that an electromagnetic launch system, such as a rail gun, was excluded for three reasons: a lack of high-field-strength magnets, lack of a sufficiently large launch platform, and too much air resistance after the payload left the launcher.  However, two different technologies may allow all three of those things to be addressed.  So it may be possible to launch things to space with electromagnetic propulsion from a gas giant after all.

Hydrogen Sulfide Is Strange

Hydrogen sulfide, H2S, is a well-known compound.  It’s produced on Earth by bacteria in swamps and variety of other environments; and notoriously gives rotten eggs their smell.  It’s also a broad-spectrum poison to mammals, and we’re highly adapted to detecting it: the threshold for most humans smelling it is about half of a part per billion.  More relevant to the ursians’ problem; it occurs as about 40 parts per million of the atmosphere of Jupiter – chemically bonded with ammonia, to give ammonium hydrosulfide (NH4SH).  I had assigned Ursa a similar atmospheric inventory.

Efforts to identify high-temperature superconductors have recently shown that hydrogen sulfide becomes superconducting under high pressure.  This is based on both theoretical and computer modeling and direct laboratory measurements in 2014 and 2015; variously by researchers in China, Germany, Austria, France, Japan, and the United States.  The pressure conditions required here are extreme: 1.5 million standard atmospheres or about 150 GPa; with the H2S cooled to about -70 C.  There’s now a flurry of research ongoing to see if other hydrogen-rich compounds or mixes thereof are superconducting at lower pressures and/or higher temperatures.  But for the purposes of science-fiction world-building, just that H2S acts this way is enough.

The ursians could make superconducting magnets with very high magnetic field strength – provided they can compress some H2S extracted from the atmosphere and keep it under extremely high pressure.  How to do that?

Synthetic Diamond

Synthetic diamonds are a big industry.  A very small part of that is the production of diamond anvil cells, which can provide pressures up to 6 million atm / 600 GPa.  If the ursians are sufficiently adept, they can manufacture synthetic diamond using chemical vapor deposition with a low-pressure mixture of methane and hydrogen – both of which they have an abundance of.  So, you have superconducting magnets with which you want to build an electromagnetic spacecraft launch system.  Where do you build it?  Remember, whatever your building site is, it has to be able to float in a hydrogen-helium atmosphere.

Previously, I had limited the ursians to floating biological hot-air balloons and to artificial islands of vacuum-filled aerogel bricks; limited to 2-5 km and ~10 km across respectively.  But if you could produce synthetic polycrystalline diamond in truly large quantities, along with nanotube fibers and the other interesting forms carbon atoms can be assembled into, then much larger structures could be assembled.  Consider a massif made of diamond vacuum balloons.

If we can grant the ursians the ability to make enough synthetic diamond, they could construct individual massifs each a hundred kilometers on a side and 35 kilometers thick.  The horizontal limit is set by pressure and wind shear.  The vertical limit is set by the pressure and temperature profiles of Ursa’s atmosphere.  Below the 30-35 atm altitude, diamond will degrade due to excessively hot temperatures (many ursians call the lower parts of the atmosphere “The Hot”).  35 kilometers above, at the 1 atm altitude, it becomes difficult to make the diamond shells thin enough to make blocks that are bouyant in a hydrogen-helium mixture.

So we have a lot of living space: 10,000 square kilometers, which can have an average of 10 stories of ursians and their farms and homes and industries on top without overloading the massif beneath it.

Tall Towers

Although diamond shells and frameworks can’t float much above 1 atm in Ursa’s atmosphere, a large massif can support very tall structures: up to 100 km high with large safety margins.  You might build such things initially for wind power, taking advantage of vertical gradients in wind speed, or to better harvest gases at different altitudes of the atmosphere.  But interesting things happen at higher altitudes:

Sketch of what you might build in the atmosphere of a gas giant, if you could make truly outrageous quantities of synthetic diamond.

Sketch of what you might build in the atmosphere of a gas giant, if you could make truly outrageous quantities of synthetic diamond.

Fifty kilometers or so up from the massif’s surface would the operating altitude for hypersonic aircraft, if the ursians have a need for such high-speed transportation to other floaters / islands / massifs thousands of kilometers away.  And 100 km up, the atmospheric pressure is a few hundred-thousandths of what it is at the surface.  That’s low enough for an electromagnetic spacecraft launch.  And also low enough for the ursians to lob projectiles over a large fraction of Ursa’s radius, if they have a need to do that.

Ursian Spacecraft

So, what can the ursians send to space?

Traveling quickly takes energy.  If you had a linear electromagnetic launcher 100 km long, and wanted to accelerate a projectile to 50 km/s, you’d need to accelerate it in a straight line at something above 1,250 gees for a bit under 4 seconds.   Going to escape velocity requires >6000 gees for just under 2 seconds.  Allowing for the efficiency of the launcher, that would mean a few gigawatts of power for every kilogram sent into orbit – and more than 10 GW / kg for anything sent on an escape trajectory.  Allowing for both solar and wind power, the entire massif would have no more than 10 GW to work with, so some way of spreading out the load is required.

A circular launcher can do that, with careful management of angular momentum to prevent the whole massif from spinning and with careful payload design to handle the jerk when it is let go from the launcher.  The payload still undergoes a lot of acceleration: For a 30-km radius loop, like I sketched above, acceleration while in the loop would peak at 8,500 gees while building up to 50 km/s speed.  But with nearly all of that acceleration at right angles to the speed, the power required is much lower.  In principle, the ursians of one massif could launch 10 tons to low orbit and 2 tons to escape each (Earth) day.

These spacecraft can’t carry the ursians themselves.  8,500 gees is a huge load, like that on a bullet hitting a strike plate; and the ursians would be squashed into goo.  But we build both electronics and moving mechanical parts that can handle such accelerations, and even some terrestrial biological systems can do so (Wikipedia has an interesting list of examples).

So if they were motivated to make the investment in relevant technologies, the ursians could launch diamond-framed robotic spacecraft into low temporary orbits around Ursa.  Chemical rockets could provide just enough of a periapse raise to stop them from falling out of the sky.  Then they have Ursa’s rings and moons to interact with.  Or they could send smaller packages out very quickly, into solar orbits around HIP 66461.

Sending the ursians themselves off-planet would require many more technological innovations than I have described.  But is this all enough to say that hiding aliens on gas giants does not by itself resolve the Fermi Paradox?

Addendum: As with the previous posts in this series, I encourage anyone who wants to do so to lift the setting or elements of it for use in stories in any medium.  Just let me know if you do, so that I can see the story concerned.

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  1. 2015/12/13 at 1:19 pm

    Escape velocity is the square root of 2 times orbital velocity, so about 70 km/s.

    Once you get significant quantities of materials into orbit, you can make subsequent launches easier with electrodynamic tethers. An untapered, rotating, carbon nanotube tether could potentially boost a payload by as much as 13 km/s–a tapered one a little more. That won’t get you to escape velocity in one shot, but stepping up with multiple tethers at different altitudes could do the trick. Add a chemical booster, and launching at as little as 30 km/s from the circular launcher might be feasible to get all the way to escape velocity quickly.

    A few centuries later, a robotic, nuclear-powered spaceship built in the local asteroid belt returns to pick up some Ursan space travelers…or maybe I’m missing some crucial physics at that point.

    • michaelbusch
      2015/12/13 at 2:02 pm

      Good catch on the square root of 2. Fixed.

      Re. the tether-booster idea:

      I don’t know how durable long strands of carbon nanofiber would be in an environment with lots of particle radiation and high magnetic field strength (even if I handwave and say that Ursa’s radiation environment is more like Saturn’s than Jupiter’s, due to the lack of a large inner moon like Io feeding in plasma). Not to mention the dust from the ring system. More distant orbits would be much safer.

      Re. the longer-term progression of an Ursian space program:

      A lot depends on how adept robots built using carbon-based structural elements and carbon/arsenic/gallium electronics that have to survive the launching process can be. And I don’t know the answer to that.

  2. michaelbusch
    2017/11/05 at 10:55 am

    A four-years-belated addendum:

    The circular launcher concept is not original to me. It’s even been suggested for launching things from Earth, under the impressive name “Slingatron” – https://books.google.co.uk/books/about/Slingatron.html?id=fB_VPQAACAAJ&redir_esc=y .

    A major difficulty for using it on Earth: The atmospheric pressure on the surface is such that the payload would be slowed down too much, and probably vaporized by much of the kinetic energy getting turned into heat, as soon as it left the launcher. As for Ursa, you’d have to build very tall towers to put the launcher far enough up for the payload to survive and get to orbit. For Earth, there are far easier ways to get to space. For Ursa, I do not think there are.

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