Voyage… the details
Stephen Baxter’s book “Voyage” is a fictional recreation of a 1986 mission to Mars as it might have happened, based on real events in our own time-stream. Hence it is “historical fiction” more than it is “science fiction”, but interesting to me for its proposed system for getting to Mars.
At first “Voyage” follows the post-Apollo recommendations for a Mars mission as an ultimate goal. In the original Space Transportation Group’s plans Saturn-Apollo was to be extended in its capabilities by a nuclear-powered third-stage, set to come into service by 1978 or 1980. In the “Voyage” alternate history NASA suffers a disastrous set-back when its new Apollo-N suffers an explosive reactor rupture, killing the astronauts slowly with a high radiation dose. Nuclear space-tugs are decisively put on indefinite hold after that.
The original Mars plan was for all nuclear propulsion, and chemical propulsion ruled out. But this wasn’t an absolute necessity. By halving the crew to just three and using a Venus swing-by to boost the vehicle the Ares mission goes ahead anyway. How do they manage it?
Some important up-grades are needed to the basic Apollo-Saturn core:
Launching a Saturn MS-IIC, the modified second stage isn’t ask big a task as I initially thought – that’s basically what Skylab did in 1973. Skylab itself was a modified S-IVB third-stage, which carried no engines or propellant – it was a ‘dry’ Saturn Orbital Workshop. Hence the S-IIC did most of the work of putting itself and Skylab into orbit, virtually the core of Ares orbitted in one go.
Which makes me wonder: how easy would it have been to orbit a S-IVB stage around the Moon as a ‘wet’ workshop? Pretty simple really. Of course Apollos launched by Saturn-Vs would be the only ones able to reach it, but a real ‘Moonlab’ would have been an enduring reminder that humans had been to the Moon.
Bombing Triton II
After Nereid smashes into Triton “hot” debris will rain down all over that little moon, probably sublimating into gas all the ices bar water ice. A massive shockwave will break-up an immense area of crust. According to at least one researcher Triton shows all the signs of having a thin ice crust over an ocean-thickness of water/ammonia – the surface is poorly cratered compared to similar moons and has huge disrupted areas already. Perhaps Nereid wouldn’t be the first super-impactor in Triton’s history?
Makes sense since a whole cloud of cometoids as big as Nereid orbit just beyond Neptune and some even cross its orbit. If any planet and moon will show signs of giant comet strikes it will be Neptune/Triton. But what melted a whole half a moon? That’s how much area looks like refrozen strawberry ice-cream! Hence after impact a huge ocean will be revealed, shrouded in a thick greenhouse blanket, perhaps enough to keep things warm for a few millennia.
Triton orbits around Neptune in an orbit opposite to Neptune’s own rotation – a retrograde orbit – indicating a catastrophic past. Something disrupted Neptune and its moons… and you can probably guess the usual suspects. Pluto and Charon, planet and moon, just beyond Neptune’s orbit have been indicted for planetary disruption. Here’s a good summary of just how…
…but they’re not the only candidates for planetary collision. Marvin Herndon suggests a rather curious origin for the Earth – and Venus, since it is virtually identical – in this recent paper…
…you might wonder what is distinctive about ‘protoplanet’ in the title. An old term describing a gaseous cloud condensing directly into a Jovian planet under its own gravity. Hence a model contrary to the prevailing core-accretion model which builds up planets from small chunks of ice/rock until they’re big enough to pull in H/He gas in space around them. That’s right – Earth and Venus are the cores of a Jupiter scale planet.
So how did they lose all that gas??? Planetary collision is the only mechanism I know of that can supply the energy needed, but how did they collide? Michael Woolfson’s Capture Theory is the only current model that explicitly produces Earth and Venus via collision. Stephen Droxley did a PhD on modelling the process, available here…
…but he doesn’t describe the collision itself. Here’s an article from the Royal Astronomical Society’s magazine…
Stephen Baxter in his “Manifold: Space” novel has a character cause the collision of two of Neptune’s moons, Nereid and Triton, to give Triton an energy boost, making it more habitable for Aboriginal refugees from Earth. So I ask: what would such planetary engineering require?
|Name||Semimajor axis (10^3 km)||(Neptunian Radii)||Orbital Period (Days)||Inclination (degrees)||Eccentricity||Orbital Velocity|
|Triton (NI)||354.76||14.328||5.876854R||157.345||0.000016||4.39 km/s|
|Nereid (NII)||5,513.4||222.67||360.13619||7.23||0.7512||1.11 km/s|
As you can see Triton is about as close to Neptune as Earth’s Moon is to Earth, but about 4 times quicker. Nereid is a long, long way out. Such an orbit would be unstable around the Earth and it would become a Near Earth Asteroid.
An important feature of Nereid’s orbit is its high eccentricity – it actually orbits between 1.3717 million km and 9.655 million km, reaching 2.953 km/s at the low point and a mere 0.4195 km/s at the high point. Quite a roller-coaster ride. Also the eccentricity means that Nereid spends ~ 74% of its orbital period higher than the average distance, and a mere 26% of the time closer to Neptune.
To crash Nereid into Triton requires lowering the low-point (the periapsis) by losing velocity at the high point (the apoapsis.) To lower Nereid into Triton’s orbital path – conveniently opposite to Nereid’s – it needs to shed ~ 196 m/s. How much would that require in time and rocket thrust?
Firstly, Nereid is 340 km across and masses ~ 30 quadrillion tons. Deccelerating it by 196 m/s takes ~ 6×10^21 Newtons of force in total. That’s a lot of force! Baxter deploys huge ion engines to do the job, but they’re probably the worst for the job. Rockets are characterised by exhaust velocity and their jet-power. Ion drives generally have a high exhaust velocity but a low jet power – maybe 100 megawatt for advanced ones. Nuclear Thermal Rockets however have a medium exhaust velocity and a very high jet power – up to terawatts of power for very large NTRs.
A high-end NTR can get 10,000 m/s jet velocity, hence a velocity change of 200 m/s will need ~ 1/50th of the total mass to be moved, expelled as propellant. In Nereid’s case that’s ~ 600 teratons of mass – probably a mix of methane, ammonia and water ices extracted from Nereid directly. Spread over several years of thrusting (~ 60 million seconds), that’s 10 million tons/second, and a 500,000 terawatt jet-power. An incredible figure. Baxter, we might say, glossed over the difficulties involved.
Alternatively we might deploy a fusion engine of some sort – probably a pulse drive – with a very high exhaust velocity and jet power. For the same thrust-time a fusion drive with ~ 10,000,000 m/s exhaust velocity has a power of 500 exawatts – 1000 times the previous power-levels. Insane amounts of energy. Like exploding 120,000 megaton bombs every second…
Assuming we do bomb Triton with Nereid, what will it be like? The impact speed is ~ 10.4 km/s, and the total energy released is ~ 1.6×10^27 Joules… or about 400 billion megatons TNT equivalent (FYI… TNT packs about 4.2 megajoules/kilogram, hence a megaton is ~ billion times that, some 4.2 petajoules.) Assuming Triton’s ice-crust is at ~ 38 K about ~ 700 km of crust would melt all over the moon. However that simplifies far too much. If Triton were just ice and rock then it’s about 56% rock, collected in the core, and the ice is just ~ 240 km thick. Nereid’s remains would probably penetrate the ice and heat-up the core via direct collision.
However there are good chemical arguments that suggest Triton has a lot of carbon dioxide and methane ice, plus an outer layer of nitrogen/ammonia ices. The resulting ferment would leave Triton with a much thicker atmosphere for a long, long time.
Three new Neptune-mass planets announced in the last week and many more to come I am sure. Quite a large time of observation precedes declaring a planet in a journal article – the official point at which a discovery is accepted – and I would guess dozens of candidates are being watched.
More mini-planets, Kuiper Belt cometoids, are being discovered all the time too. Here’s the latest listing from the IAU [the International Astronomical Union]…
…as you can see there are heaps of them – 813 when I accessed the page to write this, doubltess more next time you or I look back. A related group are the Centaurs and Scattered Disk Objects here…
…Sedna [its unofficial name] is 2004 VB12 or Minor Planet 90377, since it just graduated to having a number. The IAU is deliberating on ‘Sedna’ as its name, so calling it that is still ‘informal’. A recent news-flash is the curious prospect that Sedna is an exoplanet captured off a passing brown- or red-dwarf star…
…Sky&Telescope news-links get archived pretty quick so follow it now. Wouldn’t it be cool if it was an exoplanet on our cosmic doorstep??? But there is an even more curious possibility. Another group of small objects is listed here and currently without a category… what if there’s a few old alien probes in amongst the jumble? With sufficient engineering a probe could be self-repairing and indefinitely powered by solar and/or fusion power. It might even be able to self-replicate, albeit slowly else there’d be a few missing planets by now. Perhaps it or they watch us, waiting for our venturings forth into their territory.