Category: Sol Space

Venus orbits the Sun at ~ 0.72 AU and receives roughly twice the insolation as Earth – with very low variation because its orbit is nearly circular. Its mass, radius and surface gravity are very close to our own – 0.815 Earth masses, 6052 km and 8.87 m/s^2 respectively. Its orbit around Sol lasts 243.1 Earth-days and its rotation on its axis is 224.7 Earth-days, but retrograde. This means the solar-day – time between sunrise and sunrise – is 116.8 Earth-days. Thus, without centrifugal force countering gravity, the Venusian globe is very nearly spherical, unlike all the other large planets, Earth and Mars included.

The most striking feature of Venus is its atmosphere – opaque, very reflective and very massive. By volume it’s 96.5% carbon dioxide and 3.5% nitrogen, and little dashes of everything else. The clouds are a practically unbroken haze of fuming sulfuric acid (H2SO4 + SO3 in solution with a bit of water) and opaque because they’re very deep, many kilometres. Surprisingly, if condensed, they would amount to only a few centimetres of acid. Beneath the haze banks the air is clear, though there are unidentified particles floating around that might be decomposed acid (i.e. grains of sulfur.) From the visible top-deck of the clouds to the surface is about 70 km and the surface pressure is a very high 92 bar. The surface temperature is ~ 735 K or 462 C/864 F, and would glow a dull red if the sunlight didn’t sufficiently penetrate the clouds to give a hellish eternal glow.

So what’s available on such a nasty planet? Let’s do an inventory

(1) Atmosphere – carbon dioxide is about a quarter carbon, which is the high-strength material of the future. Nitrogen is more abundant than on Earth – 2.7 times as much in fact. Combined the atmosphere could supply C,N,O for millions of space-cities, but that’s an as yet non-existent market.

(2) Surface – while hot there are materials that happily tolerate such conditions and retain strength, so teleoperated machines would work just fine, especially using high temperature electronics developed by the US DoD. Venus is similar in bulk density to Earth so its mineral resources will be akin, but differently distributed. Water has played a major role in concentrating ores on Earth, powered by plate tectonic processes. Venus doesn’t seem to have enough water in its mantle and upper crust for the same processes to occur. It might, however, do something completely different yet with similar results. We don’t yet know what so much water-free chemistry with a hot surface might do.

(3) Energy – deuterium is 150 times more prevalent than on Earth and might need removal from what hydrogen is available in the clouds to make potable water. Thus it’s a natural fusion fuel resource, though diffusely spread throughout the clouds. Above the cloud decks the energy flux from the Sun is almost twice Earth’s – and almost as much is reflected back up by the clouds as rains down from above. Thus solar energy is abundant.

(4) Gravity – Venus’s surface gravity is 90% Earth’s, so human health issues of ‘low gravity’ will be non-existent. Gravity also makes some industrial processes easier and that will be a boon.

(5) Space – at the 1 atmosphere level of the atmosphere Venus has 90% of Earth’s surface area, thus 3 times Earth’s dry-land area. Of course any colonies will need to float, but breathable N2/O2 mixtures are lighter than the ambient CO2/N2 mix. Thus vast inflated habitats will float naturally in the nicest part of the planet. The super-rotating atmosphere will mean the effective day-night cycle will be just 4 Earth days, not the 116.7 days of the rocks below. Thus a vast area for building habitats, if so desired.

(6) Any suggestions? Make a comment and let me know.

Arthur C. Clarke wrote some absolutely classic space-travel stories in the late 1940s and early 1950s, which were amongst the first to give a “real life” account of space-travel and its challenges. Some were novels – “Prelude to Space“, “Islands in the Sky” and “The Sands of Mars“ – while others were short stories, like “The Sentinel”, “Earthlight” and “Breaking Strain”. The last story I mentioned was collected in “The Sentinel” – a gorgeously illustrated collection of shorts from 1981 spanning across his career, from “Rescue Party” (Arthur’s first serious sale) to “Songs of Distant Earth” (the movie outline version.)

“Breaking Strain” seems like an odd pick in the selection because it doesn’t involve alien contact, just two spacemen condemned to a slow death by an emptied oxygen tank on their way to Venus. Their vessel, “Star Queen“, is a nuclear powered cargo ship – two spheres separated by a 100 metre boom, a prototype of the “Discovery” from “2001: A Space Odyssey“. Their orbit is a low energy Hohmann transfer orbit which, to Venus, is only 145 days long – much less than the equivalent 258 day trip to Mars. With just 30 days to go the oxygen liquefaction system, which keeps their stored air liquid by cooling it in the ship’s shadow, is punctured by a meteoroid. Their air purifier system is imperfectly able to scrub the ship’s air, but with 10% losses on each cycle through. Thus they have air for 20 days, for the two of them. Thus the conflict that makes a story.

In the course of the telling Arthur mentions that passenger ships make the same crossing to Venus in a third the time for 10 times the propellant expenditure. So what does that involve? To work it out I assumed the passenger ships were on an orbit with the same semi-major axis, a, which simplified the search for an orbital solution. What changed with each new orbit tried was the perihelion (closest point to the Sun) and the aphelion (furthermost point from the Sun.) The assumed starting orbit was Low Earth Orbit at 1,000 km and the destination was a 1,000 km orbit around Venus – and Earth was assumed to be at 1 AU and Venus at 0.7233311 AU. The results don’t vary much with slight variations to these figures. A perihelion of 0.5833311 AU and an aphelion of 1.14 AU gives an angle of 36.25 degrees to the (assumed circular) orbits of the Earth and Venus, and a total velocity change of 46.13 km/s. The Hohmann orbit, for comparison, has an angle of zero at both by definition, and a total delta vee of 13.59 km/s. To get a ten-fold propellant increase between the two means the exhaust velocity is 22.5 km/s – a figure only extractable via iteration.

I’ve no idea if Arthur ever computed the orbits in as much detail. As president of the British Interplanetary Society he would have discussed these sorts of issues over and over again. Probably over beer at the “White Hart”. Whether he remembered the exact figures – if such ever existed – is unknowable now he has left us, but the challenge of describing realistic space-travel will remain with us. Until we actually do it, of course. But no one will seriously contemplate Hohmann transfers, except for robotic vessels. The time penalty is hefty and leaving any crew in space to soak in cosmic rays is detrimental to all. Oxygen recyclers are also somewhat more efficient now, so we may never have a “Breaking Strain” scenario…

At the edge of our Solar System is a Belt of small ice-worlds – the well-known Pluto and Charon, the unofficial Tenth Planet, Eris, and several other largish objects that the media doesn’t say much about. One of the latter is 2003 EL61, which is yet to get a real name, as the discovery code-name makes clear. It’s roughly Pluto-sized, but flattened into a football shape due to a very high rotation rate. It’s also much denser than its fellow ice-worlds, which led its discoverers to conclude it had formed in a violent collision with another ice-world. Supporting that theory are small objects in the Belt that have the same spectroscopic colour as 2003 EL61. In the Main Belt of asteroids that ALWAYS means the objects are related – for example, we have pieces of the asteroid Vesta that came to Earth as meteorites so we could check.

Computer simulations of the big smash-up support the idea, but there seemed to be no way of dating just when it occurred – bar one. Ice mixed with methane changes colour over millions of years as the methane is broken down by cosmic-rays and its break-down products recombine as “tholins” – pinkish organic material. Older ice-worlds in the Belt show these colours, indicating their icy crust once contained methane and now is diluted with a layer of tholins.

BUT 2003 EL61, and the orbitting bits of it, don’t show those tholin colours….

The Youthful Appearance of the 2003 EL61 Collisional Family

…which basically concludes that the whole group of parent body and fragments either have an anomalously low level of methane or their collision occurred less than 100 million years ago. Either conclusion is quite startling, as ice-worlds everywhere else in the Belt seem to have methane. Either the smash-up was so violent and hot that all the methane was driven off – but why didn’t the water explode as a great cloud of steam? Or the methane is there and the smash-up occurred, at the earliest, in the days of the dinosaurs. In which case a rogue dwarf-planet careened through the Belt in what is “recent time” for big planetary collisions.

In the early 1970s John Lewis modelled the interiors of the outer planet satellites and discovered they might have sub-surface oceans of ammonia/water…

Lewis, J.S. (1971) Science, Volume 172, Issue 3988, pp. 1127-1128

Lewis, J.S. Icarus, Volume 15, Issue 2, October 1971, Pages 174-185

Abstract
Steady-state thermal models for the icy satellites are constructed in which the energy released by radioactive decay in the interiors of the satellites is exactly balanced by the net radiative loss from their surfaces. It is shown that the Galilean satellites of Jupiter and the larger satellites of Saturn, Uranus, and Neptune very likely have extensively melted interiors, and most probably contain a core of hydrous silicates, an extensive mantle of ammonia-rich liquid water, and a relatively thin crust of ices. Consequences of this model relating to the Galilean satellites and the rings of Saturn are briefly described.

The atmospheric compositions and densities of the large icy satellites and certain features of the retention of volatiles during accretion are discussed.

Thus we’ve known about these dark, inner Oceans for over 3 decades or so. Opinion, in science, is never constant without data, and some estimates of the heat-transfer via solid-state convection of ice has meant the liquid interior models have fallen out of favour – at least until more data for the oceans came to hand from Galileo and Cassini. Galileo discovered that Ganymede, Callisto and Io had detectable magnetic fields – the case of Callisto, the field seemed localised to a thin layer of salt-containing fluid just under the outer ice, enwrapping the ‘mud’ mantle below. Cassini has discovered that Titan’s outer crust is decoupled from the inner layers, probably because of a liquid mantle of ammonia/water or ammonium sulfate.

But what about other moons? Europa very probably has an ocean as its crust looks like Arctic sea-ice, and Enceladus’s geysers are hard to explain via any other cause. Further afield? Here’s an interesting paper from Paul Schenk and Kevin Zahnle…

Schenk & Zahnle, Icarus, Volume 192, Issue 1, 1 December 2007, Pages 135-149

Abstract
New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton’s orbital motion (i.e., all are on the leading hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker–Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton’s small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of 50 Myr on the age of the more heavily cratered terrains, and of 6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton’s is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration.

…which seems to indicate a very dramatic thermal history for Triton, with a more global melting of its crust than the apparently localised melt on Enceladus’s south pole. If so, then the sub-surface ocean is potentially very close to the surface and liable to burst through in cryovolcanic events, making the moon a very interesting target for future investigation.

A recent study of young stars reveals a bunch with dust disks. Both New Scientist and BBC Science-Nature News report on the find…

Planet Hunters Set for a Big Bounty

Many Earth-like planets may exist in Milky Way

…with the basic conclusion that 20% of young Sun-like stars observed have dust disks, and thus may have planets like the rock-iron planets in our system. The researchers concluded that up to 60% of stars like the Sun might have rock-iron planets – like Earth in bulk composition, not necessarily like Earth in being habitable and inhabited that is.

Interestingly both news services mention Alan Stern’s opinion about thousands of planet-like bodies in the Kuiper Belt and Oort Cloud, though New Scientist muddlees his views about the number of Mars-to-Earth mass objects…

Alan Stern, a planetary scientist at NASA, says 1000 or more additional planetary bodies may be lurking at the outermost edges of our solar system, in the mysterious band of icy debris beyond Neptune called the Kuiper Belt, and in the more distant Oort Cloud, where some comets are believed to originate.
“If you like change hold on to your hat, because the view of our solar system is changing dramatically,” says Stern. “I fully expect in the deep outer solar system that we will find objects the size of Earth, Mars and potentially larger as this century unfolds.”

…while the BBC piece says…

Some astronomers believe there may be hundreds of small rocky bodies in the outer edges of our own Solar System, and perhaps even a handful of frozen Earth-sized worlds. Speaking at the AAAS meeting, Nasa’s Alan Stern said he thought only the tip of the iceberg had been found in terms of planets within our own Solar System. More than a thousand objects had already been discovered in the Kuiper belt alone, he said, many rivalling the planet Pluto in size.

“Our old view, that the Solar System had nine planets will be supplanted by a view that there are hundreds if not thousands of planets in our Solar System,” he told BBC News. He said many of these planets would be icy, some would be rocky, and there might even be objects with the same mass as Earth.

“It could be that there are objects of Earth-mass in the Oort cloud (a band of debris surrounding our planetary system) but they would be frozen at these distances,” Dr Stern added. “They would look like a frozen Earth.”

…which clarifies the size range he expects – anything from 1000 km across to 10,000 km across, not “1000s of [Earth-sized]planets” as the other report could be misread. For theoretical reasons there could be dozens of Earth-to-Mars sized objects out there, scattered by the surviving Ice Giants as they migrated out from closer to the Sun.

Such Worlds would be ripe for terraforming if we had an efficient way of piping light-and-heat from the Sun. Perhaps collector stations close to the Sun could lase the energy to the Oort planets, though I’m not sure anyone would want the Solar System being spanned by 100 petawatt laser beams. Their intensity at their destination need only be akin to what Earth receives normally, but the inverse-square law means their intensity will be much higher close to the Inner planets. But perhaps a fairly diffuse beam could be sent, then refocussed by a lens parked in the planet’s sub-solar L1 point?

Helium-3 is often seen as a profitable material to “mine” the Lunar regolith for – it’s a potential fusion fuel, but currently a fuel without a market. No current reactors in the works (i.e. ITER) are big enough (!) to burn the stuff, and no 2nd and 3rd Generation Fusion reactors are likely before c.2100, at current pace of development.

BUT let’s assume there was a market – He-3 burns quite well in IEC reactors only a bit bigger than D-T burning IECs, so once Doc Bussard’s Whiffle-Ball is demonstrated a market might appear over-night (5-10 years.) If so, how much is on the Moon? According to this reference there’s 2.5 million tons embedded in the upper layer of Lunar regolith (typically 4-12 metres deep, depending on locale.) Sounds like plenty, but you have to process a lot of moon-dust to get at it – there’s 38 million sq. kilometres of Moon and so just ~ 66 kg He-3 per sq.km, some ~ 8 million cubic metres of regolith to process for just that.

How much is 66 kg of He-3 worth then? Fusing He-3 generates ~ 57 million kW.hr of energy of which about 60-80% can be electricity with the right converters. Call it ~ 60% and 66 kg of He-3 is 2.266 billion kW.hr of power – about $113 million @ $0.05/kW.hr. Using current technology this would be unprofitable, but a few things could be done to improve the economics. For example, Jerome Pearson’s Lunar Beanstalk would eliminate the need for rockets to launch material to Earth, and could deliver ~ 200kg per trip to the Earth-Moon L1 point to be retrieved by low-thrust inter-orbital vehicles for return to LEO. Mining would have to be fully automated, of course, and processing millions of cubic metres of soil per year would be required, but this might not be onerous.

Eventually the supply will run-out. Globally we currently use ~ 15 TW of power, with growth steadily heading up, even with efficiency gains. If everyone used energy like an American or Australian (11 kW/capita) then currently 74 TW would be needed. That’s 74 billion kW.hr per hour, some 649 trillion per year. Some 286,000 tons of He-3 per year. The Moon would be exhausted in a decade. That’s a rather unlikely rate of use, but it does show the Moon’s resource potential is very limited. Within ~ 100 years we would be looking further afield. So where next?

Bryan Palaszewski’s 2006 study for NASA (available via the Glenn Technical Reports Server) looks at the options for mining the Gas Giants. For Uranus and Neptune, which have quieter atmospheres, should be accessible to balloon-borne factories, like the Daedalus report advocates – Bryan actually uses that design for analysis. On Jupiter and Saturn, with greater turbulence, actual aircraft will be needed. What does seem problematic is getting the stuff into orbit as that requires sustained hypersonic flight by the vehicles, something yet to be achieved reliably.

All that could change, at least on Earth, as Alan Bond’s Reaction Engines Limited advocates a hybrid SSTO called SKYLON, and a non-orbital version for hypersonic passenger flight. If SKYLON were developed successfully an immense amount of hypersonic experience would be gained, ultimately allowing mining of the Gas Giants. SKYLON would also enable other power-sources, like SPS, so it’s worth pursuing by itself.

Back in the late 1960s Wernher von Braun developed an architecture for colonising the Red Planet and the Moon, as a by-product, using Apollo-style boosters and NERVA-style nuclear rockets. Basically the Interplanetary vehicles were composed of three independent components – the Primary Propulsion Modules, the Planetary Mission Modules and the Mars Excursion Modules/Vehicles. A single Interplanetary Vehicle was composed, usually, of 3 PPMs, 1 PMM and 1 MEM, but an extra-PPM would be needed for some mission configurations. This was quite different to an earlier mission architecture which favoured using 4 PPMs, minimum, and an EEV (Earth Entry Vehicle) for direct returns, Apollo-style, to Earth. Von Braun believed that an Orbital Receiving Laboratory, at a 50 person Orbital Operations Centre (“Space Base”), was needed to isolate possible biological samples from Mars – von Braun had discussed the possibility of intelligent Martian life in his literature on Mars from the early 1960s, so he was being consistent.

The heaviest components were the PPMs, as fully fuelled they massed nearly 246 tons each. Each PPM was shrouded in a heavy meteoroid shield and staging components until they fired. They would be launched into orbit via a modified Saturn V, the Saturn V-25 (S)U Earth Launch Vehicle, designed to lift a maximum of 249 tons when the basic core was wrapped in 4 large Solid Rocket Boosters.

Stephen Baxter’s fictional account of a 1986 Mars Expedition, “Voyage”, explodes one such booster configuration in a launch accident in 1981 – due to a flaw in the SRBs, just like the real “Challenger” disaster of 1986. In “Voyage” that launch disaster and a core explosion in the Apollo-N nuclear test causes NASA to adopt chemical propulsion. While that was an option for a single-shot mission, von Braun’s long-term plan was to colonise Mars. Each vehicle was to carry 6 personnel, and two vehicles would carry 12 people to Mars. In one option two MEMs would depart for the surface and place 6 people on the surface, but another option was for a single lander and orbital operations amongst the Martian moons using a Space-Tug, itself capable of missions to both moons.

But eventually more people would need to be based on Mars and to do so specially designed freighters were required. These would carry Base Modules – the surface equivalent of a PMM – and Descent Modules, essentially the lander-half of an MEM. A single freighter could carry four of either at a time, and they would be combined in Mars orbit for delivery to the surface. A single Base Module could accommodate 12 personnel, just like the PMM and the core Space Station module that the PMM was to have evolved from.

On that issue, the Space Station, the original was to have launched in c.1975, to carry the “Skylab” experiment even further, and provide training in long-duration in-space activities for 12 astronauts, male and female. “Apollo” itself was to have finished with Apollo XX in 1975, to be succeeded by the much more powerful Space-Tug. The Space-Tug was to have been launched via a single Saturn-V for 14-day missions to the Lunar Surface with a crew of 3. Eventually a Space Station module was to have been landed using a Space-Tug Propulsion Module and to form the nucleus of a Moon Base. To sustainably operate a Base the Nuclear Shuttle was to have been introduced in 1978 to service the Base, carrying multiple loads of Base Modules, Propulsion Stages and freight modules. Eventually the Base was to be powered via nuclear reactors and/or solar power, depending on the applications.

You can see how the Moon Base was to be a practice run for Mars – power reactors, Nuclear Shuttles and PMM/Base Modules. A very crowded schedule with a first Mars Mission launching in 1981…