Thursday, March 12, 2015

The Importance of Dog Bones and Doughnuts: Revisiting the Freeluna Space Program

Given the success of SpaceX and Elon Musk's and Mars One's intent to colonize Mars, I think it's high time to revisit the notion of a rotating space station in low earth orbit, and eventually using said station (or something like it) to transport people to Mars. Before we go colonizing other planets, we really need to find out if humans can stay healthy in a partial gravity (less than 1 g but more than 0 g) environment, such as what humans will encounter on Mars (0.377 g) or the Moon (0.165 g). To the best of my knowledge, there has been absolutely no research done in these two gravity regimes. Our knowledge of zero gravity (microgravity) is extensive, and we have known for decades that exposure to it causes substantial physiological decay. What we need to find out is how badly or if Mars colonists will suffer decay within the gravity field of Mars.

To this end, let me introduce you to the concepts used to design a rotating space station created for the purpose of testing partial gravity fields on the human body. In the old 50's sci-fi movies, we saw plenty of examples of spinning wheels in space that represented space stations. In the movie 2001: a Space Odyssey, Dr. Heywood R. Floyd first finds himself heading for a double wheeled space station spinning in Low Earth Orbit, before he heads to the moon. The principle of artificial gravity used in these films is just an application of conservation of angular momentum, commonly called centrifugal force. Spinning a person around in circles can cause problems such as dizziness for some people, but almost all people can tolerate a spin rate of 2 rpm and some people can tolerate a spin rate as fast as 7 rpm. Depending on the RPM and the gravitational pull one is trying to induce, the radius of the spinning space station varies. The formula for calculating the radius for earth-like gravity is as follows:

Where 9.8 is acceleration of gravity at Earth's surface in meters/second^2 (use 3.7 for Mars and 1.6 for the moon), 'pi' is 3.14159.., and so on. With this calculation in hand, you can derive various habitat radii for different RPM.

If one so desired, a apparatus could be set-up on earth to simulate these spin rates and radii to test for human spin tolerance, such as with a vehicle connected to a central pivot via appropriate length cables and forced to drive around in circles to simulate station spin. The induced gravitational pull would be somewhat over 1 g, but should allow observation of inner ear phenomena, nonetheless. A graph below shows the various speeds of such a system...

Building a complete space station ring could get quite expensive, but building just a pole with small habitats on either end of it would be far less of a challenge. For a Mars equivalent gravity in a station spinning at 2 RPM, the length of the pole would need to be about twice the radius shown in the first table, or about 169 meters long. This could be reduced by offsetting the center of mass of the station towards one end of the pole. By placing most of the mass of the station at one end of the pole, the pole length to the Mars habitat would still need to be 84.4 meters from the center of mass of the station, but the pole length extending the opposite direction could be much shorter. The extra mass could consist of the station's batteries, rocket engines, fuel, water, solar, panels, radiators, etc. It might also contain a lunar habitat, using the shorter radius of 36.5 meters, thus allowing testing of both Martian and Lunar gravity effects in one station. A crude layout of such a station, which I am calling "the Dog Bone", is shown below:

The martian habitat would be on the left end of the station's central boom, and the lunar habitat would be on the right end of that boom. The extra pod to the right of the lunar habitat represents the extra weight of various station components which offsets the station's Center of Mass or Center of Gravity (I'm using the terms interchangeably here. Sorry, sue me if you must.). If it can be shown that subjects can tolerate a higher RPM, then the central boom of the station shrinks significantly.

I don't think such a station need be incredibly expensive to build. The central boom is the most awkward piece of the station, and I believe it could be assembled in orbit. The boom is, after all, not much more than a big aluminum pipe. Assuming 3 boom sections could be put in orbit per flight, with each boom being approximately 13 meters long, it would take about 3 flights just for the boom sections, potentially two more flights for habitat sections, and two more flights for the "counterweight" and docking interface, so somewhere around $480 million for launch costs for the station. Assuming my numbers are off by a factor of three, this is still a very inexpensive space project, and the payoff for having such a station is huge.

Friday, March 06, 2015

Dawn Spacecraft is Ceres-ly Orbiting a Dwarf Planet

It has been announced that the Dawn Spacecraft has successfully transitioned to orbiting around the dwarf planet Ceres. Someone thought it amusing to label this accomplishment as a first, as Dawn is now orbiting a dwarf planet, rather than orbiting some other type of celestial rock, which we have orbited before. But seriously, it wasn't until 2006 that Ceres was even referred to as a dwarf planet. Before then, it was called an asteroid, and an orbit of an asteroid was already achieved on February 14th, 2000 (Mission was NEAR, the asteroid was 433 Eros). With any luck, the IAU will redefine Ceres as something like a dormant, mega-comet at some point, and a new NASA mission can be sent there to claim yet another astronomical first.

The real first is that Dawn has accomplished is to achieve orbit at two separate celestial destinations during one mission (Vesta and now Ceres), and this is due to its use of ion propulsion. Ion engines are about ten times more efficient than conventional bi-propellant engines, when it comes to using fuel. As a result, they allow spacecraft a greater wander range than a conventional engine would allow. In this case, increased engine performance results in much smaller fuel tanks, which results in a smaller spacecraft with more range.

Ion engines put out a miniscule amount of thrust -- it takes the Dawn spacecraft 4 days to accelerate from 0 to 60 miles per hour, but unlike chemical propulsion, Dawn can maintain that thrust continuously for months at a time. The Ion engines are remarkably simple and sturdy, and can run for upwards of five years at full thrust without breaking.

Now that Dawn has arrived at Ceres, we will get the chance to investigate this peculiar planetoid. So far, scientists have speculated that Ceres has a thick crust of ice surrounding a rocky core. It appears as though Ceres' surface is coated with a fairly thick layer of dust, and there appears to be some activity that has revealed the underlying ice layer. We should be in for an exciting several months of science ahead.