Monday, November 28, 2005

Slow Posting

I may not post much for the next few days. Holidays are over, and exams are piling up.

Friday, November 18, 2005

The Cassini-Huygens Mission

The Cassini mission is a mission launched to Saturn on Oct 15 1997, and entered orbit in the Saturn system on June 30 2004. Its purpose is to study the planet, moons, and rings of the Saturn system.

Out of Respect for Copyrights - I've posted links to pictures instead of the linked images themselves.

The Cassini Probe

Its launch generated a lot of hype back in the 1990's. It carried a small nuclear battery, called a radio-isotope thermoelectric generator or RTG, which provided power and heat to the probe. Deep space probes operating far from the sun cannot get solar energy to run their instruments, and conventional batteries don't last years at a time, so RTGs are usually employed. This RTG caught the attention of the enviro-mental community, which latched onto the fact that it used a nuclear material (2kg of plutonium) to generate heat to run the device. The screaming protest was spectacular. They were prophesizing that if it were to crash over the gulf of Mexico, hundreds of thousands of people could die from nuclear contamination, and it would wipe out sea life in the area (2 kg of plutonium!).
Info about RTGs.
Of course, such concerns were absurd, first of all because of the extremely low amount of the nuclear fuel (think about it: if plutonium is that dangerous, why do we insist on weaponizing it and building atomic bombs at all? Why not just spray parts-per-billion powder over a country?), and second of all, because it is designed to break up in the upper atmosphere to disperse harmlessly. It's also an alpha emitter.

In any case, despite the hoopla, and protestors in Darth Vader masks, the mission launched successfully, did a swing-by of Venus, then Earth, then Venus again. This fortuitous and intricate trajectory, and its associated launch window enabled the probe to make the voyage, boosting its velocity enough to send it on a trajectory for Saturn.

The Interplanetary Trajectory

When the probe arrived in the Saturn system, it detached the Huygens lander on 24 December 2004. The Huygens lander landed on Titan on 14 January 2005. The probe successfully took and transmitted pictures of the surface, atmospheric and wind data, and even sounds from the surface of the moon.

Titan is a moon of Saturn, with a radius of 2580km (for comparison, our moon has a radius of 1700km and Mars has a radius of 3400km). When Voyager I first flew by this moon in 1980, it could not penetrate the hazy atmosphere to detect anything about the surface. The atmosphere of Titan is a dense smoggy layer of methane. Titan can maintain an atmosphere this dense because of its very low temperature, being far from the sun in the outer solar system.

A Comparison of the Sizes of the various Moons.

When Huygens landed, it found that Titan has a surface sculpted by liquid methane and other simple hydrocarbons. The planet looks superficially Earthlike, at least in black and white, with river deltas and lakes. But the rivers and lakes are methane and hydrocarbons, and the rocks are water-ice and nitrogen. Titan's surface has a temperature of about 94K (-290F).

Titan's sea from Huygens as it descends.

Color Image of Titan's Surface

Conquering the Solar System

Conquering the Solar System is going to be part of the theme of this blog, hence the title. I do believe that it is a goal acheivable with realistic technology, a good portion of which we already have. I think that it is a reasonable (in the sense that it is doable) goal to begin in our lifetimes, and to continue through next few generations: to colonize the solar system and set up a space-faring civilization.

But first, an interlude. I'll be posting about some different topics.

The Real Prospects for Interstellar Travel (Part II)

Basically, conventional rockets won’t be able to get us up to light speed. There are other ways to work the problem, however:

1. Don’t go so fast: Build a generation-ship. Generation ships are ships that only accelerate up to a small fraction of the speed of light, and then coast sloooowly towards another star system. If you had that fusion propulsion thing worked out, and only wanted to go 5% the speed of light, you could get mass ratios of 148 or so. A base built on a comet that uses its water as a fuel source might be a potential example of one of these. Of course, they’re called generation ships, because they would have to accommodate multiple generations of astronauts! A trip to Alpha Centauri no longer takes 5 years, but 100!

2. Bussard Ramjet – It might be possible to scoop fuel from interstellar space. A bussard ramjet works by accelerating conventionally up to some fraction of the speed of light, where it then turns on a giant magnetic scoop. This creates a huge magnetic field in front of the vehicle, several tens to hundreds of miles wide. Through some sort of ionizing laser or radio pulse, the ramjet ionizes the interstellar hydrogen in front of it (if it isn’t already ionized) and draws it into the vehicle, where it is fusioned and expelled out the back, faster than it was drawn in. In order to work, the fuel must be expelled at a greater velocity than it is taken in. The interstellar medium is very sparse, however, and so this vehicle must attain a good running start.

3. Don’t shoot the fuel from the spaceship – shoot it to the spaceship! If you fire the fuel at the spaceship in the form of a neutral particle beam, you don’t have to keep it onboard, or use an onboard power source to accelerate it. The fuel can be accelerated from a gun located back in the home star-system. The fuel exhaust velocity can be however high you want it to be, due to the fact that you can put any amount of external energy into it as it leaves the accelerator gun. The vehicle would grab the fuel by ionizing it and magnetically reacting against it, as it comes in, thus gaining velocity. The vehicle’s design could then focus on bearing and supporting payload and astronauts, rather than packing in fuel energy. The amount of fuel required to accelerate the spacecraft no longer depends exponentially on the speed you want to achieve, but rather in a different non-linear way, so that the limitations that rockets have are no longer relevant.

The governing differential equation is dv/dt = mdot(ve-v)/m, assuming complete deceleration of the particle beam by the spacecraft. Mdot is the rate of mass expulsion at the solar system. Ve is the velocity of the particle beam. M, and v are mass and velocity of the spacecraft. This doesn’t take relativity into account.


Interstellar travel is difficult to accomplish, and requires a lot of technology that we don’t have yet. Does this mean we can’t do it? No, I don’t think that it does. For one thing, even if we’re limited to the technology that we know of today, we can still, just barely, make these sorts of voyages. And there’s no telling what we will discover, or what refinements to our technology will be made in 1000, 2000, 10,000 years of human history. There’s no way that the ancient Chinese could have made it into orbit or to the moon, even though they were the inventors of the rocket. There’s no reason to suspect that we’ve made the last inventions or discoveries in science or technology, or that we’re converging on the “end of human knowledge”, as some would like to believe.

But I also don’t expect it to happen in our lifetime, or our children’s lifetimes. It will take a long hard climb up in our space travel capabilities. Before we start launching spacecraft to the stars, we first have to be capable of conquering the solar system. We have to learn to crawl, before we can run. We have to be able to make 3 year voyages, 5 year voyages, 10, ect before we can start zipping off to the stars in 20+ years trips. We have to be able to construct decent bases and stations in space before we can start constructing giant accelerator guns to propel starships. We have to be able to make 30 kps dv missions before we can make 3*10^5 kps dv missions. So we have a long way to go, and I think that space travel in our lifetimes, in this generation, and those immediately after ours will have more to do with conquering the solar system than with zipping off to other star systems.

The Real Prospects for Interstellar Travel (Part I)

I’ll talk a bit about the prospects for interstellar travel: While science fiction is filled with tales of predominantly interstellar exploits, and ships that zip from one star system to another like it was a piece of cake, realistically, based on the physics that we know right now, interstellar travel is a difficult proposition, even theoretically.

For one thing, unless some savant finds a way around the speed of light limit, and overthrows the limitations of mass, energy, and momentum allowing us to take such shortcuts around that limit, we are confined to travel at or below the speed of light. The nearest star, Alpha Centauri is 4.3 LY away. Other nearby stars are Barnard’s Star at 5.94 LY away, Lalande 21185 is 8.315 LY away. Sirius at 8.6 LY away. Procyon at 11LY, ect. If all it meant was to accelerate up to light speed, then journey for 5-20 years to reach the new star system, such a voyage would hardly be an insurmountable task. Difficult, yes, time consuming, yes, but quite doable. Foreseeable advances in technology, such as the possibilities of inducing hibernation, or of indefinitely lengthening lifespan through some genetic or medical technology (in which case we might not care as much about the duration of such a trip) could help ease the voyage. Or we could suck it up and deal with the isolation.

However, the critical assumption here is that we will be able to easily accelerate our spacecraft up to near light speed. This is a task of extraordinary magnitude, when viewed as a problem involving conventional rocketry.

The mass that a rocket has in fuel is related to the total amount of change in it’s own velocity (or delta v) that it can make, and to the exhaust velocity of the propellant. In idealized cases, such as gravity free, drag free flight, this is a good measure of how far and fast a given spacecraft can go.

Tsiolkovsky’s equation.
Mpayload/mrocket = exp(-delta v/exhaust velocity)

Our everyday chemical rockets have exhaust velocities on the order of 4000 m/sec. They can achieve delta vs of about 9000 m/sec – 11000 m/sec (with the use of staging). If we were to look at the mass ratio required to get from 0 up to the speed of light, delta v would be 3E8 m/sec.

Ln(Mrocket/Mpayload) = 3E8/4000 = 75000. In other words the mass of the rocket would be 10^32572 greater than the mass of the payload. In other words – no starship for you, smack! It’s clearly a ridiculous number. Chemical propulsion won’t cut it.

Nuclear fission propulsion, such as nuclear thermal propulsion can improve this number a bit. Exhaust velocities of 12,000 m/sec (for solid-core nuclear thermal with oxygen augmentation), 40,000 m/sec (for nuclear electric propulsion), 100,000 m/sec (for more exotic and theoretical forms) are possible. These would yield mass ratios of 10^10000, 10^3257, and 10^1302 respectively. Still, not anywhere near good enough.

Fusion propulsion, through reaction and expansion through a magnetic nozzle, promises very high Isps (Isp is effective exhaust velocity/9.8m/sec^2, and is a common measure of rocket fuel efficiency). Estimates vary wildly because we don’t have the technology yet to produce energy from a fusion reaction. Isps of 300,000 sec are given for IEC fusion at Mass ratio: 10^43. 10^43 is still problematic, due to the fact that you would have to have a tank for all that hydrogen. The tank must be less than 43 orders of magnitude lighter than the hydrogen fuel.

Anti-matter propulsion is the ultimate fuel source for a rocket. It packs the maximum possible energy (hence impulse) per unit of fuel. When anti-electrons react with electrons, only gamma rays and neutrinos are left over. But realistic anti-matter propulsion needs a way to direct the product particles out the back of the spacecraft, hence they need to be charged. Anti-proton-nucleus reactions do this much better. However, to do this, you need a large mass of anti-matter. Anti-matter is currently produced at great expense in particle accelerators, or trickles in very slowly in the form of some cosmic ray types. Anti-matter would get you mass ratios as small as 20. This is quite manageable, but for a payload of 1000 tons, you would need 19000 tons of an even matter/anti-matter mix!

Basically, conventional rockets won’t be able to get us up to light speed. There are other ways to work the problem, however:

Thursday, November 17, 2005

The Absurdity of a Galactic Empire

One of the things that has always made me laugh and shake my head was the abundance of galactic empires in science fiction. In many science fiction movies, from the Star Wars series, to the new Serenity movie, as well as in books like Asimov’s Foundation series, there are fictional nations that lay claim (or at least claim to lay claim) to all or most of the entire galaxy. With names like “The Galactic Empire”, or “The Universe Alliance”, these organizations manage to control (and even to oppress and enslave) hundreds of planets across interstellar distances. Controlling a single planet is a feat in and of itself (hasn’t been done yet), expecting two or more to bend knee and settle into cultural uniformity is ridiculous. The difficulty of traveling over interstellar distances isn’t even the primary problem, though I’ll discuss that in a section following this one.

To paraphrase Arthur C. Clarke, even if we someday have a technology that allows us to travel between star systems as easily as dialing a telephone, we still have to face the fact that the galaxy has 100 billion star systems. Even though man may one day manage to colonize the entire thing, after millennia of effort and unrelenting expansion, we could never be said to have truly conquered or tamed it any more than the ants have conquered and tamed the Earth. The galaxy is far too large, let alone the universe.

So, it’s far too large to control or ever truly conquer. If we had the capacity to construct a starship for every family on the face of earth and launch them off each to their own star system tomorrow (6.5 billion people/ about 5 people/family) = 130 million colony ships. If we radiate the population of the earth away in such a manner, and “colonize”, if we can call it that, as many systems as we could with just 1 family apiece, we would still only have set foot on less than 1% of the entire galaxy!!

So, as far as galactic empires go, the sheer size of the galaxy renders our human notions of empire and domination absurd.

The Known Extent of the Universe, Part II

The distances to other galaxies cannot be measured the way we measure most other distances. Parallax won’t work for any but the closest star systems. You cannot reflect signals off even the closest stars, as the travel time would be years. So to measure the distances to other galaxies, a different method is used.

There is a class of star called a Cepheid Variable Star, an enormously bright type of star with a periodic fluctuation in the intensity of its output. The period of these stars happens to be related to their power by a known function. See
  • Cepheid Variable Stars
  • . By measuring the apparent magnitude of these stars in other galaxies, and correlating them to the output that we expect, we can determine the vast distances between the galaxies.

    An interesting phenomenon was discovered by an astronomer named Slipher: almost every galaxy in the sky is red shifted with respect to us – the light reaching us from these galaxies is redder than it should be – their spectra are shifted. (Astronomy: A beginner’s guide to the universe, pg 438, Chaisson McMillan). Not only is this the case, but the red shift is tightly correlated with the distance from our own galaxy. Red shifts and blue shifts are indicators of radial speed under relativity. Great enough speeds tend to increase or decrease the energy of the photons emitted by an object with respect to the observers. The red shift, interpreted as a measure of relative velocity, gives a surprising result: Almost every galaxy is moving away from us at a speed proportional to their distance from us: recessional velocity = H0 x distance. This doesn’t just go for our own galaxy (this phenomena does not require us to be at the center of the universe), but every galaxy is moving away from every other galaxy at a distance proportional rate. Hubble’s constant = 75 km/sec/Mpc (Mpc is a megaparsec, or 3.3*10^6 LY).

    This startling discovery leads to the big bang theory of the origin of the universe – if every galaxy is moving away from every other galaxy at a speed proportional to distance, then it follows that at one time in the distant past, the universe was packed much closer together.

    There are several modes to the expansion of the universe which are possible – we have only one space-time cross section which we are capable of observing (a light-cone extending backwards in space and time 1 year for every lightyear). These different modes of expansion are primarily dependent on a number called omega which is a ratio relating to the density of matter and energy in space.

    Omega < 1 would yield a universe that should, according to our understanding of general relativity and gravity, expand indefinitely and with ever increasing speed, leading to an eventual heat death.

    Omega > 1 would yield a “closed universe” which would be bounded in space as well as in time. The space would eventually curve back in on itself, and time would as well, leading the universe to accelerate back together in a “big crunch”. The concept of a bounded finite universe that will end someday has enjoyed tremendous popularity, though I have no idea why. The possibility depresses me. For philosophical reasons of my own, I would be extremely disappointed if the universe should turn out to be finite in either space or time.

    Omega = 1 would yield a universe that would expand at an exponentially decaying rate. Eventually it would stabilize, and continue to exist indefinitely, neither exploding outwards into a cold heat death, or collapsing back in on itself.

    Based on Hubble expansion alone, any of these scenarios is possible. Based on information from the cosmic microwave background radiation (radiation from the very early universe), astronomers believe that omega is, against all probability, extremely close, perhaps equal to 1. If this is the case, then the universe should have a spatial geometry that is globally “flat”, or which doesn’t curve back in on itself, and should therefore be literally infinite in extent space-wise (we can’t see all of it yet, because when we look, we look along a light cone, and eventually run into the big bang).

    But all of the mass that we see in the universe today couldn’t possibly account for the mass and energy density required to make omega = 1. Going off of the visible mass, our universe should have a much lower omega, and should be in the process of exploding violently outwards.

    This, along with certain other oddities in the rotation rates of galaxies (which are larger than they should be based on our estimates of the mass of stars), has led astronomers to the idea that there might be “dark matter” in the universe. Dark matter is a name for mass that we haven’t found yet, or cannot account for, in terms of the gravitational behavior of objects on a large scale. It could either take the form of matter which we have no means of detecting, or it could be some sort of correction term that is required in our understanding of gravity over large distances, but to make our models of the universe work, this missing mass has to be present. Some have wanted to replace the big bang theory with something else on account of this hole, but to date nothing else explains the Hubble red shift as well as the big bang theory has.


    In any case, it is quite possible that the universe is literally infinite in extent. Even if it is not literally infinite (in which case, I’d be somewhat disappointed), it is still for all intents and purposes practically infinite in extent, of which we can see 14.7 billion light-years, tens of billions of galaxies, each of which have hundreds of billions of stars.

    Wednesday, November 16, 2005

    The Known Extent of the Universe

    Since this blog's theme is going to be my long time interest in all things related to space exploration, I thought I would start out with a review of where we are in space, and the structure of the universe. Most space enthusiasts probably already know all this stuff, but novices might find it interesting:

    Everyone knows we are on Earth, orbiting the sun along with 10+/-2 planets (depending on the mood of the astronomy community - pluto/sedna/charon/qauoar regularly change designation from planet to comet and back!). The other stars in the sky are indeed other suns, with planetary systems of their own. We are beginning to detect planets in orbit around other star systems, though our ability to detect them is still coarse and limited to very large planets. What perhaps most people don't have a very good grasp of is the vast extent of the universe. The universe is enormous! It is impossible to convey with mere words how huge it is, but perhaps hurling numbers at the problem can help:

    Our star exists in one spiral arm, off center of our galaxy, which is our local pie shaped group of stars. The milky way itself has a radius of about 50000LY, our star being 30000LY from the center. The galaxy contains over 100 billon stars. That's just our galaxy though.

    At one time not very long ago (70 or so years) it was thought that our galaxy was the universe. But the galactic scale is only the beginning of what is observable. We discovered that some of the off-plane nebulae were actually extraordinarily distant conglomerations of stars, and that these objects were like the disk which our own star inhabited. The observable universe extends out quite a ways, and currently is jam-packed with about 40 billion galaxies. (Astronomy: A beginner’s guide to the universe, Chaisson McMillan, pg 419). While stars are very far from each other on the scale of planetary systems, galaxies, on a galactic scale, are about as close to each other as plates on a dinner table.

    A few amazing Hubble pictures as an interlude, and to drive the point home:
  • Image 1

  • Image 2: The Tadpole Galaxy: One of my favorite desktop backgrounds

  • Image 3

  • These are from the Hubble deep field images. They are from minute areas of the sky exposed over hours (the photons come in very slowly from such distances).

    To be continued: More to come on this topic.

    Tuesday, November 15, 2005

    Exams! Exaaaams! Aahhhggg!

    My Econ exam is now done. I only have the 364 controls exam and an aero project meeting left before I'll have the time to begin seriously posting.

    I wonder how many people wander by a mostly empty blog like this? Comment if you happen to view this post please.

    Monday, November 14, 2005

    Solar Empire Blog

    This is my first post on my first blog. If you haven't guessed from the title, I'm a space nut. This is just to test the page and to get things started.

    BTW - To anyone who happens by this blog, I'm also an engineering student and have no time to be doing this. So I'm not going to post very frequently.