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Halo Science 101

In an article from the new 'Halo Effect' book, Battlestar Galactica science advisor Kevin R. Grazier takes a look at the hard science behind Bungie's Halo series, calculating the reality of surviving and fighting on a planet-sized spinning metal ring.

[EDITOR'S NOTE: An earlier published version of this article contained incorrect data from a pre-production version of the book. This has now been corrected.]

Several years ago, after I’d performed a pair of planetarium shows at Santa Monica College, several of the audience members and I retired to a local restaurant to prolong our evening of astronomical fellowship. The topic of conversation turned from the stars and planets to a round-robin discussion of movies—in general, what kind of movies everyone enjoyed, and, specifically, what we had seen lately. By the sheer fact that these people chose to spend their Friday evening attending planetarium presentations to learn more about the universe, they obviously enjoyed exercising their grey matter in their spare time. It was no surprise, then, that the movies this crowd chose to see also tended towards the intellectual.

Because of our seating arrangement, and the order of the topic’s progression, I would be the last to speak. Since I was the only person at the table with a Ph.D., there was an elevated air of expectation. What would he say? Would he reveal a little-known documentary? Perhaps a stimulating foreign film? Would he list one of the classics as his all-time most cherished movie? In retrospect, the collective disappointment to my less intellectual—and more “blue collar”—reply was astoundingly amusing. I simply said, “You know, I get enough intellectual stimulation at work, so when I go to the movies, I want to see things explode.”

Given my taste in movies, it isn’t much of a stretch to imagine that I’m quite a fan of many of today’s first-person-shooter video games. I’m a big fan of Doom, and all its incarnations, for example. So when Halo: Combat Evolved arrived on the scene—a video game that appeared on the surface to be a cross between Ringworld (one of the first science fiction books I ever read) and Aliens (my all-time favorite “shoot-’em-up” sci-fi movie), I was all over it.

In fact, the case has been made—on several Halo-related websites, for example, that there isn’t much about Halo’s plotline that is original. There are elements of numerous science fiction books, movies, classical mythology, and even biblical references. Halo is an amalgam of all of these. In fact, we can find allusions to the Alien movies when the game is barely underway: the Sergeant “motivating” the soldiers on the UNSC Cruiser, Pillar of Autumn, is remarkably similar to Sgt. Apone from Aliens, and if you look closely enough at the bulletin board behind the bridge on Pillar of Autumn, you can even make out a flyer for a missing cat named Jonesy. Whether or not the Halo games are the epitome of originality or not, who cares? Just as with my movies, I want my video games to be rampant escapism with an overdose of adrenaline. If I’m vicariously thrown into scenarios that just happen to be reminiscent of favorite sci-fi movies, and lots of things explode, then all the better!


Bungie and Microsoft's Xbox classic Halo: Combat Evolved

While science fiction can be used to examine the human condition and to make social commentary—the original Star Trek , Starship Troopers, and even Battlestar Galactica v. 2.0 are excellent examples here—science fiction can also serve as unbridled escapism. The viewer or reader or game player—the participant—isn’t preoccupied with day-to-day problems if the story successfully transports him to distant worlds or future times. Of course, the participant has a role in this as well. It is the duty of the author to create a situation interesting enough to be worthy of the time invested in a visit, but it is incumbent upon the participant to be amenable to be taken on the journey. The term is “willing suspension of disbelief,” originally coined by Samuel Coleridge in 1817.

Fans of science fiction media willingly allow ourselves to believe that the Enterprise can transport people by converting them to energy and subsequently reconstructing them, that Galactica has artificial gravity, and that the Millennium Falcon can, in fact, make the Kessel run in less than twelve parsecs. We accept a measure of unproven (faster-than-light travel), or even highly implausible (light sabers), science and technology if it’s interwoven with a ripping good yarn.

At the same time, if the science fiction work includes too many obvious technical gaffes—especially if they are easily circumvented and the story equally as entertaining if done accurately—the participant is “taken out” of the story, suspension of disbelief itself suspended, and the dramatic impact lessened or lost. With millions of computers in service today, coupled with the accessibility of the internet, we have an increasingly tech-savvy population: a population that largely appreciates technical accuracy in stories and who, more to the point, notices when things are amiss. To this end, Hollywood is increasingly using technical advisors in science fiction television and cinema to ensure that the science part of science fiction is depicted as accurately as possible and that the audience stays within the action.

If the universe, characters, or story is particularly compelling, one might choose to wander that universe of his or her own accord. The internet is full of bulletin boards where members compare and contrast the capabilities of the Viper Mark II with the Mark VIII, or debate whether or not you would take the blue pill or the red one. Of course, this is just a high-tech version of science fiction fellowship and escapism that has already existed at science fiction conventions for decades. Succinctly put, it can be fun to play in somebody else’s sandbox.

The Halo universe, detailed in the video games, novels, and upcoming movie, is a richly detailed one and lends itself well to such musings. An entire book could be written about the science and physics, both explicit and implied, within the Halo universe, but with only a little scientific knowledge we can have a lot of fun simply musing about a spinning ringed megastructure—suspended between a planet and its moon—that doubles as a research facility and a superweapon.


This is the Way the World . . . Begins

The term “megastructure” refers to a huge artificial structure for which one of its three special dimensions is 100 kilometers or greater. Both SF and speculative science have contemplated large-scale constructs for years, such as the Dyson Sphere and Star Trek’s Borg Unimatrix; even the planet Earth, as represented in The Hitchhiker’s Guide to the Galaxy, would qualify. The first literary use of a ring-shaped megastructure occurred in Larry Niven’s 1970 Hugo and Nebula award-winning novel Ringworld. Niven elaborates on the details of such a Ringworld, also known as a Niven Ring, in his 1974 book A Hole in Space:

“I myself have dreamed up an intermediate step between Dyson Spheres and planets. Build a ring 93 million miles in radius—one Earth orbit—which would make it 600 million miles long. If we have the mass of Jupiter to work with, and if we make it 1,000 miles wide, we get a thickness of about a thousand meters. The Ringworld would thus be much sturdier than a Dyson Sphere.”

Although Forerunner Halos are also huge ring-shaped habitats, they are comparatively smaller by several orders of magnitude: the radii of the Halo megastructures are a “mere” 5,000 kilometers—more similar to Earth’s average radius, 6,371 kilometers, than that of a ringworld. In fact, because the Halos we have seen to date orbit gas giant planets instead of encircling stars, they are less ring worlds than they are ring satellites.

A 5,000 kilometer radius would yield a circumference of roughly 31,400 kilometers. If the Halos had a width-to-radius ratio similar to that of Niven’s Ringworld, they would be approximately 5.37 kilometers wide. They are significantly wider, though, at 320 kilometers. The Halos, then, would have a surface area of 10 million square kilometers— slightly larger than the surface area of Canada, and approximately 2 percent of the surface area of Earth. Of course, since we know that there are lakes, seas, and rivers on the Halos, the livable surface area would be fractionally less.

What raw materials would it take to construct a Halo, and in what quantities? In order to determine the amount of raw materials required, and what elements may exist in the necessary abundances, we first must calculate the volume of the structure. While a Halo is proportionally wider than a Niven Ring, it is thicker in absolute measure. Niven proposed that a Ringworld be 1 kilometer thick, whereas the Halos are quite a bit sturdier at 22.3 kilometers thick. The total volume of a Halo would be roughly 224 million cubic kilometers, a bit more than 0.02 percent of the volume of Earth.

Of what would it be composed, then? Almost since the genre began, science fiction authors have resorted to the invention of new and exotic materials to endow their structures/spacecraft/armor with the desired combinations of weight, strength, and other material properties.


Halo's Ringworld-like structure

The practice is so common that a term has even been coined for fictitious substances that have such improbable combinations of material properties: unobtanium. If we dare to imagine how a Halo might plausibly be built, one constraint must be that we shy away from unobtanium and consider only materials that exist in practical abundances in the real universe. In the book Halo: Fall of Reach, however, spectroscopic analysis of the composition of Installation 04 is “inconclusive,” which seems to imply quite strongly that the Halos are, in fact, composed of unobtanium. Let’s go out on a limb, then, and assume that the Halos have a thin outer protective sheath composed of a super-strong, heretofore unknown, alloy that envelopes an internal structure composed of more universal elements.

Iron, in addition to being the principle component of the cores of terrestrial— or Earth-like—planets (Mercury, Venus, Earth, and Mars), is also common in asteroids. In fact, in the solar system many asteroids are composed almost entirely of iron and nickel. Carbon is a fairly common element as well. Then it would be a reasonable assumption that the primary Halo structure is composed of steel—which is an alloy of iron and carbon—with perhaps other elements in smaller amounts. Although less universally abundant, nickel and magnesium, also common in steel, exist in amounts abundant enough to create a very strong and comparatively light steel alloy.

We now know the approximate volume of a Halo and the density of its principle component (a reasonable average density for steel is 7.7 grams per cubic centimeter). Normally, these values would be enough to calculate its approximate mass. We need still one more quantity, though. Views of the exterior surfaces of Installations 04 and 05 clearly reveal direct-vision ports (read: windows) and what appear to be docking hatches. The obvious implication is that the inner surface of the ring is not the only habitable portion of a Halo—obviously a fraction of the ring structure itself is hollow and used for living space, laboratories, even the hardware, maintenance, and pulse generator spaces for the Halo’s weaponry. If we assume that the primary ring structure is roughly 50 percent empty space, then we end up with a total mass of a Halo of about 1.7x1017 kilograms, or 1,700 million billion kilograms.

In A Hole in Space, Larry Niven calculates that it would take the mass of Jupiter to build his ringworld. A major complication, however, is that jovian, or Jupiter-like, planets represent the bulk of the mass of a planetary system like ours, yet they are composed largely of very light materials such as hydrogen and helium. Each has several Earth masses worth of solid material, rock and metals, at their core, but the sum total of all the rock and metal in the solar system—that of the inner planets, the asteroids, the jovian planets and moons—would equal less than one-sixth of one Jupiter’s mass worth of potential construction materials. The mass we calculated for a Halo is approximately twice the mass of Ceres (the largest asteroid in the solar system's Asteroid Belt), a bit less than Pluto's moon Charon, or the mass of a sphere of solid iron roughly 57 kilometers in radius. The entire asteroid belt between Mars and Jupiter would have just about enough mass to construct one Halo.


The Neighborhood: A Halo’s Place in Space

Though it is likely that Halos heretofore unseen may exist in different environments, Installations 04 and 05 were both in orbit around jovian planets. In Halo: Combat Evolved, Installation 04 orbits the superjovian gas planet Threshold (Earth Survey Catalog B1008-AG), which, in turn, orbits the star Soell. Like Jupiter, Threshold is a gas giant with clouds of ammonia (white) and ammonium hydrosulfide (reddish brown) crystals. Unlike Jupiter, though, the diameter of Threshold is given at 214,604 kilometers, exactly half again as large as Jupiter (it is unlikely this is a coincidence, more likely a conscious choice on the part of the game designers).

The Halo game designers have exhibited an amazing attention to detail throughout the games. It is therefore likely that this is a result of recent astronomical discoveries more than any other reason, but jovian planets like Threshold are unlikely to exist in the real universe. Jupiter is about as large as a jovian planet can be. If increasingly more mass were added to Jupiter, it would begin to contract, collapsing under its own weight—becoming smaller even as it increased in mass. If a gas planet the size of Threshold did exist, and it had approximately Jupiter’s density, it would “weigh in” at a bit less than 3.8 Jupiter masses, or around 1,070 Earth masses. In reality, though, an object of 3.8 Jupiter masses would be smaller than Jupiter, while an object the radius of Threshold would be a medium-sized brown dwarf.


Jupiter, Image Courtesy NASA/JPL-Caltech

The debate of the late 1990s and early 2000s regarding Pluto’s status as a planet was less about Pluto than it was the definition of the cut-off point of what defines a planet at the low-mass end of the spectrum. A cut-off for the high-mass end of the spectrum has been in existence for quite some time, however. Large planets, like stars, tend to be composed largely of hydrogen. If an object has enough mass to sustain the nuclear fusion of hydrogen, thus generating its own light and heat, it is considered a star. To sustain hydrogen fusion and become a star, a body has to have roughly eighty-four Jupiter masses or more.

Objects between twelve and eighty-four Jupiter masses have properties that are intermediate between jovian planets and the smallest red dwarf stars and are called brown dwarfs. Although theoretically predicted to exist back in the 1960s, the first confirmed brown dwarf was viewed in 1995 and is 400 light years away from Earth in the Pleiades (a.k.a Subaru) open star cluster. Known as Teide 1, it is roughly twice the diameter of Jupiter, yet has fifty-five times Jupiter’s mass. For Threshold’s rasois to be 1.5 Jupiter radii, it would likely have at least twenty Jupiter masses of material and would appear quite differently— it would be more uniform in appearance than a jovian planet, as opposed to having multi-colored cloud bands.

Installation 05, or the Delta Halo from Halo 2, orbits the gas giant planet Substance. Less information is given about Substance than for Threshold, but based on its color, it is likely more Uranus- or Neptune-like than Jupiter-like. Uranus and Neptune are both blue, or bluish-green, in color, suggesting the presence of methane in their atmospheres. Methane absorbs the red light from the sun, and the resultant reflected light appears blue. So, we can make a logical deduction regarding the substance of Substance simply by its color.

Not only is Threshold unusual in that it appears to have properties of both a jovian planet and a small star, it is also unusual in that it has only one moon, and a very large one at that. Known gas giants have numerous moons, most of them small. By way of example, at present count, Jupiter has sixty-three moons, and Saturn has forty-eight. Even if some, even most, of the moons of Threshold had been used as construction materials for Installation 04, it is unlikely they all would have been suitable.


Uranus and Neptune, Images Courtesy NASA/JPL-Caltech

Most natural satellites of jovian planets, especially those as distant from their sun as Threshold appears to be, are composed of a mixture of ice and rock. In fact, it is normally so cold where the gas giants live that planetary scientists consider ice to be a rock—because ice is a main component of many of the solid objects in the outer solar system, and at the temperatures that exist in the outer solar system, ice is normally as hard as granite. Given that metals are comparatively rare around gas giants, it is unlikely that all of the moons would have been used as Halo construction materials. Perhaps some were melted for lakes, and some processed for atmosphere, but this still does not entirely explain the dearth of moons around such a large planet.

A likely explanation is that the region around Threshold was cleared on purpose. Alpha Halo monitor 343 Guilty Spark claims that Installation 04 is at least 101,217 years old. While that represents the blink of an eye in the cosmic timescale, it is still enough time for the Halo to have accumulated numerous impact scars, a few quite large. While the bulk of material in a planetary system is swept up and accreted as part of the planet-formation process, there are still countless small—and not-so-small—particles careening through the system. When the Hubble Space Telescope (HST) was serviced by the space shuttle Atlantis in 2002, it had literally hundreds of micrometeoroid impacts. In fact, it has been estimated that every square meter of HST receives five impacts from sand-grain-sized particles every year.


Halo's gas giant Threshold

Most impactors are small, some aren’t: there was a three-quarter-inch hole in HST as well. After a span of 100,000 years, a megastructure the size of a Halo would be scoured and likely would have suffered a major impact event or two. This would be catastrophic, since a major impact would likely release as much energy as was released at the end of the game by the fusion drive on Pillar of Autumn, and we know what happened there. Even the claim that the Halo had some sort of force field is inconsistent with what we’ve seen: human and Covenant spacecraft seemed to have no resistance in coming (both landing and crashing) and going. Given all that we’ve seen, then, it may very well be that the Forerunners found a way to clear the Threshold system of debris to ensure the safety of Installation 04 and its research.


The exception, of course, is Threshold’s lone enigmatic moon, named Basis. Basis is unusual by solar system standards. It’s a huge moon. With a radius of 11,924 kilometers, it is nearly twice the radius of the planet Earth and has over 6.5 times the volume! Just as there was a bit of a discrepancy with Threshold’s planet/brown dwarf duality, a similar statement can be made for Basis. Giant planets like Threshold radiate a lot of energy, especially infra-red energy. This means that although the light and heat from Soell is quite faint, it’s nevertheless quite toasty near a body like Threshold.

This would imply that Basis is composed primarily of rock, like Jupiter’s innermost large moon Io. Because of Threshold’s heat, icy satellites would not remain icy long. On the other hand, if we assume that Basis is, in fact, composed of rock, and if it had a density close to that of Io (which is still less than that for any of the terrestrial planets), then the gravity on the surface would be nearly 1.2 times that of Earth. Master Chief and his fellow marines might be moving a trifle slowly on Basis if this were the case. The gravity on Basis appears to be similar to that for Earth, however, and it would certainly be enough to hold an atmosphere. Further, the appearance of Basis is not dissimilar to Jupiter’s moon Europa. If we assume that Basis has a similar composition as Europa, and we assume the same density, then it has the same gravity as Earth (this, perhaps, is what the game designers had in mind). It seems, then, that Basis is a bit of a paradox. For gravity there to be the same as Earth, the moon would almost certainly have to be composed of a mixture of ice and rock, but since the surface seems to have a temperature comfortable to humans, then most of that ice should be in the liquid state.

A careful examination of the viewscreen on Pillar of Autumn shows that Installation 04 is halfway between Threshold and Basis. This is an untenable place to put a Halo for many reasons. More likely the Halo is situated near, or orbiting about, the L1 Lagrange point between Threshold and Basis, which would put it closer to Basis and not at the halfway point. In a system with two massive bodies (like Threshold and Basis, which we’ll call the primary and secondary bodies), there are five points where a third body of negligible mass, owing to a combination of gravitational attraction and orbital centrifugal force (understanding that “centrifugal force” is what physicists call a “fictitious force;” here, the more colloquial usage is adopted), would remain stationary with respect to the two massive bodies.

These are known as LaGrange points, and they are labeled L1 through L5. The L4 and L5 points, sixty degrees ahead and behind the smaller massive body in the same orbit, are stable. Objects placed in a co-planar orbit at either L4 or L5 will remain in roughly the same location relative to the two massive objects. The solar system is, in fact, full of examples. At the Sun/Jupiter L4 and L5 points are hundreds of asteroids, called the Trojan Asteroids, which co-orbit with Jupiter. The Saturnian moon Tethys even has smaller moons at its L4 (Telesto) and L5 (Calypso) points.

The L1, L2, and L3 points are called “meta-stable,” however. All three points orbit the primary at the same orbital rate as does the secondary body, and each maintains the same relative position between the two. L1 is along the line connecting the primary body to the secondary body and is situated at what a mathematician would call a “saddle point.” A ball bearing placed on a saddle would roll to the middle of the saddle from front to back but would have a tendency to roll off to one side or the other. The L1 point is “stable” in one direction (along the primary-secondary line), but unstable along its orbit, hence meta-stable. This implies that Installation 04 actively corrects its orbit to keep Threshold and Basis at the same relative locations. Though the L1 point is unstable, there exist trajectories that “orbit” the L1 point that are stable. The NASA Solar and Heliospheric Observatory (SOHO) and Genesis missions, both of which took data about the sun, were on such trajectories (which, ironically, are called “halo orbits”).

In placing a structure like a Halo so near a body like Threshold, other complications arise. One would be the radiation environment. The largest structure in the solar system is the magnetic environment around the planet Jupiter. If it could actually be seen, from Earth it would appear to be a few times larger than the full moon. Such a large and intense magnetic field traps charged subatomic particles like electrons, which spiral along the magnetic field lines. Some electrons can even be accelerated to relativistic speeds—a large fraction of the speed of light. This makes life near a jovian planet difficult at best. A human standing on Jupiter’s innermost large moon, Io, would receive a lethal dose of radiation from only a few minute’s worth of exposure. Surely the radiation environment surrounding a Halo in close orbit about a gas giant would be similar.

Earth is bathed in a stream of changed subatomic particles not dissimilar to those in close proximity to Jupiter: the solar wind. Earth is protected from charged particles where Io is not, owing to its magnetic field. Since electrons and other charged particles are deflected by magnetic fields, the solar wind flows around Earth, except at the poles. At Earth’s poles, there is a hole, or cusp, in the magnetic field where the solar wind can penetrate more deeply. The solar wind’s interaction with Earth’s upper atmosphere underneath the polar cusps is what creates the aurorae.

Certainly a Halo bathed in the radiation of a gas giant like Threshold would require shielding similar to that around Earth, and it turns out that to do this might be a fairly straightforward task. Magnetic fields are generated by moving charged particles like electrons, implying that any wire along which a current flows is surrounded by a magnetic field. It’s not inconceivable, then, that huge conductive cables could run the entire 31,416-kilometer circumference of a Halo, nestled within the ring structure itself. By creating electrical currents within these cables, the Forerunners could have easily created a protective magnetic environment that was strong enough to make a Halo inhabitable, but not so strong that it would interfere with the function of electronic equipment.


One Spinning Ring to Rule them All

After our brief wandering through a Halo’s space, let’s make a closer examination. What is life like, and what are some implications of life on a planet-sized spinning metal ring? Altough it is established in the Halo universe that the Forerunners, Covenant, and even humans have some degree of artificial gravity generation technology, gravity on a Halo is largely simulated by centrifugal force. Since modern-day science knows no way to generate gravity artificially, a very common technique used in both literary (e.g. Ringworld, Rendezvous with Rama) and cinematic (e.g. Mission to Mars, 2001: A Space Odyssey) science fiction is to use the centrifugal force of a spinning ring or cylindrical structure to simulate gravity. More germane to this topic, Larry Niven describes how gravity would be simulated on a ringworld:

”There are other advantages. We can spin it for gravity. A rotation on its axis of 770 miles/second would give the Ringworld one gravity outward.”

The apparent gravity on Installations 04 and 05 is close to that of Earth. For a Halo with a radius of 5,000 kilometers to simulate one Earth gravity, it would have to spin with a tangential speed of slightly over seven kilometers per second. That implies that the Halo would rotate once every hour and fifteen minutes, or 19 ¼ times a day.

The concepts of day and night would, therefore, take on entirely different meanings on a Halo than they do on Earth. For that matter, they would even be entirely different than that for Niven’s Ringworld. In Niven’s novels, the ringworld has “shadow squares,” almost a mini ringworld, encircling the central star at a smaller radius:

”Set up an inner ring of shadow squares—light orbiting structures to block out part of the sunlight—and we can have day-and-night cycles in whatever period we like.”


The view from the surface of a Halo

The shadow squares are connected with thin but extremely strong filament. By alternately passing and obscuring light, the shadow squares simulate day and night—the size of both the shadow squares and their interstices dictating the duration of each. The orientation in space of a Halo would determine what percentage of each rotation would receive both sunlight and shadow; however, if Halos orbit their gas giant in the equatorial plane, as it appears they do in the game, then a Halo goes into eclipse periodically as well. While there exist tilted, or inclined, orbits that never go into eclipse, by virtue of the fact that we can see that Installation 04 orbits between Threshold and Basis, it is not in one of these orbits. Given the stated size of Threshold, and the apparent distance to Basis, it is likely that Installation 04 would be plunged into complete darkness at least once every Earth day.

An object—a soldier, an Elite, a Scorpion MBT, a Warthog recon vehicle, anything—in direct contact with the surface of the ring would perceive the centrifugal force to be the equivalent of gravity. Anything not in direct contact would tend to follow basic laws of dynamics, but laws that might seem counter-intuitive at first. On the second level of Halo: Combat Evolved (a level called “Halo,” in fact), Master Chief can see a waterfall shortly after making ring-fall.

Figure 2 shows the results of computer simulations of the trajectory of one drop of water over the waterfall if it were subject to Earth’s gravity, and the trajectory of one drop of water on a Halo—assuming that the waterfall is 305 meters (1,000 feet) high and oriented along the Halo’s spin direction. We see that a drop would fall two meters farther on a Halo than on Earth. That’s not a great diff

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