Pelion and Ossa

Two small terrestrial worlds which collided in 9526 AT

Ossa and Pelion - the collision
Image from Steve Bowers
The energy of impact was so great that it briefly outshone the local star

Pelion and Ossa

StarJD 440454221
Distance from Sol3705 ly from Sol
Mass 3.88e21 tonnes
Diameter 11016 km
Mass 2.82e21 tonnes
Diameter 10180 km2


Collisions between planet sized bodies are not uncommon during the protoplanetary phase of a planetary system; some times these colliding objects are similar in size to Earth and other terrestrial worlds. However the environment in a protoplanetary disk is already extremely hazardous, with a continuous rain of smaller objects falling to the surface of any such planet. The typical inhabitants of a protoplanetary disk are specialised miner clades and remotely controlled drones; few tourists visit these locations to experience the collisions at close quarters.

However collisions also sometimes occur in mature systems, especially where one or more of the planets orbits at a high inclination to the equatorial plane of the local star. One such system, JD 440454221, was detected in Pyxis at long range in the third millennium AT, long before the first explorer class craft reached the vicinity.

By the time the first colony ship arrived, in 7988 AT, the orbits of the planets were well known and predicted far into the future. According to these calculations, two terrestrial-sized worlds, both larger than Mars but smaller than Earth, would collide on 14 Galileo 9526 AT.

These worlds, labelled Ossa and Pelion by the exploration team, were otherwise quite good candidates for terraformation. Ossa orbited in a retrograde fashion, in an orbit inclined 131 degrees to that of Pelion. The energy that would be liberated by this collision would be very large indeed, because of the retrograde orbit of the smaller world. Unlike the collision that formed Earth's Moon, which happened at a relatively sedate pace, this event would occur at a relative speed of 42 km/s. These two planets had avoided each other for more than two billion years, but tidal interactions between the two planets and their star had slowly changed both their orbits until they intersected one another. The collision would emit a massive pulse of light energy, and shower the system in debris, destroying any conventional colonies in the vicinity.

Attempted Terraformation and Subsequent Abandonment

At first Otus and Ephialtes, the S:2 level individuals in command of the joint NoCoZo/Sophic League colonisation venture, planned to simply observe this spectacular event from a distance, and exploit the vast amount of differentiated matter that would be thrown out by the collision. But both worlds were good candidates for terraformation. After some deliberation they decided instead to divert Ossa by a few thousand kilometres, so that the two planets would continue to coexist in this system for the foreseeable future. By building a series of mass-streams that looped between the two planets and a gas giant (Dirphys) in the outer system, enough momentum could be extracted from their orbits to cause them to miss each other.

On the strength of this plan both worlds were terraformed, using advanced swarm technology, and both were declared fully habitable at the start of the tenth millennium, five hundred years before the projected date of the collision. However Ephialtes withdrew eir support from the project after immigration had already started, apparently due to a temporary weakness in the NoCoZo hypereconomy. This left the junior partner Otis in sole charge of the project on behalf of the Sophic League, who were generally less enthusiastic about such megascale engineering. By 9015 the Sophic League had also withdrawn from the diversion project.

Already many millions of colonists had migrated to these newly opened worlds, with many more intending to arrive shortly. The colonisation venture was forced to recompense most of these due to breach-of-contract, and relocate those who had already arrived. Eventually much of this loss would be recovered from the mineral wealth generated by the destruction of both worlds, but for now the system was a financial disaster. At the end of this relocation the two doomed planets were deserted, with a number of newly built, spacious, empty cities gathering dust beneath the new, oxygen-rich skies.

The Death-seekers

As the date for the collision approached, a new population of migrants began to arrive; individuals who wished to experience life on the doomed worlds and in the dust-blown cities. Most of these migrants intended to leave the system before the devastation occurred, but some intended to stay to experience the collision first hand. The duration of this event would be several standard minutes, and some observers might expect to survive for a reasonable fraction of that time. This event had been simulated many times and had proved very popular, if somewhat traumatic virtual experience. Now some were determined to be present at the real thing.

The colonisation project now began to generate income from this influx of the curious and the morbid. Some who intended to observe the impact sent full or partial copies of their personality, in physical bodies which were often augmented to be especially observant and sensitive to the experience. At the last possible moment these copies would transmit their final sensations to receivers located at a safe distance from the impact site.
Others were determined not to survive, and regarded this event as an opportunity to terminate their existence voluntarily in the most spectacular way possible. Since the Regor supernova explosion, very few previously-predicted destructive events had been allowed to occur within the Terragen Sphere. Certainly a number of unexpected and very destructive events (such as the Gehenna Incident) had occurred, but these had happened without warning, so no interested citizens were able to arrange to be present when they happened . Now a long-anticipated disaster was about to happen, and many long-lived or jaded individuals who wished to die in a dramatic way were drawn to this pair of doomed worlds. Some who would otherwise have made the pilgrimage to Threshold or other popular termination locations instead made their way to JD440454221 for the final countdown.

The final months before the impact were particularly anarchic times on these worlds. The Sophic League provided spiritual advice to those who intended to die, and practical advice to those who were not entirely committed to remaining until the last moment. The NoCoZo declared both worlds to be free zones, and some migrants took this to mean that they could abandon all morality. Murderous gangs stalked the wide boulevards, determined to assist those who had already decided to die by killing them before the event. Open warfare broke out in several cities between those that wanted to experience the thrill of killing and those who wanted to die (but not as murder victims).

Other conflicts arose between those who wished to expire in a dignified fashion, and those who wished to go out in a blaze of debauchery. A number of unwilling victims were rescued at the last moment from gangs or cults who had smuggled them to the doomed worlds, intending to let them be killed by the collision while recording their final moments for their entertainment. It is by no means certain that all such victims were discovered in time.


At last the two worlds approached one another, looming as swelling spheres in each other’s skies. Just before contact, observers on each world began to feel the effect of gravity from the other planet, which increasingly shifted their sense of up and down, and caused some buildings to start to collapse as if the landscape was becoming tilted. When the atmospheres made contact with each other they became compressed and heated, instantly killing those below. Then the planets themselves came into contact, crushing the crust into liquid at more than forty kilometres per second. A vast wave of planetary material was instantly ejected into a shallow cone that expanded between the worlds.

For a few moments after the two worlds began to merge together, observers on the far side could feel no effects, except a steady increase in gravity and a displacement from the vertical in some regions. But the brilliant light from the impact reflected back to the surface from the interplanetary medium, causing the skies on the dark side to glow eerily. Supersonic shock waves liquefied the surface of both worlds at the point opposite the impact, in a circular region that rapidly expanded.

The shallow cone of white-hot ejecta soon spread into a vast disc between the worlds, irradiating the surface of Ossa and Pelion so that soon no life could persist on either. Finally the last remnants of the formerly spherical crust were thrown far into space above the merged surface of the worlds. Initially the fireball was opaque, but as it cooled to about 6000 K (the temperature at which silicate vapor becomes optically transparent), it released its thermal energy for more than 30 hours. The energy from this impact totalled 2.49 x 10e33 joules, a burst of light that outshone the local star considerably, and was seen later by colonies in this volume as a bright nova.

The last transmissions sent by those who remained to see the last moments of this cataclysm were swamped by the emissions from the white hot ejecta sprayed out from the impact; some individuals who were attempting to transmit their final mindstates offplanet during this period lost much, or all of their vital data. Other, more cautious individuals used transapient datacache technology to preserve their mindstates during the peak of this cataclysm.

Some commentators point to the fact that in the final reckoning the JD 440454221 System has returned a healthy profit from the legions of the voluntarily terminated and the thrill-seekers who escaped before the end (and some who did not), and subsequently from the mineral wealth and liberated energy produced by the impact. This profit has more than offset the cost of terraformation and the compensation paid out to the few colonists who arrived in the early 9000s. It seems self-evident to such commentators that this whole series of events was planned from the outset.

Others point to the apparent friction between the Sophic League and NoCoZo concerning these events, suggesting that matters are not quite as clear cut as some might believe.

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Development Notes
Text by Steve Bowers, with suggestions from Radtech497 and l0b0_t0mmy
Initially published on 10 March 2012.