2I's species

2I's species is the alien species that the entity 2I/2054 D1, generally called 2I or the Object, belongs to. 2I, apparently a typical member of its kind, is a space-dwelling animal so large that it was initially mistaken for a spacecraft.

External
Externally, 2I resembles an extremely large spacecraft or asteroid. 2I is roughly cylindrical, about 10 kilometers (6 miles) wide at its thickest and 80 kilometers (48 miles) long. Like a spindle, its body tapers down to small points at both ends. The "north" pole, facing in the direction of 2I's motion, bears a spherical structure 50 meters (160 feet) across that bears an aperture and rotates inward, serving as an airlock. The "south" pole has a much smaller airlock, about 10 meters (33 feet) across, which is actually contained in 2I's gigantic sealed mouth. The entire rest of the outer surface is coated in fractal pyramidal or conical structures that serve to maximize its surface area, probably in order to gather interstellar hydrogen. Prolonged exposure to cosmic rays has colored 2I's exterior dark red.

Internal
Most of 2I's internal volume is occupied by a hollow drum that contains its organs and its supply of water and air. The drum rotates around 2I's central axis, producing a centripetal force of about 0.8 g's toward the walls. During its interstellar flight and hibernation, 2I's internal atmosphere consists almost entirely of argon at subzero temperatures and very low air pressure, with only trace amounts of water vapor. When it "wakes up," the atmosphere rapidly becomes warmer, thicker, wetter, and more oxygenated. Eventually, the atmosphere becomes a mixture of 20% oxygen and 80% argon at tropical temperatures and Earth-like pressures. This atmosphere is breathable to humans and is apparently the ideal atmosphere for 2I's larvae. 2I's internal water supply is concentrated in a "sea" within the drum, which is frozen during hibernation but liquid during activity. The sea contains most of 2I's organs.

Since 2I is so large, most of its organs are distributed around its body. It has hundreds of gigantic hearts, each one roughly egg-shaped and hundreds of meters across. It is thought to have lungs, which presumably breathe its internal atmosphere, distributed across its interior in a similar manner. Most of 2I's internal surface is covered by tendrils that combine the function of nerves and blood vessels. Ganglions are distributed around the body, each one resembling forests of fleshy trees with branches that end in structures resembling human hands. 2I's skeletal structure is highly complex, culminating in a massive cage near its south pole. This cage contains 2I's brain, a ring-shaped structure consisting of the hand-like structures found in its ganglions.

2I's interior is completely dark, with the exception of occasional flashes sometimes produced by its organs.

Diet
During their larval phase, 2I's species are voracious omnivores. They will devour anything organic and many things that are not, including rock. Their first meal is always the flesh of their parent.

As adults, 2I's species subsist partly on the massive amounts of nutrients they consumed in their larval stage. They supplement this by gathering interstellar hydrogen and simple organic molecules.

Propulsion
2I's species propel themselves on "wings" of magnetic fields, which they use to push against solar wind and interstellar gas. They can achieve speeds of dozens of kilometers per second, enough to escape the gravity wells of solar systems.

Reproduction and Life Cycle
While 2I's species spend most of their long lives in interstellar space, they still require organic matter, oxygen, and water. Therefore, some of their life cycle must take place on planets with these resources.

2I's species begin their lives as larvae within their parent's internal drum. Each adult produces millions of larvae, which hatch from eggs that grow on the adult's bones. These larvae are worm-shaped, with jointed, clawed legs and circular maws containing three rotating rings of razor-sharp teeth. While some hatch at very small sizes, most worms are at least 20 meters (66 feet) long, with many reaching the size of skyscrapers or ocean liners. The larvae hatch inside their parent as it awakens from hibernation and immediately begin to eat its tissues. The larvae initially hold off on attacking the parent's brain, but attack it with redoubled effort once they sense that the adult has died.

The dying adult ends its life by crashing into a planet which will serve as a "nursery." 2I's species targets planets rich in complex organics, free oxygen, and water for this purpose. While the impact is slow compared to most asteroid strikes, it is still sufficient to kill the adult and devastate the target planet's surface. The adult is broken open by the impact, and its millions of larvae spill across the surface. They consume what remains of the biosphere and much of the planet's crust, eventually growing into 80-kilometer adults.

Once all available nutrients have been consumed, the newly mature adults take off from the devastated planet. They then set courses for other life-bearing planets in other solar systems in order to continue the cycle. The adults' metabolism remains fully active until they exit the solar system, at which point they descend into hibernation. While hibernating, adults retain some measure of awareness, but think and react very slowly relative to their full activity level. It is unknown how long this interstellar hibernation can last, but it is likely thousands or even millions of years - 2I's age was never determined, but it was certainly at least many thousands of years old.

Senses
During both its larval and adult phases, 2I's species is completely blind. The creatures instead seem to detect their environment through an electromagnetic field sense. Larvae can home in on electronic equipment. The adults seem to have a spectroscopic sense of some sort, allowing them to sense exactly what elements and chemicals a given object is made of and to sense what kind of radiation it is emitting. 2I was also able to sense the curvature and expansion of space-time through unknown means.

History
2I's species is unimaginably old and has existed since "before the trilobites," meaning that they evolved at least 520 million years ago. They have consumed and destroyed millions of planets during this time.

In 2017, the interstellar object 'Oumuamua passed through the Solar System. It is implied to be related to 2I, since it entered the Solar System from the same direction and is similar in shape and coloration, but it is much smaller and was not observed to move under its own power - it may have been a defective or dead specimen, or possibly a natural object that happened to resemble 2I's species.

2I itself was born on a water-rich planet orbiting an orange sun. After many thousands of years, it entered the Solar System sometime before 2054 on a course for Earth. It intended to crash into Earth and release its larvae there, which would lead to the end of the human race and the biosphere. As seen from Earth, it came from near the star Vega in the constellation Lyra, much as 'Oumuamua did. 2I was first seen in 2054 by an employee of the KSpace company, who realized that it was moving under its own power like a spacecraft. This employee alerted NASA to the find.

NASA and other space agencies across the world, believing that 2I was a crewed spacecraft, attempted to contact it with radio messages for several months, receiving no reaction or response. 2I, having only animal-level intelligence, did not understand the messages and probably did not even detect them. Since 2I was headed directly toward Earth and refusing to respond to messages, it was treated as a likely threat. In September 2055, NASA and KSpace sent concurrent crewed missions to 2I in order to make direct contact. Both crews arrived in October.

The two crews separately entered 2I in search of its "crew." The KSpace crew entered first, and the NASA crew followed to rescue them when they failed to return. The members of the NASA crew eventually managed to determine that 2I was not a spacecraft, but instead a gigantic living creature. 2I's internal environment and biology proved extremely dangerous, and several crew members were killed.

Both missions concluded at 2I's brain in the final stages of the creatures life cycle. Of all the members of both crews, only one member of the NASA crew survived and escaped. The destruction of Earth was averted, however, by a NASA crew member who stayed behind and managed to divert 2I toward Mars. The existence of 2I, the events aboard it, and its nature as an alien were kept hidden from the public until after the crisis had ended, after which the global populace was informed of what had happened.

2I would ultimately die when it crashed into Mars, releasing its larvae onto and into the planet.