Formation and evolution of the Solar System

The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

This model, known as the nebular hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Its subsequent development has interwoven a variety of scientific disciplines including astronomy, chemistry, geology, physics, and planetary science. Since the dawn of the Space Age in the 1950s and the discovery of exoplanets in the 1990s, the model has been both challenged and refined to account for new observations.

The Solar System has evolved considerably since its initial formation. Many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later to have been captured by their planets. Still others, such as Earth's Moon, may be the result of giant collisions. Collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System. Beyond Neptune, many sub-planet sized objects formed. Several thousand trans-Neptunian objects have been observed. Unlike the planets, these trans-Neptunian objects mostly move on eccentric orbits, inclined to the plane of the planets. The positions of the planets might have shifted due to gravitational interactions. Planetary migration may have been responsible for much of the Solar System's early evolution.

In roughly 5 billion years, the Sun will cool and expand outward to many times its current diameter (becoming a red giant), before casting off its outer layers as a planetary nebula and leaving behind a stellar remnant known as a white dwarf. In the distant future, the gravity of passing stars will gradually reduce the Sun's retinue of planets. Some planets will be destroyed, and others ejected into interstellar space. Ultimately, over the course of tens of billions of years, it is likely that the Sun will be left with none of the original bodies in orbit around it.

## History

Ideas concerning the origin and fate of the world date from the earliest known writings; however, for almost all of that time, there was no attempt to link such theories to the existence of a "Solar System", simply because it was not generally thought that the Solar System, in the sense we now understand it, existed. The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had been developed for millennia (Aristarchus of Samos had suggested it as early as 250 BC), but was not widely accepted until the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704.

The current standard theory for Solar System formation, the nebular hypothesis, has fallen into and out of favour since its formulation by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century. The most significant criticism of the hypothesis was its apparent inability to explain the Sun's relative lack of angular momentum when compared to the planets. However, since the early 1980s studies of young stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts, which has led to its re-acceptance.

Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power. Arthur Stanley Eddington's confirmation of Albert Einstein's theory of relativity led to his realisation that the Sun's energy comes from nuclear fusion reactions in its core, fusing hydrogen into helium. In 1935, Eddington went further and suggested that other elements also might form within stars. Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen and helium in their cores. When a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems.

## Formation

### Presolar nebula

The nebular hypothesis says that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud, most likely at the edge of a Wolf-Rayet bubble. The cloud was about 20 parsecs (65 light years) across, while the fragments were roughly 1 parsec (three and a quarter light-years) across. The further collapse of the fragments led to the formation of dense cores 0.01–0.1 parsec (2,000–20,000 AU) in size. One of these collapsing fragments (known as the presolar nebula) formed what became the Solar System. The composition of this region with a mass just over that of the Sun () was about the same as that of the Sun today, with hydrogen, along with helium and trace amounts of lithium produced by Big Bang nucleosynthesis, forming about 98% of its mass. The remaining 2% of the mass consisted of heavier elements that were created by nucleosynthesis in earlier generations of stars. Late in the life of these stars, they ejected heavier elements into the interstellar medium. Some scientists have given the name Coatlicue to a hypothetical star that went supernova and created the presolar nebula.

The oldest inclusions found in meteorites, thought to trace the first solid material to form in the presolar nebula, are 4,568.2 million years old, which is one definition of the age of the Solar System. Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that only form in exploding, short-lived stars. This indicates that one or more supernovae occurred nearby. A shock wave from a supernova may have triggered the formation of the Sun by creating relatively dense regions within the cloud, causing these regions to collapse. The highly homogeneous distribution of iron-60 in the Solar System points to the occurrence of this supernova and its injection of iron-60 being well before the accretion of nebular dust into planetary bodies. Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars, possibly similar to the Orion Nebula. Studies of the structure of the Kuiper belt and of anomalous materials within it suggest that the Sun formed within a cluster of between 1,000 and 10,000 stars with a diameter of between 6.5 and 19.5 light years and a collective mass of . This cluster began to break apart between 135 million and 535 million years after formation. Several simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produced anomalous orbits observed in the outer Solar System, such as detached objects.

Because of the conservation of angular momentum, the nebula spun faster as it collapsed. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, converting their kinetic energy into heat. The center, where most of the mass collected, became increasingly hotter than the surrounding disc. Over about 100,000 years, the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disc with a diameter of about 200 AU and form a hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre.

At this point in its evolution, the Sun is thought to have been a T Tauri star. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of . These discs extend to several hundred AU—the Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter in star-forming regions such as the Orion Nebula—and are rather cool, reaching a surface temperature of only about 1,000 K (730 °C; 1,340 °F) at their hottest. Within 50  million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until hydrostatic equilibrium was achieved. This marked the Sun's entry into the prime phase of its life, known as the main sequence. Main-sequence stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a main-sequence star today.

As the early Solar System continued to evolve, it eventually drifted away from its siblings in the stellar nursery, and continued orbiting the Milky Way's center on its own. The Sun likely drifted from its original orbital distance from the center of the galaxy. The chemical history of the Sun suggests it may have formed as much as 3 kpc closer to the galaxy core.

### Solar system birth environment

Like most stars, the Sun likely formed not in isolation but as part of a young star cluster. There are several indications that hint at the cluster environment having had some influence of the young still forming solar. For example, the decline in mass beyond Neptune and the extreme eccentric-orbit of Sedna have been interpreted as a signature of the solar system having been influenced by its birth environment. Whether the presence of the isotopes iron-60 and aluminium-26 can be interpreted as a sign of a birth cluster containing massive stars is still under debate. If the Sun was part of a star cluster, it might have been influenced by close flybys of other stars, the strong radiation of nearby massive stars and ejecta from supernovae occurring close by.

### Formation of the planets

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation. The currently accepted method by which the planets formed is accretion, in which the planets began as dust grains in orbit around the central protostar. Through direct contact and self-organization, these grains formed into clumps up to 200 m (660 ft) in diameter, which in turn collided to form larger bodies (planetesimals) of \~10 km (6.2 mi) in size. These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.

The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large. The terrestrial embryos grew to about 0.05 Earth masses () and ceased accumulating matter about 100,000 years after the formation of the Sun; subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes.

When terrestrial planets were forming, they remained immersed in a disk of gas and dust. Pressure partially supported the gas and so did not orbit the Sun as rapidly as the planets. The resulting drag and, more importantly, gravitational interactions with the surrounding material caused a transfer of angular momentum, and as a result the planets gradually migrated to new orbits. Models show that density and temperature variations in the disk governed this rate of migration, but the net trend was for the inner planets to migrate inward as the disk dissipated, leaving the planets in their current orbits.

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, which is the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. Planetesimals beyond the frost line accumulated up to within about 3 million years. Today, the four giant planets comprise just under 99% of all the mass orbiting the Sun. Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused the material to accumulate rapidly at \~5 AU from the Sun. This excess material coalesced into a large embryo (or core) on the order of , which began to accumulate an envelope via accretion of gas from the surrounding disc at an ever-increasing rate. Once the envelope mass became about equal to the solid core mass, growth proceeded very rapidly, reaching about 150 Earth masses \~105 years thereafter and finally topping out at . Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume.

T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. As a result, those planets accumulated little hydrogen and helium—not more than each. Uranus and Neptune are sometimes referred to as failed cores. The main problem with formation theories for these planets is the timescale of their formation. At the current locations it would have taken millions of years for their cores to accrete. This means that Uranus and Neptune may have formed closer to the Sun—near or even between Jupiter and Saturn—and later migrated or were ejected outward (see Planetary migration below). Motion in the planetesimal era was not all inward toward the Sun; the Stardust sample return from Comet Wild 2 has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt.

After between three and ten million years, the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets.

## Subsequent evolution

The planets were originally thought to have formed in or near their current orbits. This has been questioned during the last 20 years. Currently, many planetary scientists think that the Solar System might have looked very different after its initial formation: several objects at least as massive as Mercury may have been present in the inner Solar System, the outer Solar System may have been much more compact than it is now, and the Kuiper belt may have been much closer to the Sun.

### Terrestrial planets

At the end of the planetary formation epoch, the inner Solar System was populated by 50–100 Moon- to Mars-sized protoplanets. Further growth was possible only because these bodies collided and merged, which took less than 100 million years. These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape. One such giant collision is thought to have formed the Moon (see Moons below), while another removed the outer envelope of the young Mercury.

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto-terrestrial planets, which would have needed to be highly eccentric to collide, produced the remarkably stable and nearly circular orbits they have today. One hypothesis for this "eccentricity dumping" is that terrestrials formed in a disc of gas still not expelled by the Sun. The "gravitational drag" of this residual gas would have eventually lowered the planets' energy, smoothing out their orbits. However, such gas, if it existed, would have prevented the terrestrial planets' orbits from becoming so eccentric in the first place. Another hypothesis is that gravitational drag occurred not between the planets and residual gas but between the planets and the remaining small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by the larger planets' gravity, formed a region of higher density, a "gravitational wake", in the larger objects' path. As they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits.

### Asteroid belt

The outer edge of the terrestrial region, between 2 and 4 AU from the Sun, is called the asteroid belt. The asteroid belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of planetesimals formed there. As with the terrestrials, planetesimals in this region later coalesced and formed 20–30 Moon- to Mars-sized planetary embryos; however, the proximity of Jupiter meant that after this planet formed, 3 million years after the Sun, the region's history changed dramatically. Orbital resonances with Jupiter and Saturn are particularly strong in the asteroid belt, and gravitational interactions with more massive embryos scattered many planetesimals into those resonances. Jupiter's gravity increased the velocity of objects within these resonances, causing them to shatter upon collision with other bodies, rather than accrete.

As Jupiter migrated inward following its formation (see Planetary migration below), resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other. The cumulative action of the resonances and the embryos either scattered the planetesimals away from the asteroid belt or excited their orbital inclinations and eccentricities. Some of those massive embryos too were ejected by Jupiter, while others may have migrated to the inner Solar System and played a role in the final accretion of the terrestrial planets. During this primary depletion period, the effects of the giant planets and planetary embryos left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed mainly of small planetesimals. This is still 10–20 times more than the current mass in the main belt, which is now about . A secondary depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below).

The inner Solar System's period of giant impacts probably played a role in Earth acquiring its current water content (\~6×1021 kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently delivered from outer, colder parts of the Solar System. The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter. A population of main-belt comets discovered in 2006 has also been suggested as a possible source for Earth's water. In contrast, comets from the Kuiper belt or farther regions delivered not more than about 6% of Earth's water. The panspermia hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely accepted.

### Planetary migration

According to the nebular hypothesis, the outer two planets may be in the "wrong place". Uranus and Neptune (known as the "ice giants") exist in a region where the reduced density of the solar nebula and longer orbital times render their formation there highly implausible. The two are instead thought to have formed in orbits near Jupiter and Saturn (known as the "gas giants"), where more material was available, and to have migrated outward to their current positions over hundreds of millions of years.

The migration of the outer planets is also necessary to account for the existence and properties of the Solar System's outermost regions. Beyond Neptune, the Solar System continues into the Kuiper belt, the scattered disc, and the Oort cloud, three sparse populations of small icy bodies thought to be the points of origin for most observed comets. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU, and the distant Oort cloud begins at about 50,000 AU. Originally, however, the Kuiper belt was much denser and closer to the Sun, with an outer edge at approximately 30 AU. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20 AU), and in 50% of simulations ended up in opposite locations, with Uranus farther from the Sun than Neptune.

According to the Nice model, after the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 resonance: Saturn orbited the Sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, possibly causing Neptune to surge past Uranus and plough into the ancient Kuiper belt. The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward. Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud; those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc. This scenario explains the Kuiper belt's and scattered disc's present low mass. Some of the scattered objects, including Pluto, became gravitationally tied to Neptune's orbit, forcing them into mean-motion resonances. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune near-circular again.

In contrast to the outer planets, the inner planets are not thought to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts.

Another question is why Mars came out so small compared with Earth. A study by Southwest Research Institute, San Antonio, Texas, published June 6, 2011 (called the Grand tack hypothesis), proposes that Jupiter had migrated inward to 1.5 AU. After Saturn formed, migrated inward, and established the 2:3 mean motion resonance with Jupiter, the study assumes that both planets migrated back to their present positions. Jupiter thus would have consumed much of the material that would have created a bigger Mars. The same simulations also reproduce the characteristics of the modern asteroid belt, with dry asteroids and water-rich objects similar to comets. However, it is unclear whether conditions in the solar nebula would have allowed Jupiter and Saturn to move back to their current positions, and according to current estimates this possibility appears unlikely. Moreover, alternative explanations for the small mass of Mars exist.

### Late Heavy Bombardment and after

Gravitational disruption from the outer planets' migration would have sent large numbers of asteroids into the inner Solar System, severely depleting the original belt until it reached today's extremely low mass. This event may have triggered the Late Heavy Bombardment that is hypothesised to have occurred approximately 4 billion years ago, 500–600 million years after the formation of the Solar System. However, a recent re-appraisal of the cosmo-chemical constraints indicates that there was likely no late spike (“terminal cataclysm”) in the bombardment rate.

If it occurred, this period of heavy bombardment lasted several hundred million years and is evident in the cratering still visible on geologically dead bodies of the inner Solar System such as the Moon and Mercury. The oldest known evidence for life on Earth dates to 3.8 billion years ago—almost immediately after the end of the Late Heavy Bombardment.

Impacts are thought to be a regular (if currently infrequent) part of the evolution of the Solar System. That they continue to happen is evidenced by the collision of Comet Shoemaker–Levy 9 with Jupiter in 1994, the 2009 Jupiter impact event, the Tunguska event, the Chelyabinsk meteor and the impact that created Meteor Crater in Arizona. The process of accretion, therefore, is not complete, and may still pose a threat to life on Earth.

Over the course of the Solar System's evolution, comets were ejected out of the inner Solar System by the gravity of the giant planets and sent thousands of AU outward to form the Oort cloud, a spherical outer swarm of cometary nuclei at the farthest extent of the Sun's gravitational pull. Eventually, after about 800 million years, the gravitational disruption caused by galactic tides, passing stars and giant molecular clouds began to deplete the cloud, sending comets into the inner Solar System. The evolution of the outer Solar System also appears to have been influenced by space weathering from the solar wind, micrometeorites, and the neutral components of the interstellar medium.

The evolution of the asteroid belt after Late Heavy Bombardment was mainly governed by collisions. Objects with large mass have enough gravity to retain any material ejected by a violent collision. In the asteroid belt this usually is not the case. As a result, many larger objects have been broken apart, and sometimes newer objects have been forged from the remnants in less violent collisions. Moons around some asteroids currently can only be explained as consolidations of material flung away from the parent object without enough energy to entirely escape its gravity.

## Moons

Moons have come to exist around most planets and many other Solar System bodies. These natural satellites originated by one of three possible mechanisms:

- Co-formation from a circumplanetary disc (only in the cases of the giant planets);
- Formation from impact debris (given a large enough impact at a shallow angle); and
- Capture of a passing object.

Jupiter and Saturn have several large moons, such as Io, Europa, Ganymede and Titan, which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun. This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries also makes formation from collision debris unlikely. The outer moons of the giant planets tend to be small and have eccentric orbits with arbitrary inclinations. These are the characteristics expected of captured bodies. Most such moons orbit in the direction opposite to the rotation of their primary. The largest irregular moon is Neptune's moon Triton, which is thought to be a captured Kuiper belt object.

Moons of solid Solar System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are thought to be captured asteroids. The Earth's Moon is thought to have formed as a result of a single, large head-on collision. The impacting object probably had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon. The impact was probably the last in a series of mergers that formed the Earth. It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth–Sun Lagrangian points (either or ) and drifted from its position. The moons of trans-Neptunian objects Pluto (Charon) and Orcus (Vanth) may also have formed by means of a large collision: the Pluto–Charon, Orcus–Vanth and Earth–Moon systems are unusual in the Solar System in that the satellite's mass is at least 1% that of the larger body.

## Future

Astronomers estimate that the current state of the Solar System will not change drastically until the Sun has fused almost all the hydrogen fuel in its core into helium, beginning its evolution from the main sequence of the Hertzsprung–Russell diagram and into its red-giant phase. The Solar System will continue to evolve until then. Eventually, the Sun will likely expand sufficiently to overwhelm the inner planets (Mercury, Venus, and possibly Earth) but not the outer planets, including Jupiter and Saturn. Afterward, the Sun would be reduced to the size of a white dwarf, and the outer planets and their moons would continue orbiting this diminutive solar remnant. This future development may be similar to the observed detection of MOA-2010-BLG-477L b, a Jupiter-sized exoplanet orbiting its host white dwarf star MOA-2010-BLG-477L.

### Long-term stability

The Solar System is chaotic over million- and billion-year timescales, with the orbits of the planets open to long-term variations. One notable example of this chaos is the Neptune–Pluto system, which lies in a 3:2 orbital resonance. Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10–20 million years (the Lyapunov time) into the future. Another example is Earth's axial tilt, which, due to friction raised within Earth's mantle by tidal interactions with the Moon (see below), is incomputable from some point between 1.5 and 4.5 billion years from now.

The outer planets' orbits are chaotic over longer timescales, with a Lyapunov time in the range of 2–230 million years. In all cases, this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer becomes uncertain). Still, in some cases, the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more—or less—elliptical.

Ultimately, the Solar System is stable in that none of the planets are likely to collide with each other or be ejected from the system in the next few billion years. Beyond this, within five billion years or so, Mars's eccentricity may grow to around 0.2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the Solar System altogether or send it on a collision course with Venus or Earth. This could happen within a billion years, according to numerical simulations in which Mercury's orbit is perturbed.

### Moon–ring systems

The evolution of moon systems is driven by tidal forces. A moon will raise a tidal bulge in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary. If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In this situation, angular momentum is transferred from the rotation of the primary to the revolution of the satellite. The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time.

The Earth and its Moon are one example of this configuration. Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows one face to the Earth. The Moon will continue to recede from Earth, and Earth's spin will continue to slow gradually. Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons) and most of the larger moons of Saturn.

A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates or is revolving in the direction opposite the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks. In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out). In both cases, tidal deceleration causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a planetary ring system, or crashes into the planet's surface or atmosphere. Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years), Triton of Neptune (in 3.6 billion years), and at least 16 small satellites of Uranus and Neptune. Uranus's Desdemona may even collide with one of its neighboring moons.

A third possibility is where the primary and moon are tidally locked to each other. In that case, the tidal bulge stays directly under the moon, there is no angular momentum transfer, and the orbital period will not change. Pluto and Charon are an example of this type of configuration.

There is no consensus on the mechanism of the formation of the rings of Saturn. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, data from the Cassini–Huygens spacecraft suggests they formed relatively late.

### The Sun and planetary environments

In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. As the Sun burns through its hydrogen fuel supply, it gets hotter and burns the remaining fuel even faster. As a result, the Sun is growing brighter at a rate of ten percent every 1.1 billion years. In about 600 million years, the Sun's brightness will have disrupted the Earth's carbon cycle to the point where trees and forests (C3 photosynthetic plant life) will no longer be able to survive; and in around 800 million years, the Sun will have killed all complex life on the Earth's surface and in the oceans. In 1.1 billion years, the Sun's increased radiation output will cause its circumstellar habitable zone to move outwards, making the Earth's surface too hot for liquid water to exist there naturally. At this point, all life will be reduced to single-celled organisms. Evaporation of water, a potent greenhouse gas, from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner. During this time, it is possible that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen under the surface regolith will release into the atmosphere, creating a greenhouse effect that will heat the planet until it achieves conditions parallel to Earth today, providing a potential future abode for life. By 3.5 billion years from now, Earth's surface conditions will be similar to those of Venus today.

Around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell. This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU (180×10^6 km; 110×10^6 mi)—256 times its current size. At the tip of the red-giant branch, as a result of the vastly increased surface area, the Sun's surface will be much cooler (about 2,600 K (2,330 °C; 4,220 °F)) than now, and its luminosity much higher—up to 2,700 current solar luminosities. For part of its red-giant life, the Sun will have a strong stellar wind that will carry away around 33% of its mass. During these times, it is possible that Saturn's moon Titan could achieve surface temperatures necessary to support life.

As the Sun expands, it will swallow the planets Mercury and Venus. Earth's fate is less clear; although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out. If it were only for this, Venus and Earth would probably escape incineration, but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun's weakly-bound outer envelope.

Additionally, the Sun's habitable zone will move into the outer Solar System and eventually beyond the Kuiper belt at the end of the red-giant phase, causing icy bodies such as Enceladus and Pluto to thaw. During this time, these worlds could support a water-based hydrologic cycle, but as they were too small to hold a dense atmosphere like Earth, they would experience extreme day–night temperature differences. When the Sun leaves the red-giant branch and enters the asymptotic giant branch, the habitable zone will abruptly shrink to roughly the space between Jupiter and Saturn's present-day orbits, but toward the end of the 200 million-year duration of the asymptotic giant phase, it will expand outward to about the same distance as before.

Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. At this point, the density and temperature will become so high that the fusion of helium into carbon will begin, leading to a helium flash; the Sun will shrink from around 250 to 11 times its present (main-sequence) radius. Consequently, its luminosity will decrease from around 3,000 to 54 times its current level, and its surface temperature will increase to about 4,770 K (4,500 °C; 8,130 °F). The Sun will become a horizontal giant, burning helium in its core in a stable fashion, much like it burns hydrogen today. The helium-fusing stage will last only 100 million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers. It will expand a second time, becoming what is known as an asymptotic giant. Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about 3,500 K (3,230 °C; 5,840 °F). This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula. The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars and planets.

This is a relatively peaceful event, nothing akin to a supernova, in which the Sun is too small to undergo one as part of its evolution. Any observer present to witness this occurrence would see a massive increase in the speed of the solar wind, but not enough to destroy a planet completely. However, the star's loss of mass could send the orbits of the surviving planets into chaos, causing some to collide, others to be ejected from the Solar System, and others to be torn apart by tidal interactions. Afterwards, all that will remain of the Sun is a white dwarf, an extraordinarily dense object, 54% of its original mass but only the size of Earth. Initially, this white dwarf may be 100 times as luminous as the Sun is now. It will consist entirely of degenerate carbon and oxygen but will never reach temperatures hot enough to fuse these elements. Thus, the white dwarf Sun will gradually cool, growing dimmer and dimmer.

As the Sun dies, its gravitational pull on orbiting bodies, such as planets, comets, and asteroids, will weaken due to its mass loss. All remaining planets' orbits will expand; if Venus, Earth, and Mars still exist, their orbits will lie roughly at 1.4 AU (210 million km; 130 million mi), 1.9 AU (280 million km; 180 million mi), and 2.8 AU (420 million km; 260 million mi), respectively. They and the other remaining planets will become dark, frigid hulks, completely devoid of life. They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Two billion years later, when the Sun has cooled to the 6,000–8,000 K (5,730–7,730 °C; 10,340–13,940 °F) range, the carbon and oxygen in the Sun's core will freeze, with over 90% of its remaining mass assuming a crystalline structure. Eventually, after roughly one quadrillion years, the Sun will finally cease to shine altogether, becoming a black dwarf.

## Galactic interaction

The Solar System travels alone through the Milky Way in a circular orbit approximately 30,000 light years from the Galactic Center. Its speed is about 220 km/s. The period required for the Solar System to complete one revolution around the Galactic Center, the galactic year, is in the range of 220–250 million years. Since its formation, the Solar System has completed at least 20 such revolutions.

Various scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of mass extinctions observed in the Earth's fossil record. One hypothesis supposes that vertical oscillations made by the Sun as it orbits the Galactic Centre cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker; as it re-enters the galactic disc, as it does every 20–25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact.

However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore, the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms. Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giants, which live for relatively short periods and then explode violently as supernovae.

### Galactic collision and planetary disruption

Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of the Local Group of galaxies, is heading toward it at about 120 km/s. In 4 billion years, Andromeda and the Milky Way will collide, causing both to deform as tidal forces distort their outer arms into vast tidal tails. If this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy. After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%, the galaxies' supermassive black holes will merge. Eventually, in roughly 6 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy. During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy. This may lead to a short period of intensive star formation called a starburst. In addition, the infalling gas will feed the newly formed black hole, transforming it into an active galactic nucleus. The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions.

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. Although it is true that the gravity of passing stars can detach planets into interstellar space, distances between stars are so great that the likelihood of the Milky Way–Andromeda collision causing such disruption to any individual star system is negligible. Although the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed.

However, over time, the cumulative probability of a chance encounter with a star increases, and disruption of the planets becomes all but inevitable. Assuming that the Big Crunch or Big Rip scenarios for the end of the Universe do not occur, calculations suggest that the gravity of passing stars will have completely stripped the dead Sun of its remaining planets within 1 quadrillion (1015) years. This point marks the end of the Solar System. Although the Sun and planets may survive, the Solar System, in any meaningful sense, will cease to exist.

## Chronology

The time frame of the Solar System's formation has been determined using radiometric dating. Scientists estimate that the Solar System is 4.6 billion years old. The oldest known mineral grains on Earth are approximately 4.4 billion years old. Rocks this old are rare, as Earth's surface is constantly being reshaped by erosion, volcanism, and plate tectonics. To estimate the age of the Solar System, scientists use meteorites, which were formed during the early condensation of the solar nebula. Almost all meteorites (see the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, suggesting that the Solar System must be at least this old.

Studies of discs around other stars have also done much to establish a time frame for Solar System formation. Stars between one and three million years old have discs rich in gas, whereas discs around stars more than 10 million years old have little to no gas, suggesting that giant planets within them have ceased forming.

### Timeline of Solar System evolution

Note: All dates and times in this chronology are approximate and should be taken as an order of magnitude indicator only.

## See also

Modelling its structure and composition


BMX XXX

BMX XXX is a 2002 sports video game developed by Z-Axis and published by Acclaim Entertainment under their AKA Acclaim label for the Xbox, PlayStation 2 and GameCube. While primarily a BMX-based action sports title, the game places a distinct emphasis on off-color and sexual humor, and allows the player to create female characters that are fully topless. The game also features unlockable live-action footage of real-life strippers courtesy of Scores, a New York-based stripclub.

BMX XXX began development in 2001 as a traditional entry in the Dave Mirra Freestyle BMX series, and was announced as Dave Mirra Freestyle BMX 3. The executives of Z-Axis and Acclaim — influenced by a crowded action sports game market, a dire financial situation, and the commercial success of the Grand Theft Auto series — decided to insert nudity and mature humor into the game to increase publicity and sales. Although Dave Mirra initially supported the pitched concept, his name was eventually dissociated from the title following its unveiling at E3. The development team members were displeased with the change in direction, with some attempting to distance themselves from the production.

Despite an aggressive marketing campaign, the game's distribution was impeded by various circumstances; major retailers refused to stock the title, Sony Computer Entertainment refused to publish the PlayStation 2 version unless the topless female nudity was censored, and the game was initially banned in Australia. Upon release, BMX XXX received mixed reviews from critics, who felt that the game lacked innovation despite its content. Although the control scheme and voice acting were complimented, opinions on the soundtrack were mixed, and reviewers faulted the camera, level design, mission objectives, visuals, and humor. As a result of its limited distribution and loss of celebrity endorsement, BMX XXX was a commercial failure. It subsequently became a factor in a series of lawsuits against Acclaim by Mirra and the company's shareholders, and was cited as one of a number of failures that contributed to Acclaim's 2004 bankruptcy and liquidation.

## Gameplay

BMX XXX is a freestyle BMX sports game with an emphasis on off-color and sexual humor. The player character can be customized by name, gender, and physical attributes, or selected from a number of pre-made characters; the ability to create topless female riders is enabled when the single-player campaign is fully completed. The player's rider can perform a variety of tricks in midair with the combined input of a direction on the D-pad or left thumbstick and a button. The player can also grind on rails, ledges, or other likely surfaces, and can exit a grind by jumping into the air or falling out of balance. The player is awarded points by performing complete tricks and landing while the bike is properly oriented. The player character will be ejected from their bike if they are not oriented for a successful landing or if they crash into something with a part of the body or bike other than their feet, wheels, or grind pegs during a trick. The player character will also be ejected if they are riding off-balance and hit an obstacle too fast or at a harsh angle. In the event of a crash, the player's score is reset.

The single-player campaign is divided into eight levels, six of which are based on a series of challenges that the player must complete to advance to the next level. Challenges are often initiated by interacting with a character within the level, who will give the player an objective to fulfill. Completing ten challenges within a level will grant access to the next level. Scattered within each level are four collectible bike parts; accumulating complete sets of six parts within the campaign unlocks upgraded bikes that enhance the player's performance. Each level also features a series of 45 collectibles such as coins, as well as 20 gaps in the terrain to discover. Completing certain challenges unlocks full-motion video sequences of strippers, with videos unlocked in later levels displaying an increasing amount of nudity. Two of the levels are competitions that require the player to perform a variety of tricks and earn a medal.

The game includes three multiplayer modes in which two human players compete against each other. In "Strip Challenge", players aim to achieve the highest-scoring trick combination. When a player breaks the record, the opponent's character loses a piece of clothing, and the game ends when one player renders their opponent naked. In "Skillz", players compete to achieve the highest score over a two-minute run, and in "Paintball", one player must collect all the boomboxes within a level while their opponent attempts to snipe them.

## Development

In 2001, Dave Mirra Freestyle BMX developer Z-Axis began production on the third installment of the series, aiming to expand the technology and include such features as a story, voice-acting, and mission-based gameplay. Lead artist Mark Girouard described the game's original narrative as centering on a BMX team on tour throughout the United States. In January 2002, series publisher Acclaim Entertainment announced Dave Mirra Freestyle BMX 3, which would have showcased freestyle BMX rider Dave Mirra alongside sixteen other professionals.

Around this time, the action sports genre had become crowded, prompting executives of Z-Axis and Acclaim to ponder new angles for increasing publicity. During a meeting between the parties, someone suggested adding strippers to the game in jest. While this initially elicited laughter, the group began seriously considering the idea, with Acclaim hoping that appealing to an older audience would increase sales. Acclaim marketing coordinator Zach Smith noted that the company's decision to insert nudity in the game was influenced by their dire financial situation at the time, and Acclaim executive producer Shawn Rosen additionally cited the commercial success of the Grand Theft Auto series as the catalyst for backing an adult-oriented title. Director Glen Egan acknowledged the increasing success of M-rated games, and was encouraged to pursue the rating by his irritation at having to excise drug references and profanity from the soundtracks of the previous two Dave Mirra titles.

In March 2002, Mirra and Acclaim began discussions about attaching his name to the title, which Mirra felt would be a more tongue-in-cheek and mature game comparable to the parodic film Airplane!. Rosen claimed that while Mirra found the pitched concept humorous, sponsors warned Mirra that the game would be harmful to his image. Acclaim looked to sex comedies and Jackass as inspiration for the game's sexual humor and raunchy dialogue. Egan also cited Bam Margera's Camp Kill Yourself video series as an influence on the game's bold and irreverent humor and action. For the game, Acclaim formed a partnership with the New York-based stripclub Scores, with footage of its employees being included as unlockable content. Lead designer Tin Guerrero postulated that this decision was influenced by the popularity of Howard Stern at the time, with Scores apparently being his favorite stripclub. The footage was filmed by Acclaim without Z-Axis's involvement.

The game's change in direction required Z-Axis's development team to redesign a significant amount of content they had completed thus far, retooling what would have been Dave Mirra Freestyle BMX 3 into Dave Mirra BMX XXX. The team members were dismayed by the decision, with Girouard recalling it as "the biggest creative shock I've ever experienced in games through all these years". Some members tried to distance themselves from the production by leaving their full names out of the credits, opting to either abbreviate their surnames or use the names of historical figures such as Fletcher Christian. The team was unable to object due to the company management's support of the direction and their multi-game contractual obligation to Acclaim concerning the Dave Mirra series. Despite being frustrated by their circumstance, the development team would not work perfunctorily, and remained driven to create a satisfying game. The game runs on the same engine previously used by Z-Axis's Aggressive Inline. The script was written by Happy Tree Friends co-creator Warren Graff.

On August 19, 2002, Acclaim announced that Dave Mirra's name had been removed from the title, and that he and the other professional riders, as well as licensed equipment, would not be featured in the game. Jeff Gerstmann of GameSpot speculated that the move was made by Acclaim to preserve creative control over the game's content while preventing damage to the images of the previously involved riders and equipment manufacturers. While Egan was perplexed by Acclaim's decision considering Mirra's supposed importance to the property, Guerrero alleged a souring relationship between Acclaim and Mirra as a factor. Regardless, Acclaim's licensing deal with Mirra stood intact, with a Dave Mirra Freestyle BMX 3 title planned for release. The game would be released for the Game Boy Advance on November 25, 2002.

## Marketing and release

Dave Mirra BMX XXX was showcased at E3 in May 2002. IGN PS2, in its "Best of E3 2002 Awards", named it runner-up in the "Biggest Surprise" category, describing it as "a surprise in every sense of the word" and proclaiming that "any title that's working hard to perfect 'Boob-Jiggle Technology' deserves a questionable double take".

In September 2002, a rumor circulated that Sony Computer Entertainment would not approve the PlayStation 2 (PS2) version of the game unless certain sexual content was removed. On November 4, Acclaim representatives Alan Lewis and Tara Blanco confirmed the rumor, stating that the PS2 version would be edited to eliminate the topless nudity. An anonymous Sony representative claimed that the topless female imagery posed a detrimental threat to Sony's console brand, and believed that it was not fundamentally crucial to the gameplay experience.

Acclaim launched an aggressive \$3-4 million advertising campaign for the game, featuring irreverent jokes and the tagline "This is BMX?". On October 17, 2002, Acclaim unveiled the game's soundtrack, which consists of a mixture of classic and modern rock and hip hop. On November 7, Acclaim launched a "Ms. BMX XXX" competition, in which female contestants submitted a digital photo of themselves or a friend, which was subject to a public vote. The winner was flown to New York City and escorted to Scores by Gary Dell'Abate and K.C. Armstrong of The Howard Stern Show.

On October 14, 2002, Reuters reported that Walmart, Toys "R" Us, and KB Toys had refused to stock the title. Although Guerrero regarded Walmart's motion as a "dagger blow", he found humor in the retailer's contemporary distribution of guns and ammunition. BMX XXX was released in North America for the Xbox on November 15, 2002, for the PlayStation 2 on November 16, and for the GameCube on November 25. In the United Kingdom, it was released for GameCube and Xbox on December 6 and for the PlayStation 2 on February 7, 2003. It would be Z-Axis's last game for Acclaim before their acquisition by Activision, which was announced at the same time as the game's unveiling at E3. Those who pre-ordered the game from EB Games or GameStop received a free t-shirt. The game was initially banned in Australia, but was released in the country after the removal of its sexual content.

## Reception

BMX XXX received "mixed or average" reviews on all platforms according to video game review aggregator Metacritic, with some reviewers concluding that the game was lacking in innovation outside of its vulgarity and sexual content. Todd Zuniga of Official U.S. PlayStation Magazine further denounced the game as "the worst example of what PS2 games are capable of", and described it as milquetoast in all gameplay aspects. However, Steve Steinberg of GameSpy and the reviewers of Nintendo Power considered it to be a solid gaming experience despite its crude content, with Steinberg pointing out that the game's "twisted novelty" could not function in other genres, and worked because of the game's fundamental playability. Scott Steinberg of Playboy also opined that the title was at its strongest outside of its comedic elements, attributing the gameplay's refinements over the course of a more mundane franchise.

The control scheme was mostly found to be comfortable and intuitive, though Bryn Williams of GameSpy and Justin Nation of Planet GameCube considered the stunt system to be simplified compared to other extreme sports titles such as Tony Hawk's Pro Skater 4. Although Jeff Gerstmann of GameSpot was impressed by the variety of tricks, he felt that none of them seemed special or important, and pointed out that the game engine's unrealistic quirks reduced the sense of challenge. Scott Alan Marriott of AllGame, Dan Leahy of Electronic Gaming Monthly, and AM Urbanek of TechTV observed a lack of inertia in the player's bike, which contributed to a sense of inconsistency in the controls. Although Steve Steinberg commended the camera as solid and logically placed, others criticized it as jumpy and jerky, with Marriott and IGN's Matt Casamassina citing a tendency to bounce off walls and barriers, and Zuniga expressing frustration at having to stop the bike to look around. The levels were said to not be as large and creative as those in Z-Axis's previous efforts, and their design was faulted as barren, with an abundance of spaces bereft of features. Reviewers complained of the vague challenge objectives, which were exacerbated by the lack of any source of direction or orientation; Steve Steinberg and Gerstmann cited a specific instance of an early mission that tasked the player with running down a "fruitbooter" while giving no indication that the term is derogatory slang for an inline skater. The necessity of returning to a non-player character's position to retry a failed challenge was an additional annoyance, with Matt Helgeson of Game Informer noting that the characters do not always stay at their post. Helgeson, fellow Game Informer reviewer Justin Leeper, and Aaron Boulding of IGN were unenthused by the high amount of scavenger hunt missions, though Leeper found some objectives humorous, and Casamassina complimented their presentation. Socrates of TeamXbox enjoyed the multiplayer mode, but felt that the lack of the single-player campaign's lewdness made the mode feel indistinct from other extreme sports titles. Steve Steinberg was uneased by the Paintball mode, and wondered how it was left intact after several games were delayed or reworked after the September 11 attacks. The stripper videos were noted to be insufficiently explicit for the "XXX" title, and reviewers felt that they were too difficult to unlock to be worth the effort, particularly for adults who have easier means to access explicit content.

Assessments of the visuals were generally unfavorable, with some reviewers finding them comparable to a PSone title. While the animation and frame rate were commended for their smoothness, the characters were criticized for their blocky models, stiff body movements, and blurry textures. Some pointed out that the rough character models worked against the game's attempt at sex appeal, with Tom Bramwell of Eurogamer remarking that "not since Orchid in the original Killer Instinct have I seen such angular assets". Nation further observed that a topless female biker in a third-person video game served little function beyond novelty due to the camera's position behind her back as well as her hunched position. Dan Elektro of GamePro and Casamassina cited occasional collision issues in which riders temporarily sink through the ground or a wall. While Gerstmann and Casamassina described the environments as large in scope, Nick Valentino of GameZone felt that they were not as large or impressive as those in Aggressive Inline, and he and Bramwell found some areas to be plain and uninteresting. Marriott noticed some distant pop-up, and felt that the rider customization was insufficiently extensive. Socrates commented that the environments "look like they have been coated in a layer of Vaseline", and he considered the stripper videos to be the game's best-looking element. Boulding and Valentino also complimented the videos' professional quality, with Boulding stating that they "might convince you that you're watching an episode of G String Divas but without all of that talking and emoting". Nation, however, determined the production to be low-budget, saying that it "looks like it was copied off of a copy of an older Super 8 tape".

The soundtrack was classified as a combination of rock, punk, hip hop and ska that was typical of the extreme sports genre, with Casamassina remarking that the selections, though well-executed, "seem to appear on every extreme sports soundtrack in the universe". While Marriott, Elektro, and fellow GamePro reviewer Tokyo Drifter admired the soundtrack, Bramwell and Helgeson were not taken with it, with Bramwell being thankful for the Xbox version's option to insert a custom playlist. Valentino found the background music's downplayed prominence odd, and Gerstmann was perplexed by the soundtrack's editing of racial slurs and drug references considering the marketing campaign's emphasis on obscenity. Boulding was annoyed by an audio flaw that caused custom playlists to start again from the first track any time the music was paused for a gameplay reason, such as initiating a challenge. The sound effects were regarded as unremarkable and monotonous. Reactions to the voice acting were mostly positive, with Socrates elaborating that the humorous voices and accents successfully conveyed the game's light-hearted tone, though Boulding and Urbanek noted that the game's small pool of actors was evident, and the commentary from pedestrians was derided as repetitive. Marriott and Gerstmann were more negative, describing the voice-overs as grating and detrimental to the game's attempts at humor.

The humor was generally dismissed as juvenile and off-putting, though Williams and Casamassina found some of the jokes effective. Helgeson remarked that the content was "nothing that would raise an eyebrow on HBO", and concluded that those who were old enough to purchase the title would be too old to be shocked by it. Urbanek also saw irony in the demographic that he believed would find the game funny being too young to purchase it. Nation elaborated that the game's jokes, on top of being tired, were lacking in context and delivery, and he concurred that the game's lack of sophistication in its humor or sexual content, combined with the unremarkable gameplay, left its audience unclear. Marriott deemed the concept of a Mature-rated sports title nonsensical, and found the game's presentation of a seedy urban tone unpleasant, proclaiming that "being bombarded with obnoxious vendors and annoying pedestrians everywhere you turn should be considered a form of mental anguish". Nation, Zuniga and Urbanek accused the game's content of misogyny and racism, with Nation pondering "Why objectify women and stereotype several races and social classes so callously, taking a ton of criticism in the process, and then settle for such a poor excuse for a pay-off on all levels?".

## Legacy

In February 2003, Mirra filed a \$20 million lawsuit against Acclaim claiming that BMX XXX damaged his image. He explained that Acclaim allegedly used his name and likeness to promote BMX XXX after both parties agreed to disassociate his name from the product. The suit comprised a total of 11 claims, including unfair competition and injury to business reputation and dilution. Acclaim's public relations director Alan Lewis denounced the lawsuit as baseless and declared that Acclaim would fight vigorously against it. In the following month, two additional lawsuits were filed by Acclaim's shareholders alleging that the company's management misled it and the public on five accounts of misinformation relevant to the company's operations, including inadequate disclosure of the company's plans to publish mature-themed games. The suits claimed that titles such as BMX XXX "materially impeded the company's ability to access broad-based retail channels" and damaged revenue projections. On October 27, 2003, Acclaim announced that Mirra's suit had been settled with no monetary or other damages being paid by either side, and that Mirra's licensing agreement would continue until 2011. Additionally, Acclaim confirmed the development of a new Dave Mirra BMX game for next-generation systems.

The game's limited distribution and loss of celebrity endorsement resulted in BMX XXX becoming Acclaim's lowest-selling BMX title to date. According to Egan, the game sold a little over 160,000 copies by December 2005, making slightly under \$5 million in its lifetime. The financial failure and lack of mass market appeal of BMX XXX among other titles was cited as a factor in Acclaim's 2004 bankruptcy and liquidation. Rosen left Acclaim soon after the game's release and eventually abandoned the video game industry, having established a koi pond business by 2017.

GameSpy included the game's conception and Acclaim's violation of its agreement not to use Mirra's name and likeness to promote the game in its list of "25 Dumbest Moments in Gaming". Mike Williams of USgamer, within his list of "10 Games That Killed a Franchise", deemed BMX XXX and Turok: Evolution to be the "two nails in Acclaim's coffin". In 2015, the game was among several titles banned from streaming by Twitch. Todd Ciolek of IGN, in a retrospective feature covering major game publisher blunders, described BMX XXX as Acclaim's "last cry for attention" in a series of desperate publicity stunts by the financially ailing company.

2002 sports video game

Multiplayer and single-player video games

Video games developed in the United States


Operation Obviate

Operation Obviate was an unsuccessful British air raid of World War II which targeted the German battleship Tirpitz. It was conducted by Royal Air Force heavy bombers on 29 October 1944, and sought to destroy the damaged battleship after she moved to a new anchorage near Tromsø in northern Norway.

The attack followed up the successful Operation Paravane on 15 September 1944, when Tirpitz was crippled by British heavy bombers. As Allied intelligence was unaware that the battleship could no longer operate at sea and warships needed elsewhere were being retained in British waters to counter her, it was decided to make another attack. After a period of planning and preparations, 38 British bombers and a film aircraft departed from bases in northern Scotland during the early hours of 29 October. The attack took place that morning, but was frustrated by clouds over the Tromsø area which made it difficult for the Allied airmen to accurately target Tirpitz. The battleship was not directly hit, but was damaged by a bomb that exploded near her hull. A British bomber made a crash landing in Sweden after being hit by German anti-aircraft fire, and several others were damaged.

The Allies remained committed to sinking Tirpitz after the failure of Operation Obviate. The plans for the attack were reused for the next raid on the battleship, Operation Catechism, which took place on 12 November 1944. Weather conditions were favourable for the attackers, and Tirpitz was sunk with heavy loss of life.

## Background

From early 1942, the German battleship Tirpitz posed a significant threat to the Allied convoys transporting supplies through the Norwegian Sea to the Soviet Union. Stationed in fjords on the Norwegian coast, the ship was capable of overwhelming the close-escort forces assigned to the Arctic convoys. Tirpitz could also potentially attempt to enter the North Atlantic to attack Allied convoys travelling to the United Kingdom, as her sister ship Bismarck had sought to do in May 1941. To counter these threats, the Allies needed to keep a powerful force of warships with the British Home Fleet, and capital ships accompanied most convoys part of the way to the Soviet Union.

Tirpitz was repeatedly attacked by British forces. Royal Air Force (RAF) heavy bombers made four unsuccessful raids on the battleship between January and April 1942 while she was stationed at Fættenfjord. From March 1943, Tirpitz was based at Kaafjord in the far north of Norway. During Operation Source on 22 September 1943, she was severely damaged by explosives placed beneath her hull by Royal Navy personnel who had used midget submarines to penetrate Kaafjord. On 3 April 1944, aircraft flying from Royal Navy aircraft carriers attacked Tirpitz during Operation Tungsten and inflicted further damage. This attack had been timed for when it was believed repairs to rectify the damage caused by Operation Source were nearing completion. A series of subsequent aircraft carrier attacks, including Operation Mascot on 17 July and Operation Goodwood between 22 and 29 August, were unsuccessful.

As it was believed that further aircraft carrier raids would be fruitless due to shortcomings with the Royal Navy's aircraft and their armament, responsibility for sinking Tirpitz was transferred to the RAF's Bomber Command. On 15 September 1944, the elite Nos. 9 and 617 Squadrons attacked the battleship at Kaafjord during what was designated Operation Paravane. This operation employed Avro Lancaster heavy bombers armed with Tallboy bombs and "Johnnie Walker" mines, and was mounted from Yagodnik in the Soviet Union. Tirpitz was struck by a single Tallboy, which caused extensive damage to her bow and rendered her unfit for combat.

## Prelude

A meeting involving Grand Admiral Karl Dönitz, the commander of the German Navy, was held in Berlin on 23 September to discuss Tirpitz. Dönitz was informed that it would take nine months to repair the ship, and that all the work had to be done at Kaafjord as the battleship would be extremely vulnerable if she tried to sail to a major port. As Soviet forces were rapidly advancing towards northern Norway at that time, Dönitz judged that it was not feasible to either return the ship to ocean-going service or retain her at Kaafjord. Instead, he decided to use Tirpitz as a floating artillery battery to defend Tromsø against amphibious landings and bolster a defensive line which was being prepared in the Lyngenfjord area. Dönitz also expressed hope that retaining the ship in commission would "continue to tie down enemy forces and by her presence ... confound the enemies' intentions".

The commander of the German Navy's task force in northern Norway, Konteradmiral Rudolf Peters, was directed to position Tirpitz at a location near Tromsø where the water was shallow enough to prevent the battleship from sinking completely if she suffered further damage. An anchorage was selected just off the coast of the small island of Håkøya, 3.5 miles (5.6 km) west of Tromsø. This location lacked the natural defences Tirpitz had enjoyed at her previous Norwegian bases where she had sheltered in fjords which had steep mountains rising from the sea; this had made it difficult for attacking aircraft to spot and target the battleship. Instead, the terrain around Håkøya was fairly flat, and it was near the sea. To prepare Tirpitz for the 170-mile (270 km) voyage south-west, a repair ship was sent to Kaafjord and helped the battleship's crew to weld steel plates over the hole in her hull.

The Allies were able to confirm that Tirpitz had been badly damaged during Operation Paravane from intelligence gained by photo reconnaissance, signals intelligence and Norwegian agents, but were unsure if this had permanently put her out of service. The decision made by Dönitz on 23 September was also not known. As a result, the Royal Navy continued to assign capital ships to the Home Fleet to guard against the prospect of Tirpitz putting to sea, despite the need to redeploy these ships to the Pacific to reinforce attacks on Japanese forces.

Tirpitz's voyage to Tromsø took place during 15 and 16 October. The battleship departed Kaafjord at noon local time on 15 October under the escort of several warships. While Tirpitz was able to move under her own power, the flotilla included ocean-going tugboats tasked with towing the battleship if her damaged bow broke off. The German force slowly proceeded south, and Tirpitz eventually arrived at her berth off Håkøya at 3 pm on 16 October. Soon after reaching Håkøya 600 sailors, mostly members of her engine room crew, were removed from the ship. This left about 1,700 sailors on board.

The Allies rapidly responded to Tirpitz's redeployment. Norwegian Secret Intelligence Service (SIS) agents in the Kaafjord and Tromsø areas provided reports during the battleship's journey, Egil Lindberg radioing the United Kingdom on 16 October to confirm the ship's arrival at Tromsø. In response to these reports, the British aircraft carrier HMS Implacable was dispatched from the Home Fleet's main base at Scapa Flow on 16 October tasked with confirming the location of Tirpitz. The RAF was also instructed to fly photo reconnaissance sorties over the Tromsø area. As a precaution in case Tirpitz was able to conduct combat operations, the battleship HMS King George V was diverted from an impending deployment to the Indian Ocean to reinforce the Home Fleet until its sole battleship, Duke of York, completed repairs.

British reconnaissance aircraft located Tirpitz during the afternoon of 18 October. The first aircraft to arrive over the area was a de Havilland Mosquito from No. 540 Squadron RAF operating out of RAF Dyce in Scotland. The Mosquito's crew took photographs of the battleship from a high altitude, and returned to base despite damage by anti-aircraft guns. Shortly afterwards Implacable's Fairey Firefly aircraft reconnoitred the Tromsø area, several taking low-altitude photographs of Tirpitz off Håkøya; these aircraft were also fired on by the German anti-aircraft guns but none were damaged. During the evening of 18 October, Implacable's commander sought permission to attack Tirpitz the next day, but this was refused by the commander of the Home Fleet, Admiral Bruce Fraser, on the grounds that the carrier had sailed without any of the Supermarine Seafire aircraft needed to suppress the anti-aircraft guns. Fraser was also aware from the attacks on Tirpitz at Kaafjord that the carrier's two squadrons of Fairey Barracuda dive bombers would probably not be able to inflict significant damage, and that further raids by RAF heavy bombers were required.

## Preparations

### British

The RAF began preparations to attack Tirpitz again immediately after she was confirmed to be at Tromsø. As the Tromsø area was within range of Lancasters flying from northern Scotland if they were fitted with extra fuel tanks and other modifications, this operation would be simpler to conduct than Operation Paravane. Nevertheless, it required a lengthy return flight of 2,252 miles (3,624 km).

Nos. 9 and 617 Squadrons' Lancasters were modified to extend their range. All of the aircraft selected for the operation were fitted with powerful Rolls-Royce Merlin 24 engines, which were hurriedly obtained from maintenance units and airfields across the UK. The Lancasters also carried two extra fuel tanks inside their fuselage: a tank usually fitted to Vickers Wellington bombers and a type of drop tank used by Mosquitos. The extra fuel put the aircraft greatly above their authorised maximum take-off weight. To address this, the bombers' forward and mid-upper gun turrets were removed, along with 3,000 rounds of ammunition from the rear turret, the flare chute, the armour plating around the pilot's seat and some of the oxygen and nitrogen bottles. The reduction in armament left the Lancasters very vulnerable to German fighter aircraft, and they would have to fly without escort as no British fighters had sufficient range to reach Tromsø.

The operational order for the attack on Tirpitz was issued by No. 5 Group on 24 October. It specified that the battleship was to be attacked by 36 Lancasters, Nos. 9 and 617 Squadrons each contributing 18. Another Lancaster from No. 463 Squadron RAAF's film unit would also take part but not carry any bombs. As the "Johnnie Walker" mines had failed during Operation Paravane, only Tallboy bombs were to be used, each bomber carrying one of the weapons. These 12,000-pound (5,400 kg) bombs were the largest in service with the RAF, and were capable of penetrating heavily armoured targets. The two squadrons were to remain at their home bases until it was judged that weather conditions over Tromsø were likely to be suitable. They would then be bombed up, and fly to RAF Kinloss, RAF Lossiemouth and RAF Milltown in northern Scotland. The operational order stated that the attack was required as "it appears likely that the Germans may attempt to get the battleship back to a base in Germany, where the necessary repairs and refit can be carried out".

After the decision was made to launch the attack, the bombers were to fly individually across the Norwegian Sea, and cross the Norwegian coast at a point between Mosjøen and Namos where No. 100 Group RAF had found a gap in German radar coverage. The Lancasters were to rendezvous over Torneträsk lake in northern Sweden; this flight path involved violating Sweden's neutrality, but was selected as it would allow the bombers to approach Tromsø from the south-east, which it was believed the Germans would not expect. After the rendezvous was completed and if weather conditions remained suitable, the bombers would proceed to Tromsø and attack Tirpitz if she could be visually spotted or, if obscured, her location confirmed relative to unobstructed landmarks. If these criteria were not met, the aircrew were not to bomb. Following the attack on Tirpitz, the bombers were to fly directly back to Scotland.

Group Captain Colin McMullen was selected to command the ground crew and aircrew ahead of the start of the attack, a role he had also played in Operation Paravane. The commanders of the two squadrons, Wing Commanders James Bazin (No. 9 Squadron) and "Willie" Tait (No. 617 Squadron), would control their units in the air. The attack was designated Operation Obviate.

Preparations were also made to recover aircraft or crews if any of the bombers ran short of fuel or suffered battle damage. RAF Sumburgh in Shetland was selected as the emergency airfield for the return journey from Tromsø. If any of the bombers experienced engine problems or lacked sufficient fuel to return to the UK, they were to proceed to the Soviet airfields at Vaenga or Yagodnik. The Soviet Government was not informed of this until 29 October, the day of the attack. No. 5 Group also requested that three Royal Navy destroyers be stationed along the return route from Tromsø to rescue the crews of any bombers forced down over the Norwegian Sea.

The British were able to draw on two intelligence sources other than photo reconnaissance flights to monitor the German forces at Tromsø. Lindberg was based there, and provided updates on Tirpitz by radio. As Lindberg worked in the local meteorological office, he also regularly reported on weather conditions. The other source of intelligence was German radio traffic decrypted by Allied codebreakers.

### German

Tirpitz was particularly vulnerable to attack in the period immediately after she arrived at Håkøya. None of the many smoke generators and anti-aircraft guns, which had protected her against air attack at Kaafjord, were initially available because they had not yet been shipped south. The only protection available was from the battleship's own armament, two flak ships anchored nearby, and several anti-aircraft batteries in the Tromsø area. British intelligence believed that there were 16 heavy and 16 light anti-aircraft guns in the area at the time of Operation Obviate. The battleship was also surrounded by torpedo nets. No fighter aircraft were stationed nearby. The depth of water below Tirpitz at her mooring was greater than anticipated, leaving the ship vulnerable to capsizing. Owing to the space needed by the torpedo nets, it was not possible to move Tirpitz closer to shore. Instead, work began on building up the seabed using earth and gravel two weeks after she reached Håkøya.

The battleship's crew expected further air attacks, and doubted that she would survive them. This, and a belief that Germany would lose the war, led to poor morale. The civilian population of Tromsø also expected air attacks after Tirpitz arrived, and were concerned about the prospect of being accidentally bombed.

## Attack

### Departure

On 26 October, Bomber Command advised the Admiralty that Operation Obviate would commence as soon as weather conditions permitted after the night of 27 October. No. 5 Group also informed the two squadrons that day to make final preparations for the mission. This included loading the Tallboy bombs. In the evening of 27 October, the aircrew selected for Operation Obviate were briefed on the plan and told they would proceed to the Scottish airfields the next morning.

During the morning of 28 October, 20 Lancasters from each of the squadrons flew from their home bases to Kinloss, Lossiemouth and Milltown. A photo reconnaissance Mosquito flew over Tromsø that morning, and confirmed that Tirpitz was still moored off Håkøya and that weather conditions remained favourable for an attack. As forecasts for the next day indicated that good weather would continue, the attack was set for 29 October. Another Mosquito flew over the Tromsø area at midnight on 28 October, and reported that conditions remained clear.

The strike force departed Scotland in the early hours of 29 October. No. 9 Squadron dispatched 20 Lancasters, the aircraft taking off between 1:18 and 2:55 am BST. No. 617 Squadron contributed 19 aircraft, which departed between 1:03 and 2:10 am BST. The No. 463 Squadron Lancaster accompanied the attack aircraft.

### Over Tromsø

The approach flight was uneventful. The aircraft flew individually across the Norwegian Sea at an altitude of 1,500 feet (460 m), and began climbing to 10,000 feet (3,000 m) after they crossed the Norwegian coast. One of the No. 9 Squadron aircraft experienced engine problems during the climb, and returned to the UK. The bombers met over Torneträsk lake, formed up into their attack formations and proceeded to Tromsø. During this flight the Lancasters climbed to their bombing altitudes of between 13,000 and 16,000 feet (4,000 and 4,900 m). The attack force was fired on by Swedish anti-aircraft guns when it passed near Abisko, but no hits were recorded.

The attack on Tirpitz was frustrated by cloud cover. While the weather remained fine during the approach to Tromsø, the surrounding area was mainly covered by cloud. Tirpitz was visible when the bombers first arrived over the Tromsø area, but was obscured before any of them were in position to release their bombs. Despite the orders to bring their Tallboys back if visual bombing was not possible, almost all the bombers attacked; historian Patrick Bishop has written that this was due to an unwillingness to make the long return flight carrying a 12,000-pound (5,400 kg) bomb.

The first bombs were dropped at 7:49 am GMT, No. 617 Squadron leading the attack. Sixteen aircraft from the squadron released Tallboys aimed at Tirpitz's estimated position, several making multiple bomb runs before attacking. One of the three Lancasters which did not bomb made four runs over Tromsø before Tait gave its pilot permission to break off the attack. No. 9 Squadron began its attack six minutes after its sister squadron, 17 Lancasters dropping Tallboys. Like No. 617 Squadron, several aircraft from No. 9 Squadron made multiple bomb runs over Tromsø, one conducting five approaches before attacking. At least two of the No. 9 Squadron crews were able to take visual aim at Tirpitz through gaps in the cloud; the others aimed at the battleship's estimated position. Of the two No. 9 Squadron aircraft which did not bomb, one made two passes over Tromsø. The final bomb was dropped at 8:07 am GMT.

The German defenders began firing on the British aircraft as they approached Tirpitz. Four No. 9 Squadron Lancasters, at least one from No. 617 Squadron, and the No. 463 Squadron film aircraft, were damaged by anti-aircraft fire. The No. 617 Squadron Lancaster lost so much fuel from two hits that its pilot judged that the aircraft would not be able to reach RAF Sumburgh or the USSR. Instead, he decided to put down in northern Sweden so that the crew could avoid being made prisoners of war. The aircraft made a crash landing in a bog near Porjus. All of the crew survived, and were eventually repatriated to the UK by the Swedish Government. The damage inflicted on the other Lancasters was not significant.

None of the Tallboys struck Tirpitz. Several landed in the water near her. The explosion of one of these bombs damaged the battleship's port-side propeller shaft and rudder, and caused flooding. Three of her crew were injured. The explosions from the huge bombs were felt by civilians in Tromsø.

## Aftermath

Most of the Lancasters' return flights were uneventful, all returning to the UK after completing flights of an average duration of 13 hours. A No. 617 Squadron aircraft made an emergency landing at RAF Sumburgh after running short of fuel; this was one of the Lancasters that had not released its Tallboy. The damaged No. 463 Squadron aircraft successfully landed on one wheel at RAF Waddington. The airmen were aware that Tirpitz had not been sunk, and were disappointed with the operation's results. No. 5 Group's commander, Air Commodore Ralph Cochrane, sent them a message stating "Congratulations on your splendid flight and perseverance. Luck won't always favour the Tirpitz. One day you'll get her."

Allied intelligence soon learned that Tirpitz had been only slightly damaged. A Mosquito conducted a photo reconnaissance flight over the Tromsø area at 12:10 pm GMT on 29 October. Its photos showed no visible damage to the battleship. German post-battle reports broadcast by radio which were intercepted and decoded confirmed that damage was limited to the propeller shaft and rudder. Lindberg recommended that the RAF "give her another salvo" in one of his reports.

The Germans believed that several Lancasters had been shot down in the attack. Tirpitz's crew attributed the failure of Operation Obviate to their ship's gunnery, leading to improved morale. They expected that further attacks would be made, and were frustrated that Luftwaffe fighters had not been available to protect the battleship. A force of 38 fighters was transferred to Bardufoss after Operation Obviate to bolster the region's air defences.

The British remained determined to sink Tirpitz as soon as possible. Shortly after the 29 October raid, it was decided to use the same plans as had been employed in Operation Obviate in the next attack, which was designated Operation Catechism. On 12 November, Nos. 9 and 617 Squadrons set out from northern Scotland again. The weather over the Tromsø area was clear when they arrived, and Tirpitz was hit by two Tallboy bombs. The damage from these bombs and several near misses caused the battleship to capsize. Between 940 and 1,204 of her crew were killed.

The Lancaster which crash-landed near Porjus, "Easy Elsie", remains in-situ. The aircraft's engines and tyres were removed and sold locally soon after it crashed. The airframe was dismantled by a scrapper in the 1960s or 1970s, but abandoned as it proved too difficult to move from the site. The tail section of the aircraft was recovered in 1984 by the Swedish Air Force on behalf of the Swedish Air Force Museum, and the crash site was opened up to visitors in the early 1990s. Proposals to return the rest of the wreckage to the UK have not been successful.

Unsuccessful British air raid in World War II

Aerial operations and battles of World War II involving Germany

Aerial operations and battles of World War II involving the United Kingdom

Germany–United Kingdom military relations

Military operations directly affecting Sweden during World War II

World War II aerial operations and battles of the Western European Theatre