Jupiter is the fifth planet in the solar system from the Sun, and by far the largest – in terms of mass, more than twice the size of all other planets combined. Viewed from Earth, the planet rivals or exceeds Mars in terms of brightness in the night sky (at least at those points when it is visible) and has fascinated astronomers and astrologers for over two thousand years.
– Composition –
Jupiter is 11 times as wide as Earth at its equator, a total of about 89,000 miles, with a mass of 318 Earths and a volume – the total space occupied – of 1320 Earths. At the centre of this enormous planet lies a rocky core which resembles the entire planet of Earth; however, this is surrounded by a massive, layered atmosphere, so large that its gravity crushes the lower levels of gas into a soupy matter which might – if we could ever reach it – resemble a liquid ocean.
The atmosphere, which is the only part of Jupiter visible from Earth, consists of about 89% hydrogen, 10% helium, and small amounts of trace chemicals, including water and organic compounds (that these have come together in Jupiter’s atmosphere created considerable speculation that primitive life forms could evolve there during the mid-20th century, but Jupiter’s gravity and radiation are both far higher than here on Earth, and would prevent recognizable life from evolving. (This does not, however, mean that some form of very different, primitive biological life could not possibly evolve there, assuming that current theories about abiogenesis and evolution are roughly accurate; it simply makes it unlikely.)
Viewed from the outside, Jupiter’s atmosphere is most notably characterized by horizontal bands of enormous storm systems. One such system is home to the Great Red Spot, a large storm occupying the same volume as several Earths which seems to be a permanent, if poorly understood, feature of Jupiter’s atmosphere. Other large storms also form and dissipate over time, but the Great Red Spot has been visible to human astronomers for as long as we have had the telescopes necessary to observe its location in the southern hemisphere.
– Orbit and Day –
The day on Jupiter lasts ten hours, less than half the length of a day on Earth. However, because it is much farther from the Sun, its orbit is substantially longer, lasting about 11.8 Earth-years. The average distance from the Sun is 483.7 million miles, or 5.2 astronomical units – that is, 5.2 times as far from the Sun as Earth is. Despite its distance, Jupiter is quite a hot planet because of the heat and radiation emitted from its core – which, according to astronomers’ estimates, actually exceeds the heat it receives from the Sun.
Jupiter’s orbit is located beyond the asteroid belt, but within that of Saturn. As such it occupies an important position in the solar system. Because of its large size, it tends to deflect the path of asteroids and comets that cross its orbit. This means that relatively few of them adopt the long-term, stable orbits that could eventually collide with Earth’s. Instead, Jupiter’s massive gravity pulls them far off course, sometimes sending them into the Sun or, alternatively, out of the solar system entirely. Earth and the other inner planets likely have fewer major impact events today in part because Jupiter has already deflected many of the bodies which would be responsible for those collisions.
– Moons and Rings –
Jupiter has a ring system, but, like those of Uranus and Neptune, it is relatively thin and insubstantial, unlike the massive and distinctive rings which orbit Saturn. As a result, scientists have been relatively less concerned with Jupiter’s rings. In part this is simply a reflection of the lack of data: the rings are so thin that they cannot be seen except by the most powerful telescopes on Earth today, though they were pictured easily by the Voyager probes and more recently by Galileo.
In addition, Jupiter has the largest number of known moons, including 63 named moons and a small number of objects believed to be moons but not yet properly characterized and, as such, not yet named. A small number of these moons are so-called regular satellites, with fixed elliptical orbits and stable properties. Most of the others are irregular, bodies either captured and pulled into orbit by Jupiter’s gravity or the remnants of old collisions between destroyed moons.
The largest four moons are Galileo, Europa, Callisto, and Io, which have been known since they were first spotted by Galileo Galilei in the 1600s. Europa is in many ways the most promising of the Jovian moons. Since the Pioneer and Voyager probes of the 1970s, scientists have known that Europa’s surface was ice, below which lies a massive water ocean. Science fiction writers and biologists alike have speculated that this ocean could be home to life, making it – alongside the planet Mars – one of the most likely candidates in the solar system for the evolution of life apart from that on Earth. A proposed joint European-American mission to Jupiter will examine the life potential of Europa in greater detail.
– Spacecraft Visits –
All of the probes that are sent to the solar system habitually swing by Jupiter, using its massive gravity as a slingshot to accelerate them on to more distant targets: for instance, Cassini used a gravity assist from Jupiter to reach Saturn, and New Horizons more recently used the same maneuver on its way to Pluto and the Kuiper Belt.
In addition, Jupiter was visited by the long-distance probes launched during the 1970s: Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2. All four of these imaged Jupiter as they flew past; the latter two, still operational, are now on their way out of the solar system entirely.
To date, however, only one spacecraft, Galileo, has actually orbited Jupiter. Galileo spent seven years in Jupiter orbit, between 1995 and 2003, during which it gathered data on Jupiter itself as well as Jupiter’s moons. In 2003, when its mission ended, scientists deliberately crashed the probe into Jupiter’s atmosphere, where it was crushed by the planet’s gravity and destroyed. This decision was made because several of Jupiter’s moons are plausible candidates for life (especially Europa), and leaving the probe in orbit meant that it might eventually crash into one of these moons, with unknown effects on any life which might exist there.
The next probe to visit Jupiter is Juno, which NASA will launch next year. It will reach Jupiter five years later and begin what is hoped to be a more advanced study of the planet that Galileo. It will be powered by solar powers rather than by a nuclear generator, hopefully giving it a much longer lifespan in orbit.
