The sky — astronomy

Jupiter: The Giant of the Solar System

Jupiter is the fifth planet from the Sun and the largest in the Solar System, a gas giant made mostly of hydrogen and helium. It is known for the Great Red Spot and for four large moons discovered by Galileo in 1610, which you can see even with ordinary binoculars.

Jupiter is the largest planet in the Solar System and one of the brightest objects in the night sky. It is not a star but an enormous world of gas, orbited by dozens of moons. Whether you are curious about the science or hoping to see it yourself, here are the essentials.

The gas giant

Jupiter is the fifth planet from the Sun. It orbits at about 5.2 astronomical units (one astronomical unit is the average distance between Earth and the Sun) and takes almost 12 Earth years to complete one lap. Its diameter is about 11 times that of Earth, and its mass is more than twice the combined mass of all the other planets.

The planet is made mainly of hydrogen and helium, which is why it is called a gas giant. It has no solid surface like Earth: deeper inside, pressure and temperature rise and the gases gradually become denser. Despite its size, Jupiter spins very fast, with a day of just under 10 hours. That rapid rotation stretches the clouds into parallel stripes, the lighter zones and darker belts you see in photographs.

Jupiter also has faint rings, but they are much thinner and darker than those of Saturn and are out of reach of amateur instruments.

The Great Red Spot

The Great Red Spot is a giant storm in Jupiter's atmosphere, an anticyclone larger than the whole Earth. Astronomers have tracked it since at least the 19th century. Observations from telescopes and spacecraft show that it has been shrinking and becoming rounder over time, but the storm is still going.

Its color comes from chemical compounds in the clouds, though their exact makeup is still being studied. The shade also varies, sometimes a deeper orange-red and sometimes paler. Binoculars will not show it; you need a telescope and steady air.

The Galilean moons

In January 1610, Galileo Galilei pointed an early telescope at Jupiter and noticed small points of light beside the planet that changed position from night to night. He had found the four largest moons, later called the Galilean moons: Io, Europa, Ganymede and Callisto. It was a landmark moment in science, because it showed that not everything in the sky circles the Earth.

• Io is the most volcanically active body in the Solar System.
• Europa is thought to hide an ocean of liquid water beneath its icy crust, making it one of the most intriguing places in the search for habitable conditions.
• Ganymede is the largest moon in the Solar System, bigger even than the planet Mercury, and the only moon known to have its own magnetic field.
• Callisto has an ancient, heavily cratered icy surface.

Jupiter has more than 90 known moons in total, but most are small, irregular bodies captured by its gravity.

Jupiter through binoculars

Jupiter is an easy target. Near opposition (when Earth lies between Jupiter and the Sun), it outshines every star and shines with a steady, non-twinkling light. Oppositions come around roughly every thirteen months.

To see the moons you will need:

1. Binoculars with about 7× to 10× magnification and 50 mm lenses (for example 10×50). For help choosing, read binoculars or telescope.
2. A steady support: rest your elbows on a railing or use a tripod. Any shake smears the tiny dots.
3. A dark spot and a little patience while your eyes adapt.

The moons look like small points of light lined up almost in a straight row on either side of the planet, because their orbits lie close to one plane. Io circles Jupiter in about 1.8 days and Callisto in about 16.7 days, so the pattern changes from night to night. Sometimes a moon is hidden behind or in front of the planet, and you will count fewer than four. The cloud bands need at least a small telescope. You can also watch for Jupiter's meetings with other worlds in a planet parade.

Missions to Jupiter

Pioneer 10 made the first flyby in 1973, followed by the two Voyager probes in 1979. NASA's Galileo became the first spacecraft to orbit the planet, from 1995 to 2003.

Today the key mission is NASA's Juno. Launched in 2011, it reached Jupiter in July 2016 and flies a long, polar orbit. Its data help scientists understand the planet's interior, magnetic field, clouds and auroras.

The European Space Agency (ESA) launched JUICE (Jupiter Icy Moons Explorer) in 2023. It is headed for Ganymede, Callisto and Europa, and is planned to arrive in the Jupiter system in 2031. NASA also launched Europa Clipper in 2024, a mission dedicated to studying Europa.

Astronomy and astrology

Everything above is scientific fact. In the astrological tradition, Jupiter carries its own symbolic meaning linked to growth and generosity, but that is a cultural interpretation, not a scientific claim. If you are curious, see Jupiter in astrology.

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