Juno to Jupiter

The main mission of the spacecraft Juno, which NASA launched in August 2011, is to unlock some of Jupiter's deepest mysteries.

Astronomers have fixed their eyes on Jupiter for millennia. The invention of the telescope inspired Galileo to observe the planet with the aid of a very small telescope he made himself and to discover its four large satellites. And then, in the course of time, very powerful ground-based telescopes, the Voyager probes, the Hubble Space Telescope and the Galileo spacecraft began to tell the story of Jupiter. Very fine images of Jupiter were received from the Cassini spacecraft as it passed the planet in 2000-01 en route to Saturn, but researchers contend they have barely scratched the surface of Jupiter.

The National Aeronautics and Space Administration (NASA) of the United States launched another spacecraft, Juno, on August 5, 2011, to peer deeper inside the planet's impenetrable atmosphere, map its gravity and magnetic fields, and reveal the gas giant's internal structure. The robotic satellite will also get a close-up view of Jupiter's poles for the first time. After a five-year voyage through the solar system, Juno will drop anchor in orbit around Jupiter on July 4, 2016. There it will weather Jupiter's intense radiation belts for at least a year on a scientific quest to address fundamental questions about the solar system's most massive planet. The ultimate objective of the $1.07 billion project, named Juno for the wife of Jupiter in Roman mythology, is to unravel how the planet formed at the dawn of the solar system 4.5 billion years ago. In mythology, the goddess Juno was able to see beyond her husband's steely and guarded exterior, revealing the king god's true self. Like its namesake, NASA's Juno mission will yield information on the planet that is hidden from plain view.

Fundamental properties

The planets beyond the asteroid belt Jupiter, Saturn, Uranus and Neptune are very different from the four terrestrial planets (Mercury, Venus, the earth and Mars). These giant planets, also called Jovian planets, are not only more massive but also less dense. These facts suggest that the internal structure of the giant planets is entirely different from that of the four terrestrial planets. Jupiter, the largest planet, dominates the sun's planetary system. Jupiter is 5 astronomical units (AU = 1.496 x 108 km) from the sun and takes 12 years to complete one revolution around the sun. By itself it contains two-thirds of the mass in the solar system outside the sun, 318 times as much mass as the earth. Jupiter is interesting because it is massive and very hot inside. Jupiter has at least 63 moons and so is a miniature planetary system in itself. It is often seen as a bright object in the night sky, and observations with even a small telescope reveal bands of clouds across its surface and four of its moons.

Jupiter is more than 11 times greater in diameter than the earth. From its volume and mass, its density has been calculated to be 1.3 grams per cubic centimetre, not much greater than the density of water, which is 1 gm/cc. The density tells one that any core of heavy elements (such as iron) that Jupiter may have takes up a smaller fraction of Jupiter's mass than the cores of the terrestrial planets make up of the planets' masses. Jupiter, rather, is composed mainly of the lighter elements, that is, hydrogen and helium. Jupiter's chemical composition is closer to that of the sun and stars than it is to that of the earth. In fact, 86.1 per cent of Jupiter's atmosphere is hydrogen and 13.8 per cent is helium so that those two gases comprise 99.9 per cent of the atmosphere. Jupiter is not solid; it has no crustal surface at all. At deeper and deeper levels, its gas just gets denser and denser, eventually liquefying. Because of Jupiter's average density, the planet must have heavier elements at its core, but little is known about them.

Jupiter rotates very rapidly, that is, once every 10 hours. Undoubtedly, this rapid spin rate is a major reason for the colourful bands of clouds that spread out parallel to its equator. The bands appear in subtle shades of orange, brown, grey, yellow, cream and light blue and are beautiful. The most prominent feature of Jupiter's visible cloud surface is a large reddish oval known as the Great Red Spot. It is about 13,000 km x 26,000 km, larger in area than the earth, and drifts about slowly with respect to the clouds as the planet rotates. The Great Red Spot is a relatively stable feature, for it has been visible for over 350 years. In 1955, intense bursts of radio radiation were discovered to be coming from Jupiter. This indicates that Jupiter has a stronger magnetic field and stronger radiation belts than the earth.

Spacecraft observations

The first close-up views of Jupiter came from the Pioneer 10 and Pioneer 11 spacecraft as they flew by in 1973 and 1974. A second revolution in the understanding of Jupiter occurred in 1979 when Voyager 1 and Voyager 2 also flew by it. The cameras aboard Voyager 1 discovered three ringlets around Jupiter. They are the darkest, simplest set of rings in the solar system. They consist of very fine dust particles that are continuously pushed out of orbit by the impact of radiation from Jupiter and the sun. When Voyager 1 glided past Jupiter in 1979, it was able to take a close-up photograph of the Great Red Spot. Voyager 2 had figured out that the Great Red Spot is an anticyclonic storm lying in the southern hemisphere of Jupiter, which means it has counterclockwise winds. Storms are called cyclones when their centres have low pressure and anticyclones when their centres have high pressure. There is still much to be learnt about the Great Red Spot. The Pioneer and Voyager spacecraft detected that Jupiter is surrounded by a vast sea of energetic charged particles, mostly electrons and protons, somewhat similar to the earth's Van Allen belts but much larger. The boundary of Jupiter's magnetic influence on the solar wind lies about three million kilometres from the planet. Direct spacecraft measurements show Jupiter's magnetosphere to be almost 30 million km across, roughly a million times more voluminous than the earth's magnetosphere and far larger than the entire sun.

Io, the densest of Jupiter's moons, is the most geologically active object in the entire solar system. An outstanding discovery was made when the Voyager spacecraft glided past Io: the moon has active volcanoes. Voyager 1 photographed eight erupting volcanoes, and six were still erupting when Voyager 2 passed by four months later.

Each spacecraft carried many types of instruments to measure the properties of Jupiter, its satellites and the space around them. The observations made with the imaging equipment were of the greatest popular interest. The resolution of the Voyager images were five times better than that of the best images obtained from the earth, and some of Galileo's images were another 70 times better.

Galileo's camera carried out the most exciting work in astronomy when the comet Shoemaker-Levy 9 crashed into Jupiter in July 1994. The results exceeded everyone's expectations. The spacecraft Cassini passed by Jupiter in 2000-01 en route to Saturn and its radar instruments captured microwaves from high-energy electrons moving in Jupiter's harshest radiation belt.

Juno mission

One of Juno's primary tasks is to help scientists get an insight into Jupiter's origin a crucial unsolved problem in planetary science. Because Jupiter is the largest planet in the solar system, understanding how it formed is crucial to knowing how the smaller planets such as the earth formed and will help scientist determine why there is no planet in the asteroid belt. Juno's detailed studies of Jupiter can also shed light on how gas giants form and influence the evolution of other planetary systems.

Juno is loaded with seven instruments to beam back data on particle fluxes, temperatures, light spectra and plasma waves. The instruments should be able to go beyond visible light and beyond human capability to be able to really see inside and understand what nature is doing. The beauty of technology and the advancement of the human species is that ways have been developed to measure things in nature that are beyond the five senses.

Juno is outfitted with a special colour camera to snap images of Jupiter's polar regions for the first time. The camera, called Junocam, was built by Malin Space Science Systems in California. Scientists say the imagery will be spectacular. Scientists will be able to measure how much water there is on Jupiter by listening to microwave signals from inside the planet and measuring its temperature very precisely at many different levels. Learning how much water and methane are inside Jupiter's atmosphere could lead to stronger theories on how such compounds happened to be there in higher ratios on the planets than in the sun.

The Juno spacecraft is itself another experiment. With three sweeping solar panels, each nearly 9 metres long, the probe will push the envelope of sun power into uncharted territory. Arranged 120 degrees apart like the rotors of a helicopter, the panels contain 18,600 solar cells to generate electricity. The collecting area is about 59 square metres. Half of Juno's electricity budget goes to the thermal system that keeps the spacecraft at a comfortable temperature. The balance goes towards communications, computers, propulsion and operating the probe's seven scientific instruments and camera.

Never before has NASA sent a spacecraft into such a treacherous environment. Juno will have to withstand radiation equivalent to more than 100 million dental X-rays, or roughly 100 times the radiation dose that becomes lethal to humans. Vital spacecraft components are protected in a washing-machine size box with inch-thick (1.27 cm) titanium walls. Several Juno instruments will study the energetic interactions that occur when charged particles caught in Jupiter's powerful magnetosphere slam into Jupiter's upper atmosphere, generating auroras that dwarf their terrestrial counterparts in scale and intensity. These results should help scientists better understand auroras on other planets as well.

When Juno arrives at the planet in July 2016, it will spend three months manoeuvring into its science orbit before starting to collect data in October. Juno's unique orbit will last for 11 days. It will fly within 5,000 km of Jupiter on each circuit of the planet. On each close approach, Juno will zip above the equator and over both the poles. The spacecraft will take advantage of the polar orbit to directly observe Jupiter's auroras.

On each of its elliptical orbits, Juno will spend the six hours nearest to Jupiter conducting non-stop observations, logging data with on-board recorders. Juno will use the time when it is farther away from the planet to transmit the data back to the earth. Juno's final moments will come sometime in late 2017, when the probe will have accomplished all its goals and the spacecraft will have been crippled by the ravages of radiation. NASA plans to guide the probe on a crash course into Jupiter's atmosphere. If all goes as planned, Juno will survive about 32 orbits. But during that relatively short period, it will unlock some of Jupiter's deepest mysteries.

Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.