Juno To Make Fewer Orbits Than Planned

Originally, Juno was to make 37 orbits around Jupiter before the planned end in February. After the first two orbits, the craft was to change course and get closer to Jupiter. But the plans were changed, because two satellites, which had the same engine a

The Juno spacecraft had been five years underway when, in the summer of 2016, after a 2.8-bn-km space journey, it approached Jupiter at a speed of 250,000 km/h. Here, the spacecraft entered an elliptic course, which sent it across the gas giant only 4,200 km above the white and red-brown stripes of cloud. On its way, the craft crossed the fierce radiation belt, while astronomers followed its journey holding their breaths. The radiation is 10,000 times stronger than on Earth, and despite the extensive safety measures, it can easily destroy the instruments in the craft.

Launched from Cape Canaveral in August 2011, the Juno spacecraft is part of the American space agency, NASA’s, prestige programme, New Frontiers. As the first craft ever, Juno will take a peek below the thick clouds that surround Jupiter and reveal the composition of the giant’s interior.

The results have altered our knowledge about the largest planet in the Solar system completely, as neither the exterior nor the interior is what the astronomers expected. JUNO USES SOLAR POWER IN THE DARK Jupiter is an old acquaintance, discovered already 3,000 years ago by ancient astronomers, who saw the gas giant with the naked eye. In recent decades, five spacecraft have given more detailed pictures of the cloudcovered surface of the gas planet. But no one has come as close as Juno.

NASA engineers worked hard for ten years to develop Juno and ended up creating the fastest spacecraft in the history of space travel. The main engine in the craft is a rocket engine, which is only ignited at critical times to give the craft more speed or apply the brakes. Travelling the long distances, Juno is powered by three solar cell panels. They each measure nine metres in length and contain a total of 18,698 solar cells. Al-though the cells deliver a mere 400 watts – which is what an ordinary handheld mixer uses – when Juno reaches Jupiter, it is actually a major sensation, as the craft is the first one to be run by solar cells in the complete darkness of the outer solar system.

Also, the craft is full of scientific equipment, which has been tailored to reveal the true colours of the gas giant. A radiometer can see several hundred kilometres beneath the top of the clouds by measuring microwaves, which penetrate the clouds from beneath. A gravimeter looks deep into the gas planet to determine its inner composition. A magnetometer measures the strength and direction of Jupiter’s extremely strong magnetic field, and three instruments examine Jupiter’s northern light. As part of every orbit, a wide-angle camera takes colour photos of the whirling clouds at the top of the atmosphere and of the poles. GIANT CORE DISSOLVING Jupiter is by far the largest planet in the Solar system and takes up as much room as 1,321 Earths. The astronomers know that the atmosphere reaches some 1,000 km beneath the top of the clouds. Underneath lies first a dense layer of electrically charged hydrogen gas and after that an ocean of liquid hydrogen. Further in, the temperature is so high and the pressure so great that, according to the astronomers, the hydrogen has been converted to liquid metal all the way to the core.

The core itself, however, has been the subject of fierce scientific debate in recent years. Classic theory has it that the giant has a small core of iron and stone, not much bigger than Earth. An alternative model says that the gas giant does not have a core at all, as the heavy elements have been dissolved in liquid metallic hydrogen due to the extreme pressure and temperatures at the core of the planet. But Juno has revealed something completely different.

The mass inside the gas giant is unevenly distributed, and this affects the planet’s

violent gravitational field. The minor variations in the gravitational field affect Juno’s path, making the craft wobble slightly. Based on the movements, the astronomers have calculated the composition of the planet and have found that the core is the size of up to 25 masses of earth. The core, however, is not well defined like that of Earth, but frayed and maybe dissolving.

CLOUDS ARE RAINING AMMONIA

The core is far from the only surprise Juno has given. Before the craft left Earth, the astronomers believed that the atmosphere would contain lots of water, because Jupiter, due to its huge size, has attracted hosts of aqueous comets, which have subsequently been dissolved in the atmosphere.

Nevertheless, the measurements showed that even though the atmosphere contains large amounts of water, it contains at least ten times more ammonia. So the clouds are raining ammonia crystals, which evaporate when passing through the atmosphere, where the pressure and temperature increase. The observations also showed that the wind system on the giant is similar to that of Earth, where hot air rises at the Equator and blows out towards the poles. Similarly, dense ammonia vapours rise at Jupiter’s equator high up into the atmosphere and blow towards the poles.

JUNO BURNS UP IN THE ATMOSPHERE

Since its arrival in July 2016, the craft has travelled in a 53-day-long elliptic orbit. In October 2016, the large engine was to have been switched on for 34 seconds to change the

orbit to a circular path, which would bring the craft close to Jupiter at 14-day intervals. But in September 2016, the American military lost two satellites due to defective engines. As Juno has similar engines, NASA did not dare ignite the engine, as the craft was at risk of ending up in a useless orbit or directly in the gas planet. Instead, it continues in its oblong path, where the craft flies close to Jupiter every 53 days.

As a result, Juno will only manage 12 orbits, even though the mission has been extended until July. Unless NASA makes another extension, Juno will, at that time, be shot down t hrough Jupiter’s atmosphere like a projectile, where it will burn up after having delivered its final data about the giant of the Solar system. Once the data are home and dry, the scientists can finally go back to their work desks and find out how the giants of the solar system were born.