'Lego passengers' on board as Juno probe gets dangerously close to Jupiter

Scientists are preparing for a bumpy ride as they send a spacecraft perilously close to Jupiter, the Solar System's largest planet.

The Juno probe is due to reach the gas giant on July 4 after a five-year, 1.4 billion-mile journey from Earth.

It will enter a long polar orbit flying to within 2,900 miles (4,667 km) of the planet's swirling cloud tops.

No previous spacecraft has orbited so close to Jupiter, although two others have been sent plunging to their destruction through its atmosphere.

To complete its risky mission Juno will have to survive a circuit-frying radiation storm generated by Jupiter's powerful magnetic field.

The maelstrom of high energy particles travelling at nearly the speed of light is the harshest radiation environment in the Solar System.

To cope with the conditions, Juno is protected with special radiation-hardened wiring and sensor shielding.

Its all-important "brain" - the spacecraft's flight computer - is housed in an armoured vault made of titanium and weighing almost 400 pounds (172kg).

Dr Scott Bolton, Juno's principal investigator from Southwest Research Institute in San Antonio, US, said: "We are not looking for trouble, we are looking for data.

"Problem is, at Jupiter, looking for the kind of data Juno is looking for, you have to go in the kind of neighbourhoods where you could find trouble pretty quick."

Juno will study Jupiter's composition, gravitational and magnetic field, and search for clues about the planet's formation and the source of its raging winds, which can reach speeds of 384 mph (618 kph).

It will also deliver stunning colour photos via its JunoCam camera, which has a wide field of view geared for panoramic images.

To provide electrical power, the spacecraft carries three huge solar panels, each 29ft long and almost 9ft wide.

The previous record for a close approach to Jupiter was set by the American space agency Nasa's Pioneer 11 spacecraft which passed by the planet at a distance of 27,000 miles (43,000 km) in 1974.

Only one previous spacecraft, Galileo, which visited Jupiter and its moons from 1995 to 2003, has orbited the planet.

Galileo made wide orbits at distances of hundreds of thousands of kilometres that kept it out of serious danger from the radiation, although it suffered a number of technical "anomalies".

The spacecraft sent a small probe on a one-way trip through the clouds of Jupiter, and was eventually itself crashed onto the planet at the end of its mission.

As a further safeguard, Juno is programmed to follow a long orbital path that avoids Jupiter's radiation belts as much as possible.

Despite these measures, the probe is not expected to last much longer than its planned lifespan of 20 months.

Chief radiation monitoring investigator Heidi Becker, from Nasa's Jet Propulsion Laboratory, said: "Over the course of the mission, the highest energy electrons will penetrate the vault, creating a spray of secondary photons and particles.

"The constant bombardment will break the atomic bonds in Juno's electronics."

Unusually for a robotic space mission, Juno is carrying passengers - three Lego figures depicting the 17th century Italian astronomer Galileo Galilei, the Roman god Jupiter, and the deity's wife Juno.

Lego made the figures out of aluminium rather than the usual plastic so they could withstand the extreme conditions of space flight.

A plaque dedicated to Galileo and provided by the Italian Space Agency is also on board.

Measuring 2.8 ins (7.1 cm) across, it shows a portrait of Galileo and a text penned by the astronomer in January 1610 while observing Jupiter's four largest moons - later to be known as the Galilean moons.

Juno was launched into space by an Atlas V rocket from Cape Canaveral, Florida, on August 5, 2011.

The mission is part of Nasa's New Frontiers programme of robotic space missions which last year saw the New Horizons spacecraft obtain close up views of dwarf planet Pluto.