Independence Day on Jupiter: What are NASA's hopes for Juno?

There are only a couple million miles and a few days left until NASA’s space explorer Juno reaches its destination, Jupiter, on Independence Day.

After five years and more than a billion miles of travel through the deepest parts of our solar system, Juno will begin to look for answers to astronomy’s many questions when it arrives at the most massive object here besides the sun on Monday. These questions include how much water is on the giant planet, whether it has a dense core below its layers of gas, and why the giant hurricane-like storm that’s been raging on the planet for centuries is shrinking?

"In many ways, what we're after is the recipe for a solar system," said Scott Bolton, the mission's lead scientist, reported the Press Enterprise. Astronomers believe that Jupiter was the first planet to form in the solar system.

During its one-year mission, Juno will come closer to Jupiter than any other spacecraft before it. Jupiter has a circumference about 11 times the size of Earth’s, and, unlike our own rocky planet, is made up mostly of hydrogen and helium, which makes it a “gas giant.” Juno will be the first to peer through the planet’s dense clouds, flying within 3,100 miles of the planet.

Juno’s predecessors include the Voyagers, the Pioneers, Galileo, Ulysses, Cassini, and New Horizons spacecraft. But most of these flew by quickly on the way to other destinations. New Horizons reached Pluto last year, for example. Only Galileo – named for the Italian astronomer who discovered Jupiter's large moons – orbited the planet and even released a probe.

Named after the insightful wife of the Roman god Jupiter, Juno is traveling with nine instruments on board that will map Jupiter's interior and study its mysterious atmosphere. Three mini LEGO figures of Jupiter, Juno, and Galileo are also traveling on Juno, as well as a plaque inscribed with Galileo's writings, donated by the Italian Space Agency.

Juno, about the size of a sport utility vehicle, is the first spacecraft to venture this far into space using only solar power. Its three tractor-trailer-size sunlight-gathering wings resemble the blades of a windmill. Because the planet is so far from the sun, previous spacecraft relied on nuclear power to travel to its neighborhood. Jupiter is the fifth planet from sun, the next one out after Mars.

The spacecraft is now traveling toward Jupiter at more than 160,000 miles per hour, which is more than 200 times the speed of sound. It is expected to fire its braking rocket at 11:18 p.m. Eastern time on Monday in order to slow itself enough to be captured by Jupiter's gravity and be swooped it into its orbit.

Juno will get close enough to send back the most detailed look at the planet's polar regions, clouds, and auroras.

The spacecraft has a camera onboard – the JunoCam – that was turned off this week to prevent any interference during its critical arrival. This means that pictures of Juno’s swing into orbit about Jupiter will not be available. But plenty of others will start being released in late August. Throughout Juno’s mission, the public can vote on where to point its camera. People can also watch Juno's simulated arrival on July 4 in real-time through a downloadable app from NASA.

This report includes material from the Associated Press and Reuters.