NASA's Juno spacecraft is going closer to the massive and mysterious planet Jupiter than any spacecraft has ever gone. The ship has flown more than 1.7 billion miles over five years, and on Thursday Juno began executing the sequence of commands that will take it right to Jupiter's clouds. On July 4th, the engines will switch modes again, and the ship will begin burning fuel to slow it down enough so that Juno can be swept into Jupiter's orbit. If it succeeds, the $1.1 billion mission will give scientists the most complete data set so far of a planet little understood.

"Juno is very special because it is one of those rare missions where it is entirely focused on looking inside Jupiter," said Curt Niebur, lead program scientist for New Frontiers, which oversees the Juno mission, in an interview with CNBC. Being covered with, and largely composed of gaseous elements, Jupiter does not have the kind of same kind of rocky surface a planet like Mars or Earth has.

"With the instruments and techniques we are using, we can unzip the planet and really peer inside it and understand what its deepest internal structure is," he said.

A desktop model of the Juno spacecraft is seen as NASA officials and the public look forward to the Independence Day arrival of the the Juno spacecraft to Jupiter, at JPL on June 30, 2016 in Pasadena, California. David McNew | Getty Images

Jupiter is an "odd planet," Niebur said, and scientists have debated for a long time what it looks like beneath those clouds.

For starters, Jupiter is huge, and if it were a bit bigger, some say it would be a star. Take all the planets in the solar system and combine them. Jupiter is still twice the size of that. It is 11-times the size of earth in diameter. It takes 12 Earth-years to orbit the sun. Jupiter rotates much faster than our planet, though. It spins so quickly its day is a mere 10 hours. And it has four large moons, as well as several smaller ones.

Though it is covered in gases, there are theories it might have a solid core. "We have almost no data to back up those theories," Niebur said. Juno will be able to get some "solid concrete data" that will help scientists sift through the various theories.

Apart from that, "one of the most general questions is really getting after what was the formation process like in the early solar system for the planets," he added, which is "something, surprisingly, we don't have a really solid feeling on." Unlike the other planets, Jupiter is so massive it has been able to hang onto much of the original material from when the solar system formed. To learn about this, Juno will measure for water and the presence of certain compounds, like ammonia. Niebur finds three things incredibly fascinating about the planet. The first is its turbulent atmosphere of colored clouds. "It is a quite beautiful planet and it changes on an hourly basis," he said.

Jupiter is also largely made up of helium and hydrogen. Scientists think that the hydrogen changes as one goes deeper into the planet, the hydrogen condenses to the point where it "behaves like a metal" — something never seen before.

Jupiter Antonio M. Rosario | Getty Images