Imagine a planet with more than 60 moons, with a metallic hydrogen subterranean ocean 25,000 kilometres deep, with no solid surface to walk on, with clouds made of ammonia, with massive storms lasting several hundred years and with windspeeds of 400 mph, and with a magnetic field 20,000 times as powerful as the Earth. Welcome to Jupiter, the new frontier of humankind’s exploration of the Solar System.
The last couple of decades have seen a flotilla of Missions to Mars — but with Juno, NASA begins a brand new trip into the mysteries of the Jovian system. Mars and Earth are similar in many aspects. They belong to the family of four planets from Mercury to Mars that are called Terrestrial Planets — thus, Mars and Earth are made of silicate rock and have broadly similar surface, interior and atmospheric processes. Hence, wind erosion, volcanic processes, the geochemical processes and the geophysics of the planetary interior on Earth and Mars bear a lot of similarities. In fact, before Mars lost its atmosphere and its water, Mars and Earth were even more similar — with flowing water from rivers, streams and catastrophic floods, and the subsequent landforms produced by water erosion.
With Jupiter, we are stepping out of our familiar surroundings of Terrestrial Planets to the relatively novel environment of the Gas Giants. Jupiter and Saturn are Gas Giants — formed beyond the frost line, where water and volatile gases can condense into solid grains and become part of planets.
In the early Solar System, planets were being formed from the gaseous nebula surrounding the Sun — planets that formed inwards of the frost line were in large measure composed of rock. Planets like Jupiter that were formed outwards relative to the frost line incorporated primarily gas and volatiles. Jupiter is formed mostly of Hydrogen and Helium, and unlike Earth and Mars, does not have a rocky surface. The planet consists of Hydrogen gas that becomes dense as we advance towards the planetary interior.
A massive ocean of metallic Hydrogen occupies the region above the core of Jupiter. Jupiter is about 300 times as massive as Earth. It has four large moons and more than 60 smaller moons. Ganymede, the largest satellite of Jupiter is also the largest moon in the Solar System. Interestingly, Ganymede is also larger in size than the planet Mercury and, therefore, could have been easily classified as a planet had it been orbiting the Sun rather than Jupiter!
Jupiter’s moons also have their own share of exciting possibilities. Europa and Ganymede, for example, seem to have massive underground salt water oceans beneath a surface layer of ice. Can the oceans on Europa and Ganymede harbour life? Both these oceans are subterranean — so they have presumably never seen sunlight. Would life survive in such an environment? Contrary to popular conception, oceans on Earth are bathed in darkness — except for the first 1000 metres of depth. Yet, in the aphotic zone of the Earth’s oceans, where light does not penetrate, we do find an amazing variety of life forms. So it is not entirely inconceivable to hypothesize that life may exist in the subterranean oceans of these Jovian moons.
There have so far been 7 fly-by missions of Jupiter: Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 and 2 (1979), Ulysses (1992), Cassini (2000) and New Horizons (2007). Just one spacecraft, Galileo, has entered orbit around Jupiter (from 1995-2003). Though multiple missions have studied Jupiter, fundamental questions related to its evolution lie unanswered. We do not know, for example, what the size of Jupiter’s core is. Is the core rocky? What is the atmospheric structure? Jupiter rotates on its axis every 10 hours despite its huge size — does it rotate as a solid body, or does the interior of Jupiter move at a different speed?
Juno, the second spacecraft to orbit Jupiter, will provide greater definition to the many unanswered questions. Juno, in a larger context, will help us understand how Jupiter was formed — and, by extension, how the Solar System was formed.
Although Juno will address questions related to science, the mission is a showcase of futuristic technology. The spacecraft will operate in an unprecedented radiation environment. As a result, the electronics would need to be protected in a titanium housing the size of a large cupboard. In contrast to most spacecraft that travel to the outer Solar System, Juno is not powered by nuclear energy, but by solar power. Operating in solar power mode is no mean achievement at a distance of 5 times the Earth-Sun distance — which means that the Sun is much smaller in the Jupiter sky than it is from Earth. As a result, to generate adequate power, the solar panels on Juno need to be comparatively large. Hence, each of the three solar panels is as high as a three-storey building. Even with very large solar panels, the power is a meagre 500 watts — or the power expended in lighting five 100 W bulbs.
The speed of Juno as it enters the orbit of Jupiter on July 4 will be about 275 times that of a Boeing 747. Juno had a unique trajectory to reach Jupiter: after launch, it flew past the orbit of Mars, returned for an Earth flyby, before heading to Jupiter. Due to this indirect approach, Juno will travel 1.8 billion miles — which is 15 times the closest distance between Earth and Jupiter. In fact, the journey to Jupiter would have taken 343 years, had the speed of the spacecraft been equal to the speed of a commercial jetliner.
Juno will hopefully throw open a new frontier in our exploration of the Solar System. Already both the National Aeronautics and Space Administration and the European Space Agency have lined up follow-up missions to the Jovian System. ESA will launch the Jupiter Icy Moon Explorer (JUICE) in 2022, and NASA has the mandate for a mission to Jupiter’s moon Europa in the 2020s. NASA’s Europa Clipper Mission will include an orbiter and a lander — which will mark the first ambitious attempt to land a spacecraft in the Outer Solar System.