Even though the true role Jupiter played, and is still playing in how the solar system is arranged today was not known until comparatively recently, the King of the Planets has been in the human consciousness from before recorded history, and it was known to all the major cultures and civilizations that ever bestrode the Earth. Given that we now know that Jupiter contains 71% all the mass in the solar system that is not vested in the Sun, it is hardly surprising that no expense (relatively speaking), has been spared to send probes to Jupiter to gather first-hand knowledge of why the planet is the way it is.
While much about the King of the planets remains unknown, much of what is known about Jupiter today was derived from data collected by several spacecraft, some of which have spent varying amounts of time investigating the planet before continuing on to other planets. Below are some details of those missions, but we would be remiss if we did not start by saying that..
Getting to Jupiter is expensive
While all missions to other planets in the solar system have been expensive, getting to Jupiter is spectacularly expensive. NASA’s Juno mission which left from Cape Canaveral on August 5, 2011 and entered an orbit around Jupiter on July 5, 2016 has cost around $1.1 billion over its life.
One of the reasons for its high cost is the fact the energy required by a space probe to reach Jupiter from a low Earth orbit is almost the same as for that craft to reach a low Earth orbit from the ground in the first place. In technical terms, the energy required to reach Jupiter (or any other planet) is known as “delta-v”, and it refers to the amount of energy/velocity that is required on top of the energy/velocity the craft already possesses while it is in low Earth orbit. Therefore, in the case of a craft wanting to reach the King of the planets, that craft needs to have an initial velocity of 9.0 – 9.5 km/sec just to reach an orbit around Earth, to which must be added an additional velocity of 9 km/sec to be able to reach Jupiter.
While fly-by’s of intervening planets have traditionally been used to add the required additional delta-v, the ion-thruster engines used on the Dawn spacecraft managed to add a delta-v value of 10 km/sec, which was sufficient to reach Jupiter without the need to use planetary fly-by’s, or gravity-assists to obtain the required velocity. Launched in 2007, the Dawn space probe then ended up costing NASA $446 million, and has been orbiting the dwarf planet Ceres since 2015, before being running out of fuel sometime next year.
Jupiter’s radiation nearly killed several space probes
Apart from the difficulties involved in just getting to Jupiter, spacecraft that do survive the ± 6-year journey have to be shielded exceptionally well to survive the harsh charged-particle environment around the planet. For instance, when the Pioneer 11 probe arrived at Jupiter, it encountered radiation levels that were more than ten times higher than its designers had anticipated, and it was only through blind luck that the craft survived at all.
However, while the craft did survive, Jupiter’s high radiation had scrambled the crafts’ imaging equipment, which caused all images of the moon Io to be lost. Moreover, in the eight years the Galileo spacecraft was in orbit around the planet, the harsh radiation caused several major malfunctions, despite the lessons learned from the Pioneer 11 mission. Some malfunctions included increased errors in the crafts’ gyroscopes, and several forced fail-safe modes caused by electrical arcing between rotating and non-rotating parts of the craft that in turn, caused all data from orbits 16, 18, and 33 to be lost.
Apart from the above, Jupiter’s radiation also caused severe phase shifts in Galileo’s normally stable quartz oscillator (clock) on several occasions, which caused severe time-keeping issues between the various scientific instruments in the craft.
First ever close-up view of Jupiter
Pioneer 11 followed about 12 months after the arrival of Pioneer 10, and approached the planet to within 34,000 km of the planets’ cloud tops on December 4, 1974. Despite issues with Jupiter’s strong radiation, Pioneer 11 did manage to obtain close-up views of the Great Red Spot, as well as obtain detailed data on the planets’ Polar regions. Pioneer 11 also determined the mass of Jupiter’s moon Callisto. Collectively, the data obtained by the two Pioneer missions served as the basis around which design improvements were made to subsequent space probes to cope with the extreme conditions around Jupiter better.
Both Voyager spacecraft visited Jupiter
Voyager 2 arrived soon after, and made its closest approach to Jupiter on July 9, 1979, when it was within 576,000 km of the planet’s cloud tops. Collectively, the Voyager missions discovered the full extent of the planets’ rings, determined that the Great Red Spot was a contra-rotating storm system, and found Adrastea and Metis, two small, and previously unknown satellites of Jupiter, making them the first moons of Jupiter to be discovered by a space probe.
In addition, the Voyager 1 & 2 missions also discovered the first evidence of current volcanic activity on a body other than Earth. Between them, the two Voyagers recorded nine volcanic eruptions on the moon Io, plus conclusive evidence that several other eruptions had occurred between the two Voyager fly-by’s.
Jupiter’s gravity placed the Ulysses solar probe into solar orbit
In order to attain a stable orbit around the Sun, the Ulysses solar probe was flown to within 451,000 km of Jupiter’s North Pole on February 8, 1992. This was done to adjust the probe’s orbital inclination to 80.2 degrees relative to the plane of the ecliptic, which was a requirement for the probe to attain an orbit around the Sun’s poles.
Essentially, Jupiter’s enormous gravity was used as a gigantic lever with which to bend the Ulysses probe’s orbit downward, and away from the plane of the ecliptic in order to impart the correct orbital path to the probe. During this maneuver, the size and shape of the probe’s orbit around the Sun were also adjusted so that the probe now maintains a separation of about five Sun – Jupiter distances at aphelion (farthest from Sun), and slightly more than one Sun-Earth distance from the Sun at perihelion (closest to Sun).
Best picture of Jupiter taken by the Cassini probe
More importantly though, data obtained by the Cassini probe determined that areas on Jupiter that were once thought to be upwelling currents from the planets’ inner regions, are in fact areas where the net motion of the atmosphere is actually downward, i.e., these areas are sinking down into the planet. Other discoveries made by Cassini include adjacent, but contra-rotating, planet-encircling winds close to the planet’s North Pole.
New Horizons collected more data on Jupiter than on Pluto
Additionally, the New Horizons probe also obtained detailed measurements of some volcanoes on Io, studied all four Galilean moons in more detail than was ever possible before its arrival, and also performed detailed studies on Jupiter’s Little Red Spot, its magnetosphere, and the structure of the ring system. In fact, New Horizons spent so much time in and around the vast Jovian system that it collected more information on this planetary system than it will on Pluto, its ultimate target.
Jupiter as seen by the Juno probe
Juno is the latest probe to investigate Jupiter, and specifically its composition, gravitational and magnetic fields, how much water the planet contains, how the planets’ mass is distributed, and whether or not the planet has a rocky core. Launched on August 5, 2011, Juno entered a polar orbit around Jupiter on July 4, 2016.
The series of images above are a composite of 14 images taken during one orbit, with the two largest images in the center (taken four minutes apart) of the panel showing what Jupiter looks like when the probe is closest to the cloud tops. The images on either end of the panel show the north and south poles respectively, with the North Pole shown on the far left of the panel. Juno is also investigating the winds that “blow” deep under the planets’ visible surface, and preliminary data show that these circulation patterns can exceed 600 km/h.
Juno will complete only 37 orbits of Jupiter
Juno is designed to complete only 37 orbits around Jupiter, each orbit taking just more than 53 days to minimize the craft’s exposure to the high radiation in the planet’s radiation belts. Essentially, the Juno craft orbits Jupiter far beyond the planets’ radiation belts, and only crosses one side of the radiation belt to zip by the planet while its instruments are gathering data, before it zips back out of the radiation belt over the South Pole.
The mission is scheduled to end sometime after February 2018, when it will be deliberately de-orbited over a period of 5.5 days to allow the craft to fall into Jupiter, thus preventing from colliding with one of Jupiter’s moons. This is in accordance with NASA’s Planetary Protection Guidelines, which have been developed specifically to reduce the impact that spacecraft used in planetary exploration might have on other planets and their satellites.
Juno is said to be cheap at $1.1 billion
When Juno was first proposed in 2003, its estimated cost was only $700 million. However, when it was eventually launched in 2011, its cost had escalated to $1.1 billion, which may be cheap or expensive, depending on your point of view. Since the design cost did not include a lander or a rover, or a landing system for the lander/rover combination as was required for the Mars missions, the $1.1 billion final cost was seen by many commentators as being too high a price to pay, considering that the Juno mission will last for only a year or so.
However, by way of comparison, the $2.5 billion spent on the Curiosity Rover that is now exploring Mars may also seem excessive, but then again, Curiosity is expected to provide high quality scientific data for several, if not for many years. In stark contrast to these two missions, the International Space Station that cost $150 billion to construct has not delivered any useful scientific data at all, so Juno might be more cost-effective than we realize.