published on 4 July 2016 in space

Discovering Jupiter!

Jupiter, the fifth planet in order of distance from the Sun, is certainly the giant of the solar system, so large that it could contain the Earth 1,300 times. Also Jupiter’s mass is considerable, amounting to 318 times the mass of the Earth, and if we were to sum the mass of all the other bodies in the solar system, excluding the Sun, we would obtain a value that exceeds the mass of Jupiter by only 30%! It is no coincidence that this planet has even taken the name of the Olympian god.
Let’s discover together the main missions launched to the discovery of giant gaseous planet!

Le Pioneer missions
In 1969, NASA approved the programme for the construction of the two Pioneer 10 and 11 probes, to be put into different orbits so that Pioneer 11 could then continue its journey to Saturn. The adventure of these two interplanetary probes began on 2 March 1972 from Cape Canaveral, Florida, when Pioneer 10 was launched toward Jupiter, reached on 4 December 1973, passing at a minimum distance of approx. 130,000 kilometres from the planet. Identically equipped, Pioneer 11 was launched one month later, on 5 April 1973, heading toward Saturn, which it reached in September 1979 by exploiting the gravitational pull of Jupiter.


Images of Jupiter sent by Pioneer 10. Credits: NASA

Pioneer 10 flew over Jupiter in December 1973, followed by Pioneer 11 exactly one year later. Pioneer 10 was the first spacecraft to cross the asteroid belt, to perform direct observations of the planet Jupiter and to use the gravitational force of this great planetary mass as fuel which, like a giant catapult, threw it to the extreme regions of the solar system, making it achieve an escape velocity such as to escape the attraction of the Sun forever. Originally designed for a mission of 21 months, the probe was active for more than 30 years. In January 2003, a weak signal was in fact received from Taurus (approx. 2 million light-years from the nearest star of this constellation, Aldebaran), at a distance equal to more than twice that between the Sun and Pluto, approx. 82 AU.
Pioneer 11 sent significant images of the Great Red Spot to Earth, the first observations of the polar regions of the planet and sufficient data to determine the mass of Callisto. The last communication of Pioneer 11 with Earth was in September 1995, when the last radio signals were received, before it was lost forever in the vastness of space. Having in fact exhausted its energy, the electric generator stopped working and therefore supplying power to the on-board instrumentation, making the spacecraft uncontrollable.

A curiosity: both probes carried a message to hypothetical extraterrestrial civilizations. They were in fact equipped with a metal plate depicting the solar system, the planetary orbits, the Sun, our planet, the spacecraft themselves, the figures of a man and a woman and references to the position in the solar system based on those of 14 pulsar stars. (see: Plaque launched with the probe Pioneer 10 in 1972)

To know more: NASA – The Pioneer Mission

Le Voyager missions
The Voyager 1 and Voyager 2 space probes were the first probes sent to explore the outer solar system (beyond the asteroid belt) still today in full operation. The observation and study of Jupiter, therefore, was not the only objective of the mission.
The two probes were launched from Cape Canaveral (Florida) in 1977. More precisely, Voyager 1 began its journey on 5 September 1977, reached Jupiter on 5 March 1979 and then Saturn on 13 November 1980, while Voyager 2, launched on 20 August 1977 (before Voyager 1), reached Jupiter on 7 August 1979, Saturn on 26 August 1981, Uranus on 24 January 1986 and Neptune on 8 August 1989. Voyager 2 took advantage of a rare planetary alignment that occurs every 189 years to catapult itself from one planet to another. Thanks to these two probes, our knowledge of Jupiter and the other three giant planets, Saturn, Uranus and Neptune, of their satellites and of their rings has become immense. The probes collected images of the planet and of the Galilean satellites at a much higher resolution compared to that of the Pioneer probes that preceded them. The first information on the physical, atmospheric and geological processes taking place on the planet, on its main satellites and affecting the Jovian magnetosphere were thus revealed. Three new satellites were discovered. Two of the biggest surprises were the discovery that Jupiter has rings and the intense volcanic activity of Io, such as to influence the magnetosphere of Jupiter.
Currently, Voyager 1 and Voyager 2 are the most long-lasting probes and will continue functioning until the RTG (radioisotope thermoelectric generator) battery runs out, an event expected in 2025.

A curiosity: both probes carry with them the Voyager Golden Record, a gold record with recordings of the sounds of the Earth and the coordinates on how to reach it should someone find it. Inside there is also an audio message from the President of the United States at the time, Jimmy Carter, which reads: “This is a gift from a small, distant planet, a fragment of our sounds, our science, our images, our music, our thoughts and our feelings. We are trying to survive our times, so that we can live until yours.”

Other close encounters with Jupiter
After the Voyager missions, Jupiter was also flown past by the Ulysses (8 February 1992), Cassini-Huygens (30 December 2000) and New Horizons (28 February 2007) probes, which took advantage of gravity to reach their main target, the solar polar orbit of Saturn and Pluto, respectively.
In February 1992, the Ulysses probe passed at a distance of 409,000 km from Jupiter. The manoeuvre, necessary to move the probe on a polar orbit around the Sun, allowed it to study the Jovian magnetosphere at latitudes not covered by earlier probes.
In 2000, the Cassini-Huygens, en route to Saturn, approached Jupiter, providing some higher resolution images of the planet. 26,000 images of the planet were collected allowing the most detailed image ever obtained of Jupiter to be produced. New Horizons (28 February 2007) which took advantage of gravity to reach their main target, the solar polar orbit of Saturn and Pluto, respectively.


The trajectory followed by the Galileo probe was called “V.E.E.G.A.” – Venus – Earth – Earth Gravity Assist. Image credits: NASA-JPL

The Galileo probe
NASA’s Galileo probe was launched into space on 18 October 1989 from the Atlantis Space Shuttle at the Kennedy Space Center. The objective of the mission, dedicated to Galileo Galilei, who in 1610 discovered the existence of the four largest satellites of Jupiter, which he baptised “Medicean planets” (Io, Europa, Callisto and Ganymede, known today as the Galilean moons), in honour Cosimo II de ‘Medici, Grand Duke of Tuscany, was to study the planet, its moons and its magnetosphere.
To take advantage of the ‘gravity assist’ effect provided by the masses of the planets and to save precious fuel, the Galileo probe first headed for Venus, then again to Earth and, after passing through the asteroid belt, sending us the first ever image of two of them (951 Gaspra and 243 Ida), returned towards the Earth for the last thrust towards Jupiter, where it arrived on 7 December 1995, after a 6-year journey.
Five months before arrival, in July 1995, Galileo released the atmospheric probe which entered Jupiter’s atmosphere without the use of aerobraking systems. This type of atmospheric entry was the most difficult ever made: the probe entered at a speed of 47.8 km/s and was slowed down by the intense friction with the atmosphere. In the fall, lasting 58 minutes, through the increasingly dense layers of Jupiter’s atmosphere, the probe transmitted data on solar radiation, heat flow, pressure, temperature, winds and atmospheric composition. The measurements show that Jupiter is affected by much larger storms than on earth, due to a vertical circulation of water in the upper atmosphere.
The eight years of the mission brought significant scientific results. In 1994, it found itself in a favorable position for observation and transmission of unique images of a collision of fragments of the Shoemaker-Levy 9 comet on Jupiter, while the telescopes had to wait until the sites of the impact were facing towards the Earth. Galileo provided evidence of an ocean of salty water beneath the icy surface of Europa, one of Jupiter’s satellites; and then confirmed extensive volcanic activity on another satellite, Io, approx. 100 times greater than that on Earth, and probably very similar to that existing on our planet during its primordial era and discovered that Ganymede had its own magnetic field. It also discovered that Jupiter’s ring system consists of tiny grains of dust coming from the collision of meteorites with the surface of four inner satellites of Jupiter.
On 21 September 2003, after spending 14 years in space and 8 years of service in the Jovian system, the mission was terminated, sending the spacecraft into the atmosphere of Jupiter at a speed of approx. 50 km/s to avoid an undesired impact with Europa and any possibility of contamination with bacteria from Earth.

To knw more: NASA – Mission Galileo

The Galileo probe until today held the record of the first and only probe to have entered the orbit of Jupiter. Today, 4 Jul , the Juno probe will become the second probe to enter the orbit of Jupiter. To learn more 4 July 2016: the Juno spacecraft reached the orbit of Jupiter and brought a lot of Italy with it

To know more and find out how to observe the sky, read: What Planets Are Visible Tonight? – 2019 Astronomer’s Guide to the Night Sky

by Benedetta Palazzo

Eni S.p.A. - P.IVA 00905811006