New images of Jupiter's moon Amalthea.

NASA has released a pair of images of Jupiter's moon Amalthea, taken by the Juno Spacecraft during a close flyby on 7 March 2024. At the time of the flyby the spacecraft was only 265 000 km from the Jovian moon (about 69% of the distance between the Earth and its Moon), though Amalthea still appears as a tiny speck, since it is only about 84 km in diameter.

Images of the Jovian moon Amalthea taken by the Juno Spacecraft on 7 March 2024. NASA/JPL/Caltech/Southwest Research Institute/Main Space Science Systems/Gerald Eichstädt.

Amalthea was discovered on 9 September 1892 by American astronomer Edward Emerson Barnard, then working at the Lick Observatory on Mount Hamilton in California. Other than the four moons discovered by Galileo Galilei in 1610, Amalthea is the only Jovian moon to have been discovered by direct observation. The remaining 90 know moons of Jupiter have been discovered either by photographic surveys or spacecraft flybys.

Amalthea is irregularly shaped, being roughly 250 km long, 146 km wide, and 128 km deep, and tidally locked to Jupiter, so that it always has the same side facing towards the planet and the same end forward, with its long axis aligned with its orbit.

Images of Amalthea captured by the Galileo Spacecraft; (left) from a distance of 446 000 km on 12 August 1999, (right) from a distance of 374 000 km on 26 November 1999. NASA/JPL/Wikimedia Commons.

Amalthea orbits Jupiter at a distance of 181 000 km, making it the third closest known moon to the planet; since the Roche Limit of Jupiter (distance below which a satellite will be torn apart by tidal forces) is about 80 000 km, it is unlikely that there are many undiscovered moons closer to the planet than Amalthea. The current orbit of the moon is extremely circular and has a very low inclination, but it has been calculated that it goes through periodic episodes of resonance with Io, the innermost of the Galilean Moons, during which its orbit becomes more eccentric and tilted.

The surface of Amalthea is extremely red, and brighter than the other inner satellites of Jupiter, with a leading hemisphere 1.3 times as bright as the trailing hemisphere. This has been theorized to be because of an accumulation of sulphur ejected from Io onto the surface of Amalthea, although without visiting the moon it is unlikely that this can ever be established.  

Amalthea as imaged by the Voyager 1 Spacecraft on 5 March 1979. Calvin Hamilton/NASA/Wikimedia Commons.

A number of large craters have been observed on the surface of Amalthea. The largest of these has been named Pan, and is 100 km wide and at least 8 km deep. Another crater, Gaea, is only about 80 km wide, but thought to be at least twice as deep as Pan.

Largely inside the orbit of Amalthea lies the Amalthea Gossamer Ring, a faint dust ring which extends from about 129 000 km above Jupiter to about 182 000 km, which reaches its maximum thickness, about 2300, close to the orbit of Amalthea. The Galileo Spacecraft passed through this ring in 2002-2003, finding that it is made of particles between 0.2 μm and 5.0 μm in diameter, with some larger bodies close to the moon. It is thought that the ring is made up of material ejected from the surface of Amalthea, and that the larger objects may represent debris from a collision.

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New images of Io.

NASA has released a new series of images of Jupiter's moon Io, following a close pass of the moon by its Juno Spacecraft on 30 December 2023. Juno passes within 1500 km of the moon, the closest any spacecraft has come to Io since the Galileo Spacecraft- made a pass at 181 km in 2001 (for comparison, the International Space Station orbits the Earth at 408 km). The flyby is the closest the spacecraft has come to the moon in a series of passes that began in July 2022, with the intention of studying Io and its volcanoes.

An image showing the day and night sides of Io. Emma Walimaki/NASA.

Io is the innermost of the four Galilean Moons of Jupiter (the four large moons discovered by Galileo Galilei in January 1610), and is one of the most distinctive bodies in the Solar System, with a surface dominated by a series of extensive volcanic fields. The volcanism is thought to be caused by tidal forces, as Io is pulled by the gravitational forces of both Jupiter and the other large Galilean Moons, deforming and heating the moon's interior. This has led to a body unlike any other in the Outer Solar System, with no significant ice or hydrocarbon deposits (presumably lost due to the heat of the volcanic activity) and a silicate rock surface surrounding an iron or iron-sulphur core.

A high colour image of Io. NASA.

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Jovian InfraRed Auroral Mapper discovers new volcanic field on Io.

Io is the innermost of the four Galilean Moons of Jupiter (the four large moons discovered by Galileo Galilei in January 1610), and is one of the most distinctive bodies in the Solar System, with a surface dominated by a series of extensive volcanic fields. The volcanism is thought to be caused by tidal forces, as Io is pulled by the gravitational forces of both Jupiter and the other large Galilean Moons, deforming and heating the moon's interior. This has led to a body unlike any other in the Outer Solar System, with no significant ice or hydrocarbon deposits (presumably lost due to the heat of the volcanic activity) and a silicate rock surface surrounding an iron or iron-sulphur core.

The Galilean Moon Io, as imaged by the Galileo Spacecraft in 1995. NASA/JPL/University of Arizona/Wikimedia Commons.

In a press statement released on 13 July 2018, scientists from NASA described the discovery of a new volcanic field on Io, close to the moon's South Pole and about 300 km from the nearest previously discovered field. This was revealed in an image of Io taken by the Jovian InfraRed Auroral Mapper instrument on the Juno Spacecraft during a flyby on 16 December 2018. 

This annotated image highlights the location of the new heat source close to the south pole of Io. The image was generated from data collected on 16 December 2017, by the Jovian Infrared Auroral Mapper (JIRAM) instrument aboard NASA's Juno mission when the spacecraft was about 470 000 kilometres from the Jovian moon. The scale to the right of image depicts of the range of temperatures displayed in the infrared image. Higher recorded temperatures are characterised in brighter colours – lower temperatures in darker colours. NASA/JPL/Caltech/Southwest Research Institute/Agenzia Spaziale Italiana/Insituto Nazionale di Astrofisica/Jovian Infrared Auroral Mapper .

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Juno Spacecraft to flyby the Earth on Wednesday 9 October 2013.

NASA's Juno Spacecraft will flyby the Earth on Wednesday 9 October 2013, gaining a gravitational boost from the encounter which will accelerate it on its way towards its eventual destination in a polar orbit around Jupiter. The probe will be at its closest at about 7.20 pm GMT, when it will be approximately 559 km above a point in the South Atlantic roughly 200 km off the South African coastline, and potentially visible to amateur astronomers in Cape Province.

The path of the Juno Spacecraft past the Earth on 9 October 2013. Heavens Above.

Juno was launched on 5 August 2011, and should reach Jupiter on 4 July 2016. It is following a somewhat complex path to the giant planet, which involves passing outside the orbit of Mars before falling back inwards in order to gain more thrust from a slingshot maneuver past the Earth, using the Earth's gravity to propel itself out to Jupiter at greater speed than could be imparted by the thrusters used to launch it. Once in orbit around Jupiter it will complete 33 orbits around the Planet, before crashing into the atmosphere in October 2017. Juno is designed to study the composition of the Jovian atmosphere.

The path of Juno from Earth to Jupiter. NASA/JPL/Wikimedia Commons.

See also Space Age archaeology: Apollo engines rise from the deep, Cassini Probe finds 400 km river on Titan, Names for nine craters in the North Polar Region of Mercury approved, India plans mission to Mars and Sally Ride, the first American woman in space, dies aged 61.

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