Category: Space Sunday

On Friday, September 15th, 2017, just one month short of the 20th anniversary of its launch, the NASA/ Italian Space Agency (ASI) space probe Cassini will plunge into the upper reaches of Saturn’s atmosphere, bringing to a close the momentous NASA / ASI / European Space Agency Cassini-Huygens mission.

It will be a bitter-sweet moment for many the world over – most of all the vast international team who devoted up to fourteen years of their time on the mission – even before it launched. The Cassini vehicle has not only revealed so much about Saturn, its myriad moons, the rich complexities of the gas giant’s ring system- it has also helped inform us on the potential for life to exist elsewhere in the solar system and has even helped test Einstein’s work. It has also over the years returned some of the most stunning and evocative images of other worlds we have yet witnessed. Many of these images have been gathered together by National Geographic and have been put together in a superb interactive web presentation on the mission by Nadia Drake and Brian Jacobs.

In all seventeen countries have been directly involved in the conception, design, construction and operation of the Cassini-Huygens mission, both in terms of the Cassini orbiter and the Huygens Titan lander and the science instruments they carry. NASA carries primary responsibility for the orbiter’s design and construction, with the Italian Space Agency providing the all-important, dual-purpose high-gain radio antenna and its associated communications equipment, together with the low-gain communications suite which would provide continuous communications with Earth through the mission. ASI also incorporated a compact radar system in to the high-gain antenna systems, allowing it to function as a synthetic-aperture radar, a radar altimeter, a radiometer, and provide the visible channel portion of the VMS spectrometer package carried by the probe.

ESA was responsible for the Huygens lander, with France designing the vehicle itself, with the descent parachute system provided by Martin-Baker of America, while the science and communications packages were supplied by several European countries and the United States.

The mission was named for the Italian-French astronomer Giovanni Domenico Cassini, who first observed the divisions within Saturn’s rings system (and after whom one of the divisions is also named) as well as for of the planet’s moons, and  Christiaan Huygens, the Dutch mathematician, physicist and astronomer, who first observed Titan, Saturn’s largest moon.

Work actually commenced on the mission in the 1980s, the goal being to develop a mission which could determine the three-dimensional structure and dynamic behaviour of Saturn’s ring system, investigate Saturn’s atmosphere and magnetosphere, determine the composition and likely structure of Saturn’s moons, including the nature and origin of the dark material on Iapetus‘s leading hemisphere, and, in conjunction with the Huygens lander, characterise Titan’s atmosphere, including the variability of the cloud haze, and characterise the moon’s surface at a regional level.

Initially, the mission was funded for a 10-year period from late 1997 through mid-2008, which included a journey of seven years to reach Saturn. The voyage took so long because at the time of launch, there was no launch vehicle combination capable of sending Cassini directly to Saturn. Instead, it completed a mini-tour of the inner solar system; six months after launch, Cassini flew by Venus, using the planet’s gravity to accelerate it into a wide elliptical orbit. A second encounter in June 2000 again accelerated the spacecraft, slinging it on to a further gravity-assist flyby of Earth in August 2000, which in turn accelerated it and bent it onto an trans-Jovian flight path.

In late 2000, Cassini reached the vicinity of Jupiter, making its closest approach to the planet on December 30th of that year. As well as using Jupiter’s gravity to sling it onwards to its final destination, Cassini used the encounter to study Jupiter and its faint system of rings. In all some 26,000 images of Jupiter, its moons and its rings were taken during the 6-month period of the flyby (October 2000 – March 2001). Cassini’s science suite was powered-up for the flyby, and resulted in some significant discoveries concerning Jupiter’s turbulent atmosphere, including breaking a long-held view. Jupiter’s banded atmosphere comprises a series of alternate bands of darker and lighter zones, in part caused by Jupiter’s rapid rotation. It had also been thought that the lighter bands were the result of the atmosphere rising upwards, giving rise to lighter cloud formations, before circulating downwards once more.

However, Cassini revealed the dark bands were peppered with individual storm cells of upwelling bright-white clouds too small to see from Earth, suggesting the vertical circulation of Jupiter’s atmosphere to be far more uniform than thought. The probe’s findings also showed that Jupiter’s thin and dusty rings to be made up from small, irregularly shaped particles, most likely created by ejacta from micrometeorites impacting the Jovian moons.

Cassini reached Saturn in 2004, officially entering orbit around the planet on July 1st of the year. Prior to doing so, the vehicle was part of a test of Einstein’s theory of general relativity. This states that any massive object like the Sun causes space-time to curve, causing a beam of light or any other form of electromagnetic radiation that passes close to it to travel farther (the Shapiro time delay). In 2003, with the Sun coming between Earth and Cassini, scientists on Earth measured the frequency shift in radio signals being received from the spacecraft. Similar experiments had been carried out with the Voyager and Viking missions, but Cassini provided for much more refined measurements to be taken, and firmly validated Einstein’s theory.

Cassini‘s primary mission at Saturn commenced as it approached the planet for orbital insertion. Although the orbiter was capable of functioning – all things being equal – through until around 2017, this primary mission was scheduled to last just 3 years and 261 days, ending in mid-2008. This was sufficient time for the primary goals of the mission to be achieved, but Cassini was always designed to achieve so much more. With this in mind, the programme was granted two funding extensions. The first, called the Equinox Mission, funded the project through until the end of 2010, and gave a particular focus on Titan (15 flybys) Saturn’s ice-covered moon Enceladus, thought two of the locations in the Saturnian system where life might have taken hold.

The second extension, granted funding in 2010 to the tune of around US $60 million a year, is referred to as the Solstice mission (as it would end a few months past Saturn’s summer solstice). It guaranteed that, avoiding any spacecraft failures, the mission would continue through to the point where Cassini’s manoeuvring propellants would be practically depleted. This phase of the mission allowed for a more extended study of Saturn, its rings and moons. It meant Cassini could witness never before seen seasonal changes in the planet’s atmosphere and study. It also meant Cassini could study Saturn’s atmosphere and magnetosphere at exactly the same time as NASA Juno mission studied Jupiter’s atmosphere and magnetosphere, allowing a direct comparison of the two. Finally, the extension would carry the mission through its 5-month “grand finale”, probing the region between Saturn and its complex ring system.

Continue reading “Space Sunday: Cassini – a journey’s end” →

August 20th 2017 marks the 40th anniversary of the launch of Voyager 2, which with its sister craft Voyager 1 (launched on September 5th, 1977) are humanity’s furthest-flown operational space vehicles, with Voyager 1 being the most distant human made object from Earth, at some 140 AU (AU= astronomical unit, the average distance between the Earth and the Sun; 140 AU equates to about 20.9 billion kilometres / 13 billion miles).

Despite being so far away from Earth, both craft are still sending data back to Earth as they fly through the interstellar medium in the far reaches of the solar system (the Pioneer 10 and Pioneer 11 craft which pre-date the Voyager programme by some 5 years, ceased transmissions to Earth in January 2003 and September 1995 respectively, although Pioneer 10 is the second most distant human made object from Earth after Voyager 1).

The Voyager programme stands as one of the most remarkable missions of early space exploration. Originally, the two vehicles were to be part of NASA’s Mariner programme, and were at first designated Mariner 11 and Mariner 12 respectively. The initial Mariner missions – 1 through 10 – were focused on studying the interplanetary medium and  Mars, Venus and Mercury (Mariner 10 being the first space vehicle to fly by two planets beyond Earth – Venus and Mercury – in 1974). Mariner 11 and Mariner 12 would have been an expansion of the programme, intended to perform flybys of Jupiter and Saturn.

However, in the late 1960s Gary Flandro, an aerospace engineer at the Jet Propulsion Laboratory (JPL) in California noted that in the late 1970s, the outer planets would be entering a period of orbital alignment which occurs once every 175 years and which could be used to throw a series of probes out from Earth, which could then use the gravities of the worlds they encountered to “slingshot” them on to other targets. This led to the idea of a “Grand Tour” mission: sending pairs of probes which could use these gravity assists to fly by Jupiter, Saturn, Uranus, Neptune and Pluto in various combinations.

Funding limitations eventually brought an end to the “Grand Tour” idea, but the planetary alignment was too good an opportunity to miss, and so elements of the idea were folded into the Voyager Programme, which would utilise Mariner 11 and Mariner 12. However, as the mission scope required some significant changes to the vehicles from the basic Mariner design, they were re-designated as Voyager class craft.

(As an aside, the Mariner class is the longest-lived of NASA’s space probe designs; as well as the ten missions of the 1960s and 1970s which carried the design’s name,  the Mariner baseline vehicle – somewhat enlarged – was used for the Viking 1 and Viking 2 orbiter missions to Mars, and formed the basis of the Magellan probe (1989-1994)  to Venus and the Galileo vehicle (1989-2003) which explored Jupiter. And uprated and updated baseline Mariner vehicle, designated “Mariner Mark II”, formed the basis of the Cassini vehicle, now in the terminal phase of its 13-year study of Saturn and its moons.)

Each of the Voyager mission vehicles is powered by  three plutonium-238 MHW-RTG radioisotope thermoelectric generators (RTGs), which provided approximately 470 W at 30 volts DC when the spacecraft was launched. By 2011, both the decay of the plutonium and associated degradation of the thermocouples that convert heat into electricity has reduced this power output by some 57%, and is continuing at a rate of about 4 watts per year.

To compensate for the loss, various instruments on each of the vehicles have had to be turned off over the years. Voyager 2’s scan platform and the six instruments it supports, including the vehicle’s two camera systems, was powered-down in 1998. While the platform on Voyager 1 remains operational, all but one of the instruments it supports – the ultraviolet spectrometer (UVS) – have also been powered down. In addition, gyro operations ended in 2016 for Voyager 2 and will end in 2017 for Voyager 1. These allow(ed) the craft to rotate through 360 degrees six times per year to measure their own magnetic field, which could then be subtracted from the magnetometer science data to gain  accurate data on the magnetic fields of the space each vehicle is passing through.

However, despite the loss of capabilities, both Voyager 1 and Voyager 2 retain enough power to operate the instruments required for the current phase of their mission – measuring the interstellar medium and reporting findings back to Earth. This phase of the mission, officially called the Voyager Interstellar Mission, essentially commenced in 1989 as Voyager 2 completed its flyby of Neptune, when the missions as a whole was already into their 12th year.

Voyager 2 was launched on August 20th, 1977. Of the two vehicles, it was tasked with the longer of the planned interplanetary missions, with the aim of flying by Jupiter, Saturn, Uranus and Neptune. However, the latter two were seen as “optional”, and dependent upon the success of Voyager 1.

This was because scientists wanted the opportunity to look at Saturn’s moon Titan. But doing so would mean the Voyager craft doing so would have to fly a trajectory which would leave it unable to use Saturn’s gravity to swing it on towards an encounter with Uranus. Instead, it would head directly towards interstellar space.

So it was decided that Voyager 1, which although launched after Voyager 2 would be able to travel faster, would attempt the Titan flyby. If it failed for any reason, Voyager 2 could be re-tasked to perform the fly-past, although that would also mean no encounters with Uranus or Neptune. In the end, Voyager 1 was successful, and Voyager 2 was free to complete its surveys of all four gas giants.

Along the way, both missions revolutionised our understanding of the gas giant planets and revealed much that hadn’t been expected, such as discovering the first active volcanoes beyond Earth, with nine eruptions imaged on Io as the vehicles swept past Jupiter. The Voyager missions were also the first to find evidence that Jupiter’s moon Europa might harbour a subsurface liquid water ocean and to return the first images of Jupiter’s tenuous and almost invisible ring system. Voyager 1 was responsible for the first detailed examination of Titan’s dense, nitrogen-rich atmosphere, and Voyager 2 for the discovery of giant ice geysers erupting on Neptune’s largest moon, Triton. In addition, both of the Voyager vehicles added to our catalogues of moons in orbit around Jupiter and Saturn, and probed the mysteries of both planet’s atmospheres, whilst Voyager 2 presented us with our first images of mysterious Uranus and Neptune – and thus far remains the only vehicle from Earth to have visited these two worlds.

The flyby of Neptune also sealed Voyager 2’s future. Scientists were keen to use the flyby of the planet to take a look at Triton, Neptune’s largest moon. However, because Triton’s orbit around Neptune is tilted significantly with respect to the plane of the ecliptic, Voyager 2’s course to Neptune  had to be adjusted by way of a gravitational assist from Uranus and a number of mid-course corrections both before and after that encounter, so that on Reaching Neptune, it would pass over the north pole, allowing it to bent “bent” down onto an intercept with Triton while the Moon was at  apoapsis – the point furthest from Neptune in its orbit – and well below the plane of the ecliptic. As a result, Voyager 2 passed over Triton’s north pole 24 hours after its closest approach to Neptune, its course now pointing it towards “southern” edge of the solar system.

Continue reading “Space Sunday: Voyager at 40” →