NASA’s Cassini spacecraft performed what is effectively its last close flyby of Titan, Saturn’s largest moon on Saturday, April 22nd, 2017, marking a final opportunity for the mission to make up-close observations of the lakes and seas of liquid hydrocarbons that spread across the moon’s northern polar region and for the probe to use its radar imager pierce the haze enveloping the moon and map its surface. The next time the spacecraft passes Titan, it will be on its way to its destruction.
It is twenty years since the mission was launched from Earth, a combined NASA / ESA attempt to explore Saturnian system and probe the mysteries of Titan. It took seven years for the vehicle, carrying the European Huygens Titan Lander to is own rendezvous with the surface of Titan. Over the last thirteen years, the Cassini vehicle, roughly the size of a small truck and massing (at launch), 5 tonnes, has revolutionised our understanding of Titan and the potentially habitable moon of Enceladus.
However, all good things must eventually come to an end. The Cassini vehicle now has limited manoeuvring fuel left in its tanks, and while its three plutonium radioisotope thermoelectric generators (RTGs) are still capable of producing around 600 watts of electrical power, a decision was made some time ago to ensure the probe ended its mission before its tanks were dry and it was left to tumble around Saturn, where it might one day collide with one of the moons and contaminate it.
Instead, it was decided to direct the probe to into a series of orbits which would eventually see it enter the upper regions of Saturn’s atmosphere to burn up. This might seem an ignominious end for such a grand mission, but it is not without purpose.
This final plunge will not occur until September 15th, 2017, and the flyby of Titan – Cassini’s 127th – was the first step in that final journey, turning as it did, Cassini’s path in towards Saturn as it loops around the planet from pole-to-pole. But before that fiery end comes, the vehicle will complete 22 more orbits of Saturn which will see it repeatedly dive between the gas giant and its series of concentric rings, giving it an unprecedented science opportunity – a dive into the unknown.
“No spacecraft has ever gone through the unique region that we’ll attempt to boldly cross 22 times,” Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington said. “What we learn from Cassini’s daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end.”
“Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft,” Earl Maize, Cassini project manager at JPL added. “But we’re also being cautious by using our large antenna as a shield on the first pass, as we determine whether it’s safe to expose the science instruments to that environment on future passes. Certainly there are some unknowns, but that’s one of the reasons we’re doing this kind of daring exploration at the end of the mission.”
In mid-September, Cassini will make a final, distant pass by Titan. Distant, but still close enough for the moon’s gravity to turn the craft into its rendezvous with Saturn’s cloud-tops. And when Cassini makes that final plunge on September 15th, it will send data from several instruments until its signal is lost.
Ahead of the April 22nd Titan flyby, Cassini captured an image of Earth as seen through the ring of Saturn. Taken on April 13th, the probe was 1.4 billion kilometres (870 million miles) from Earth. when the image was taken.
Visible in the picture are, on the right, the A ring and the Keeler and Encke gaps, with the F ring over to the left. Earth is plainly visible in the gap between the rings. During this observation, Cassini was looking toward the backlit rings with the sun blocked by the disk of Saturn. The part of Earth facing toward Cassini at the time was the southern Atlantic Ocean.
Seen from Saturn, Earth and the other inner solar system planets always appear close to the sun much like Venus and Mercury do from Earth. All orbit interior to Saturn; even at maximum elongation, they never get far from the Sun. Early this month, as viewed from Saturn, Earth was near maximum elongation east of the sun, thus an “evening star,” making it an ideal time to take a picture.
It is still the most ambitious space mission so far undertaken by humanity: forty years ago, in August and September 1977, NASA launched the twin vehicles of the Voyager Programme to study the outer Solar System. The year of launch was important, because it marked a favourable alignment of Jupiter, Saturn, Uranus, and Neptune which would allow one of the two vehicles to perform flybys of all four planets using the gravity of each to push it on towards the next, massively reducing the amount of propellant (and thus vehicle mass) required to complete such a mission to all four of the gas giants.
Today, 40 years on from those launches, both of the Voyager craft are still operational, and are tasked with exploring interstellar space. Their mission has been extended three times, and both probes continue to collect and relay useful scientific data.
Voyager 2 was actually the first of the two craft to be launched, lifting-off from Launch Complex 41 at Cape Canaveral Air Force Station on August 20th, 1977 atop a Titan III booster. It was tasked with the “long haul” trip of flying by the four outer gas giants of the solar system, and remains thus far the only human-made vehicle to visit Uranus and Neptune.
Voyager 1 was launched 16 days later, on September 5th, 1977, again from Launch Complex 41. Despite its later launch, Voyager 1 Jupiter ahead of Voyager 2, to reach Jupiter first thanks to a more favourable flights plan as it would only be visiting Jupiter and Saturn. It performed an extended flyby of Jupiter planet between January and April 1979, coming the closest to Jupiter on March 5th, 1979, when it was some 349,000 km (217,000 mi) from the planet’s centre.
In November 1980, the vehicle performed its flyby of Saturn, coming to within 124,000 km (77,000 mi) of the planet’s cloud-tops, and giving us our first close-up looks at Titan. The flight path used meant voyager 1 used Saturn’s gravity to “tip” its trajectory out of the plane of the ecliptic occupied by the major planets, to place it on an intercept with the outer heliosphere, the “boundary” of the solar system where the solar wind directly interacts with the “winds” of interstellar space.
Here are some brief bullet points to the mission in the years since both vehicles completed their surveys of the outer planets of the solar system:
- On February 17th, 1998, Voyager 1 reached a distance of 69 AU from the Sun and overtook Pioneer 10 as the most distant spacecraft from Earth.
- Voyager 1 and Pioneer 10 are the most widely separated human made objects anywhere, since they are travelling in roughly opposite directions from the Solar System
- Voyager 1 is travelling at around 17 kilometres (10.6 miles) per second relative to the Sun; Voyager 2 is travelling at around 15.4 kilometres (9.6 miles) per second relative to the Sun
- In December 2004, Voyager 1 crossed the termination shock, where the solar wind is slowed to subsonic speed, and entered the heliosheath, where the solar wind is compressed and made turbulent due to interactions with the interstellar medium
- In December 2007, Voyager 2 also reached the termination shock, about 1.6 billion kilometres (1 billion miles) closer to the Sun than from where Voyager 1 first crossed it, indicating that the Solar System is asymmetrical
- In 2010 Voyager 1 reported that the outward velocity of the solar wind had dropped to zero, and scientists predicted it was nearing interstellar space
- In 2011, data from the Voyagers determined that the heliosheath is not smooth, but filled with giant magnetic bubbles, theorized to form when the magnetic field of the Sun becomes warped at the edge of the Solar System
- In June 2012, scientists reported that Voyager 1 was very close to entering interstellar space, indicated by a sharp rise in high-energy particles from outside the Solar System
- In September 2013, NASA announced that Voyager 1 had crossed the heliopause on August 25, 2012, making it the first spacecraft to enter interstellar space
As of 2017 Voyager 1 and Voyager 2 continue to monitor conditions in the outer expanses of the Solar System. They are expected to be able to operate science instruments through 2020, when limited power will require instruments to be deactivated one by one. Sometime around 2025, there will no longer be sufficient power to operate any science instruments, and the vehicles will effectively be lost to interstellar space.
To mark the 40th anniversary of the Voyager programme, a special documentary, The Farthest, tracing the history of the programme, had its international premiere at the Tribeca Film Festival in New York on Thursday, April 20th, and the trailer is embedded below.
China Launches Resupply Mission
April 20th also marked the launch of China’s first automated resupply vehicle. Tianzhou-1 (“Heavenly Ship 1”) vehicle, based on China’s first orbital laboratory module, Tiangong-1, lifted off from the Wenchang Satellite Launch Centre atop a Long March 7 booster. At 10 metres (30 feet in length and with a diameter of 3.35 metre (11 ft), the vehicle is considerably larger than the automated resupply vehicles operated by NASA, Russia and the Japanese space agency in support of the International Space Station, and it is capable of carrying considerably more to orbit.
This class of vehicle will eventually be used in support of China’s permanently crewed space station, construction of which is due to start in 2018. In the meantime, Tianzhou-1 craft rendezvoused with the Tiangong-2 orbital laboratory on Saturday, April 22nd, and completed a successful automated docking with the laboratory.
The rendezvous and docking was the first of three expected of the mission, which will see the vehicle perform refuelling operations for the Tiangong-2 manoeuvring engines and see the two craft fly in concert for two months testing the cargo spaceship’s control of the combined vehicles. After this, Tianzhou-1 will separate from the laboratory and conduct a three-month “free flight” during which time it will perform a range of its own experiments.
A final rendezvous and docking will then be held with Tiangong-2, this time using the laboratory’s forward docking port in order to test the automated docking capabilities at both ends of Tiangong-2.
A second crewed mission to Tiangong-2 (“Heavenly Palace 2”) is expected to take place later in 2017.