Year 2015 was yet another banner stretch for planetary exploration. The year saw the farewell of NASA’s MESSENGER spacecraft at Mercury, the delayed arrival of JAXA’s Akatsuki at Venus, the amendment of Juno’s mission timeline at Jupiter, and a series of final grand moments of science for the Cassini spacecraft as it began its penultimate year at Saturn.
Farewell to MESSENGER:
While most years traditionally see the arrival of new planetary explorers or the return of science from those on-going missions, there comes a time when all missions must end.
Thus was the case in 2015 for MESSENGER – the first spacecraft to enter orbit and perform in-situ observations of the first planet in the solar system: Mercury.
Since arriving at Mercury on 18 March 2011, MESSENGER successfully completed both its primary mission (which ended on 16 March 2012) and its first mission extension (which ended on 17 March 2013) before moving into its final, two-year mission phase.
During its four years at Mercury, MESSENGER achieved numerous scientific goals, such as determining Mercury’s surface composition, revealing its geological history, discovering that its internal magnetic field is offset from the planet’s center, and verifying that its polar deposits are dominantly water ice.
But with fuel running low and an inability to maintain orbit without fuel, the third mission extension for MESSENGER would be the probe’s last.
After a final low-altitude survey mission, MESSENGER’s teams commanded the spacecraft into an orbit that would allow it to impact the planet.
On 30 April 2015, after 4 years 1 month, 14 days in orbit of Mercury, MESSENGER hit the surface of the planet at 8,750 mph.
“We are celebrating MESSENGER as more than a successful mission,” said John Grunsfeld, associate administrator for NASA’s Science Mission Directorate.
“The MESSENGER mission will continue to provide scientists with a bonanza of new results as we begin the next phase of this mission – analyzing the exciting data already in the archives, and unravelling the mysteries of Mercury.”
Akatsuki arrives at Venus:
The year also saw excitement at the second planet in our solar system as the Japan Aerospace and eXploration Agency’s (JAXA’s) Akatsuki spacecraft finally entered the orbit of Venus five years after its scheduled arrival.
Launched on 20 May 2010 from the Tanegashima Space Center in Japan, Akatsuki began its sixth month voyage to Venus, culminating in an Orbital Maneuvering Engine (OME) burn at 23:49 UTC on 6 December 2010.
That OME burn was supposed to last 12 minutes, inserting Akatsuki into a 4-day, 180,000 x 200,000 km (110,000 x 120,000 mile) orbit.
However, the OME burn lasted less than three minutes after salt deposits jammed the valve between the helium pressurization tank and the fuel tank, resulting in an oxidizer-rich burn that raised the burn temperature and damaged the throat and nozzle of the combustion chamber.
The short duration of the OME burn placed Akatsuki into orbit of the Sun and left controllers in Japan with the task of configuring the spacecraft for a five-year hibernation (it carried a predicted lifespan of 4.5 years) before it would reencounter Venus.
By September 2011, controllers determined, through a series of trial burns, that the OME was effectively inoperable and that the probe’s four hydrazine-fueled Reaction Control System (RCS) thrusters would have to be used instead to insert the probe into orbit.
With a new plan set, three peri-Venus orbital maneuvers were conducted between the 1-21 November 2011 through the RCS system for a total delta-v change of 243.8 m/s.
Four additional trajectory correction maneuvers were performed between 17 July and 11 September 2015 to place Akatsuki into the correct corridor for rendezvous with Venus.
On 7 December, JAXA controllers commanded Akatsuki to perform a pre-programmed 20-minute burn of the four RCS thrusters to insert the probe into Venus orbit.
Unlike the previous attempt in 2010, the second attempt was a success, and Akatsuki placed itself into a 440,000 km (270,000 mile) by 400 km (250 mile) orbit of Venus with a total orbital period of 13 days 14 hours.
A follow-up thruster burn is currently scheduled for 26 March 2016 to lower Akatsuki’s orbit to 330,000 km (210,000 miles) by 400 km and reduce its orbital period to nine days.
After the initial orbit insertion was successful, JAXA successfully tested three of Akatsuki’s instruments and verified their functionality despite having experienced temperatures upwards of 30 °C above the design parameters as the probe swung much closer to the Sun during its five year solar orbit than initially anticipated.
Now safely in orbit, Akatsuki’s two year primary science phase will begin in March 2016.
Juno’s team prepares for primary mission, amends orbital parameters and timeline:
For the team watching over the hibernating Juno spacecraft en route to Jupiter, 2015 marked the one year until arrival milestone and also saw an overhaul of the mission’s duration and plan once the spacecraft arrives in the Jovian system on 4 July 2016.
Juno, the first spacecraft to orbit Jupiter since the Galileo mission of the 1990s, will be the first mission to the largest planet of our solar system designed specifically to study Jupiter’s interior by mapping the planet’s magnetic and gravity field.
With a year to go before arrival in the Jovian system, Juno’s control and science teams gained final approval from NASA to alter the mission’s flight plan, changing the spacecraft’s orbital period from 11 to 14 days.
This change will allow Juno to build maps of Jupiter’s magnetic and gravity fields to provide a global perspective of the planet earlier in the mission than originally planned.
The original plan would have required 15 orbits to map these global forces, with 15 more orbits filling in gaps to make the map complete.
In the revised plan, Juno will obtain basic mapping coverage in just eight orbits, with a new level of detail added with each successive doubling of the number, at 16 and 32 orbits.
The revised plan lengthens Juno’s mission to 20 months instead of the original 15 and increases the number of orbits to 32 instead of 30.
However, the extra time does not represent additional science for the mission.
Instead, it will simply take Juno longer to collect the data it’s tasked with measuring.
Cassini: New understandings and final visits
Reaching one planet over, 2015 saw Cassini continue its unprecedented exploration of the ringed-planet as the mission itself progressed into its 11th year on 1 July 2015.
(Click here for a complete list of NASASpaceflight.com’s coverage of the Cassini mission.)
For Cassini’s science teams and researchers studying the data returned from the probe, the year began with a new understanding regarding the much talked about ice moon Enceladus.
Specifically, new research from Cassini suggested that most of the eruptions from Enceladus’ south polar region might be diffuse curtains rather than discrete jets.
“We think most of the observed activity represents curtain eruptions from the ‘tiger stripe’ fractures, rather than intermittent geysers along them,” said Joseph Spitale, lead author of the study and a participating scientist on the Cassini mission at the Planetary Science Institute in Tucson, Arizona.
“Some prominent jets likely are what they appear to be, but most of the activity seen in the images can be explained without discrete jets.”
Specifically, many features that appear to be individual jets of material erupting along the length of prominent fractures in the moon’s south polar region might be phantoms created by an optical illusion.
The discovery of curtain eruptions on Enceladus helps scientists understand the nature of the ‘tiger stripe’ ejecta plumes and provides yet further evidence for and proof of Enceladus’ subterranean ocean.
However, Enceladus was not the only Saturnian moon to reveal something surprising in 2015.
In July, NASA revealed unexplained red streaks visible on the surface of the ice moon Tethys.
The narrow, red, curved lines on Tethys’ surface are among the most unusual color features on Saturn’s moons.
Scientists are unsure of the origin of these red streaks; however, they could be exposed ices with chemical impurities or be the result of outgassing from inside Tethys.
While reddish hues are unusual on Saturnian moons, save a few small craters on Dione, they are abundantly common on the geologically young surface of Jupiter’s moon Europa.
“The red arcs must be geologically young because they cut across older features like impact craters,” said Paul Helfenstein, a Cassini imaging scientist at Cornell University.
Following the discovery of these red marks, Cassini’s controllers and scientists obtained follow-up observations of the features at higher resolutions in November when Cassini revisited Tethys – observations that should help scientists better understand the origin of the marks and how they relate to Tethys.
However, while these quick turnarounds of scientific information are useful in obtaining follow-on observations in the remaining two years of Cassini’s mission, more long term understandings – specifically, Saturn’s ring structure – emerged in 2015 from observations in 2009.
In August 2009, Saturn reached equinox, a multi-day period when the sun’s rays illuminated the planet’s rings edge-on – providing an extraordinary opportunity for Cassini to observe short-lived changes in the rings that revealed details about their nature.
This year, scientists reported that one section of the rings – the A ring: the outermost of the large, main rings – had a higher temperature during equinox than the rest of the rings.
“We can’t learn much about what Saturn’s ring particles are like deeper than 1 millimeter below the surface. But the fact that one part of the rings didn’t cool as expected allowed us to model what they might be like on the inside,” said Ryuji Morishima of NASA’s JPL.
And the results were puzzling.
Previous studies had shown Saturn’s icy ring particles to be coated by a fluffy regolith created over time as tiny impacts pulverize the surface of each particle.
However, the A ring is now understood to be composed largely of particles roughly 3 feet (1 meter) wide made of mostly solid ice, with only a thin coating of regolith.
“A high concentration of dense, solid ice chunks in this one region of Saturn’s rings is unexpected,” said Morishima.
The accumulation of dense ring particles in one place suggests that some process either placed the particles there in the recent geologic past or that the particles are somehow being confined there.
One possible explanation for this is that a moon may have existed at this location within the past hundred million years before it was destroyed. Thus, its debris would not yet have had enough time to diffuse evenly throughout the ring.
Another explanation is that rubble-pile moonlets are transporting the dense, icy particles as they migrate within the ring, with the moonlets dispersing the icy chunks in the middle A ring as they break up there under the gravitational influence of Saturn and its larger moons.
All of this could suggest that the A ring is much younger than all of the other rings of Saturn, something Cassini will directly measure in the final two years of its mission.
Nonetheless, despite increases in knowledge of the Saturnian system and the new discoveries made by Cassini this year, 2015 also represented a series of lasts for the mission.
Particularly, the year saw Cassini’s final close encounters with the moons Hyperion and Dione, on 31 May and 17 August, respectively.
But perhaps the most important final encounter for Cassini in 2015 was a series of three flybys of the ice moon Enceladus.
The first of the three close fly-bys occurred on 14 October and provided the first opportunity for close-up examination of Enceladus’ north polar region.
The encounter specifically targeted the north polar region to permit the study of that pole during the moon’s summer months, thus allowing Cassini to look for signs of ancient geologic activity similar to the geyser-spouting, tiger-stripe fractures at the opposite pole.
The first flybys helped set up the second of the three final close encounters on 28 October.
That second flyby saw Cassini dive just 49 km (30 miles) above Enceladus’ surface and plunge directly into the deepest accessible parts of the moon’s southern ejecta plumes to collect valuable scientific data regarding Enceladus’ subterranean salt water ocean.
Following the 28 October flyby, Cassini made its final encounter with Enceladus on 19 December, passing 4,999 km (3,106 miles) from the surface.
“This final Enceladus flyby elicits feelings of both sadness and triumph,” said Earl Maize, Cassini project manager at JPL.
“While we’re sad to have the close flybys behind us, we’ve placed the capstone on an incredible decade of investigating one of the most intriguing bodies in the solar system.”
The final flyby was the 22nd encounter of Enceladus by Cassini, capping an unprecedented streak of geologic discovery that has shocked many scientists and catapulted Enceladus to near top of the list of places in the solar system that could potentially harbor life.
“Cassini has made so many breathtaking discoveries about Enceladus, yet so much more remains to be done to answer that pivotal question, ‘Does this tiny ocean world harbor life?’” said Linda Spilker, the mission’s project scientist at JPL.
While the close encounters of Enceladus are behind it, Cassini will continue to monitor the icy world from a distance over the next two years as it completes its grand tour of the Saturnian system before its mission comes to an end in September 2017.
(Click here for part one of NASASpaceflight.com’s Year In Review for 2015.)
(Part three of NASASpaceflight.com’s four part Year In Review series for 2015 will be published in the coming days.)