Moving out into the asteroid belt and farther still to the gas giants, NASA’s Dawn, Juno, and Cassini missions churned away in orbit of their respective hosts – with Dawn continuing an up-close-and-personal investigation of the dwarf planet Ceres, Juno’s fantastically accurate arrival at Jupiter but frustratingly crippled start to its science mission, and the beginning of the end for the Cassini mission that’s set to end in September 2017.
Dawn – Revealing dwarf planet Ceres:
Launched on 27 September 2007, Dawn became the first spacecraft to enter orbit of two completely different celestial bodies on 6 March 2015 when it slipped into orbit of the dwarf planet Ceres after first spending 14 months in orbit of the protoplanet Vesta.
This year, Dawn marked the first anniversary of its arrival at Ceres, and Dawn’s deputy principal investigator, Carol Raymond, stated that “Ceres has defied our expectations and surprised us in many ways thanks to a year’s worth of data from Dawn. We are hard at work on the mysteries the spacecraft has presented to us.”
Among Ceres’ most enigmatic features is the mountain Ahuna Mons, which appeared as a small, bright-sided bump as seen by Dawn’s camera as early as February 2015 from a distance of 46,000 km (29,000 mi) before the spacecraft entered orbit.
As Dawn gradually lowered its orbital altitude over its first year, the shape of this mysterious feature came into focus.
From afar, Ahuna Mons looked pyramid-shaped, but upon closer inspection, it is best described as a dome with smooth, steep walls.
Dawn’s latest images of Ahuna Mons, taken 120 times closer than in February 2015, reveal a significant amount of bright material on some of the mountain’s slopes, and less on others.
“No one expected a mountain on Ceres, especially one like Ahuna Mons,” said Chris Russell, Dawn’s principal investigator. “We still do not have a satisfactory model to explain how it formed.”
But Ahuna Mons isn’t the only feature on Ceres that interests scientists.
About 670 km (420 mi) northwest of Ahuna Mons is Occator Crater – which the Hubble Space Telescope revealed to have a prominent bright patch on its surface prior to Dawn’s arrival.
Dawn’s subsequent orbital observations have revealed that there are at least 10 bright spots in this crater alone, with the brightest area on Ceres located in the center of Occator.
“Dawn began mapping Ceres at its lowest altitude in December, but it wasn’t until very recently that its orbital path allowed it to view Occator’s brightest area,” said Marc Rayman, Dawn’s chief engineer and mission director.
By late-April, Dawn returned stunning new images from its low-altitude (385 km – 240 mi) mapping orbit of the dwarf planet’s numerous bright material craters.
In particular, Dawn’s view of Haulani Crater, with a diameter of 34 km (21 mi), revealed evidence of landslides from its crater rim – indications that the crater is a relatively new formation.
“Haulani perfectly displays the properties we would expect from a fresh impact into the surface of Ceres,” said Martin Hoffmann, co-investigator on the Dawn framing camera team.
The crater’s polygonal structure is also noteworthy as most craters on planetary bodies are nearly circular, but the unique straight edges of some Cerean craters, including Haulani, are due to pre-existing stress patterns and faults beneath the surface.
Moreover, another crater, Oxo, also presents a uniqueness in that its rim is slumped – indicating an area where material has dropped below the surface – and that its crater floor contains minerals observed nowhere else on Ceres’ surface.
However, a big focused remained on Occator crater, and by mid-year, Dawn had finally returned enough information about it that scientists were gaining a better understanding of its composition.
At the end of June, NASA announced findings that Occator’s bright areas contain the highest concentration of carbonate minerals ever seen outside Earth.
“This is the first time we see this kind of material elsewhere in the solar system in such a large amount,” said Maria Cristina De Sanctis, principal investigator of Dawn’s visible and infrared mapping spectrometer.
Specifically, the dominant mineral of this bright area is sodium carbonate, a salt found on Earth in hydrothermal environments.
On Ceres, the material appears to have come from inside the dwarf planet, having been lifted to the surface by an impacting asteroid – which suggests that temperatures inside Ceres are warmer than previously believed.
More intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geologic time and that the salts could be remnants of an ocean, or localized bodies of water, that reached the surface and then froze millions of years ago.
“The minerals we’ve found at the Occator central bright area require alteration by water,” De Sanctis said. “Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator.”
This discovery announcement came just one day before Dawn completed its primary mission on 30 June.
At this time, Dawn had taken 69,000 images, completed 48,000 hours of ion engine thrusting, collected more than 132 GB of science data, completed 2,450 orbits of Vesta and Ceres, travelled 3.5 billion miles since launch, and explored two new worlds.
On 1 July, Dawn entered its extended mission, which will see the craft continue to operate in Ceres orbit into 2017 – at which point, due to its highly stable orbit of the dwarf planet, it will become a permanent artificial satellite of Ceres.
By the end of July, Dawn had returned information that helped scientists start to answer the question of what happened to all of Ceres large impact craters.
Presently, Ceres is covered in countless small, young craters, but none are larger than 280 km (175 mi) in diameter. To scientists, this is a rather large mystery given that the dwarf planet must have been hit by numerous large asteroids during its 4.5 billion-year lifetime.
“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres’ peculiar composition and internal evolution,” said Simone Marchi, a senior research scientist at the Southwest Research Institute.
Marchi and her colleagues modeled collisions of other bodies with Ceres since the dwarf planet’s formation, and these models predicted that Ceres should have up to 10 to 15 craters larger than 400 km (250 mi) in diameter, and at least 40 craters larger than 100 km (60 mi) wide.
However, Dawn has shown that Ceres has only 16 craters larger than 100 km, and none larger than 280 km across.
“Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years,” Marchi said.
One potential reason for the lack of large craters could be related to Ceres’ interior structure.
Specifically, since Ceres’ upper layers contain ice and salts – which are less dense than rock – the topography could “relax,” or smooth out, more quickly if ice or salt dominates the subsurface composition.
Moreover, past hydrothermal activity, which may have influenced the rising of salts to the surface at Occator Crater could also have something to do with the erasure of craters.
If Ceres had widespread cryovolcanic activity in the past, the ejected cryogenic materials could have flowed across the surface and possibly buried pre-existing large craters.
However, its wasn’t just Ceres’ surface features that scientists learned more about this year.
In August, a careful study of minute changes in Dawn’s orbit from the first year of its orbital mission helped scientists gain a better understanding of Ceres’ gravity field – and therefore its internal composition.
“The data suggests that Ceres has a weak interior, and that water and other light materials partially separated from rock during a heating phase early in its history,” said Ryan Park, supervisor of the solar system dynamics group at JPL.
Among the things confirmed about Ceres in this data return is that Ceres has hydrostatic equilibrium – meaning its interior is weak enough that its shape is governed by how the dwarf planet rotates.
This confirmation validated one of the reasons why the International Astronomical Union classified Ceres as a dwarf planet in 2006.
Moreover, the data indicate that Ceres is differentiated – meaning it has compositionally distinct layers at different depths, with the densest layer at the core.
Scientists were also able to confirm that Ceres is much less dense than Earth, the Moon, Vesta, and other rocky bodies in our solar system.
The data also led scientists to conclude that Ceres’ weak mantle can be pushed aside by the mass of mountains and other high topography in its outermost layer – as though the high-elevation areas “float” on the material below.
Overall, by combining this new information with previous data from Dawn about Ceres’ surface composition, scientist are beginning to reconstruct Ceres’ history – in which water must have been mobile in the ancient subsurface while the interior did not heat up to the temperatures at which silicates melt and a metallic core forms.
Following this announcement, Dawn controllers began maneuvering the spacecraft into its higher, mission extension orbit in early September.
Dawn had been – for eight months – in its low-altitude science orbit, but due to its mission extension and limited supply of hydrazine for orientation operations, controllers decided to raise Dawn’s orbit for its extended mission so that the hydrazine can be used more sparingly.
“Most spacecraft wouldn’t be able to change their orbital altitude so easily. But thanks to Dawn’s uniquely capable ion propulsion system, we can maneuver the ship to get the greatest scientific return from the remaining mission,” said Marc Rayman.
The orbit raising maneuver, which began from an altitude of 385 km (240 mi), will push Dawn to 1,460 km (910 mi) above Ceres’ surface – just about the orbit in which Dawn first slid into orbit around the dwarf planet.
Also in September, Dawn scientists released information on a possible detection of a temporary atmosphere around the dwarf planet.
The surprising finding emerged after Dawn’s Gamma Ray and Neutron Detector (GRaND) observed evidence that Ceres had accelerated electrons from the solar wind to very high energies over a period of six days.
In theory, the interaction between the solar wind’s energetic particles and atmospheric molecules could explain the GRaND observations.
A temporary atmosphere would also be consistent with water vapor detections via the Herschel Space Observatory in 2012-2013.
The electrons that GRaND detected could have been produced by the solar wind hitting the water molecules that Herschel observed, but scientists are also looking into alternative explanations.
“We’re very excited to follow up on this and the other discoveries about this fascinating world,” Russell said.
Juno – Triumphant arrival, less than stellar start to science mission:
Originally, Juno was to complete two of these 53.4-day orbits before performing a perijove burn on 19 October that would have altered its orbit to the pre-mission determined 14-day science orbit.
However, just a few days before this scheduled burn, controllers noticed a performance issue with a pair of valves that are part of Juno’s fuel pressurization system.
At the time, Rick Nybakken, Juno project manager, said, “Telemetry indicates that two helium check valves that play an important role in the firing of the spacecraft’s main engine did not operate as expected during a command sequence.
“The valves should have opened in a few seconds, but it took several minutes.”
Controllers subsequently delayed the planned orbit adjustment burn to allow time to study the issue.
As of writing, controllers have still not determined a best way forward and are currently investigating the valves’ potential link to similar failures on Akatsuki and an Intelsat satellite.
For Juno, this meant more orbits of Jupiter in its longer orbit.
Currently, the spacecraft has completed just three close flybys of Jupiter – not counting the flyby that occurred on the night of its arrival.
The third flyby occurred on 11 December, with the fourth now slated for 2 February 2017.
For comparison, when the third flyby occurred on 11 December, Juno should have been gearing up for its fifth flyby.
Now, if controllers are unable or unwilling to perform the orbit adjustment burn, the effects on the science mission as well as the mission’s planned duration are somewhat unknown.
What is known is that the amount and quality of science collected during a close flyby is not affected by the prolonged orbit – which has a much greater apojove than the standard science orbit would but a nearly identical perijove.
Nonetheless, NASA has been quiet on the effect the prolonged orbit might have on the science collected at other points in the orbit and the effect the prolonged exposure to Jupiter’s harsh radiation field will have on the craft’s instruments – which now receive just under 4 times the amount of radiation exposure between scientific close flybys of Jupiter.
Moreover, Juno’s mission is slated to only last until February 2018 – at which time it is anticipated that Juno will have to be deorbited into Jupiter’s atmosphere due to system failures triggered by the intense radiation.
Additionally, Juno mission directives call for a minimum of 7 to 10 operational flybys of Jupiter to achieve minimum mission success.
Given the safe mode pre-approach profile flown in October, the first operational flyby did not occur until 11 December – though even this wasn’t a fully operational flyby as a critical instrument, the Jovian Infrared Auroral Mapper (JIRAM), was not active due to the need to upload a software patch to allow Juno’s software to process information from JIRAM.
At present, if Juno is forced to remain in its 53.4-day orbit and if it still needs to be purposefully disposed of into Jupiter’s atmosphere in February 2018, the mission stands a high chance of not actually or just barely meeting minimum mission success criteria.
Cassini – 19 years after launch, the intrepid little probe prepares for its Grand Finale exit:
The first major event for Cassini this year was a carefully choreographed observation of Enceladus as it occulted – passed in front of, as viewed from a specific location – the star Epsilon Orionis, the central star in Orion’s belt.
Previous Cassini observations of Enceladus saw its polar eruptions spraying three times as much icy dust into space when the moon neared aposaturnium – farthest point in its elliptical orbit around Saturn.
But scientists hadn’t had an opportunity to see if the gas part of the eruptions – which account for the majority of the plume’s mass – also increased at this time.
They got that chance on 11 March… and the results were surprising.
During a carefully planned observation, Cassini set its gaze on Epsilon Orionis, and at the appointed time, Enceladus – roughly at aposaturnium – and its erupting plume glided in front of the star.
Cassini’s Ultraviolet Imaging Spectrometer (UVIS) measured how water vapor in the plume dimmed Epsilon Orionis’ ultraviolet light, thereby revealing how much gas the plume contained.
Since lots of extra icy dust appears at this point in Enceladus’ orbit, scientists expected to measure a lot more gas in the plume.
But instead of the expected large increase in gas output, UVIS only saw a bump of 20% in the total amount of gas.
“We went after the most obvious explanation first, but the data told us we needed to look deeper,” said Cassini scientist Candy Hansen.
This led Hansen and her colleagues to focus on one of Enceladus’ ejecta-spewing jets that was discovered to be four times more active than anticipated – producing 8% of the occultation-observed plume’s total gas instead of just 2% as predicted.
Thus, the occultation observation revealed that at least some of the narrow jets that erupt from the moon’s surface blast with increased fury when the moon is at aposaturnium – but why the gas in the plume was so much less than anticipated is still a mystery.
However, the new observations provide helpful insights on what could be going on with the underground plumbing – cracks and fissures through which water from the moon’s potentially habitable subsurface ocean makes its way into space.
“We had thought the amount of water vapor in the overall plume, across the whole south polar area, was being strongly affected by tidal forces from Saturn. Instead we find that the small-scale jets are what’s changing,” said Larry Esposito, UVIS team lead
Following this observation, the first major scientific release from previously gathered Cassini data occurred in April when scientists announced confirmation of a 2014 discovery that the Ligeia Mare sea on Titan was composed primarily of liquid methane.
“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart,” said Alice Le Gall, a Cassini radar team associate.
“Instead, this sea is predominantly made of pure methane.”
The confirmation of the 2014 discovery came from data collected with Cassini’s radar during flybys of Titan between 2007 and 2015.
But with this discovery – as do most things in science – came a host of new questions, one being how the methane in the lake is replenished.
“Either Ligeia Mare is replenished by fresh methane rainfall or something is removing ethane from it,” said Le Gall. “It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare. But that will require further investigation.”
The same data also revealed that the shoreline of Ligeia Mare may be porous and flooded with liquid hydrocarbons.
This hypothesis comes from Cassini data that did not show any significant difference between the sea’s temperature and that of the shore throughout the local winter to spring timeframe.
Scientists had expected that – like on Earth – the surrounding solid terrains would warm more rapidly than the sea.
“It’s a marvelous feat of exploration that we’re doing extraterrestrial oceanography on an alien moon,” said Steve Wall, deputy lead of the Cassini radar team at JPL. “Titan just won’t stop surprising us.”
And the moon did not stop surprising throughout the year.
On 25 July, Cassini performed its 122nd close flyby of Titan, observing the moon’s long, linear dunes, thought to be comprised of grains derived from hydrocarbons that have settled out of Titan’s atmosphere.
Cassini has shown that dunes of this sort encircle most of Titan’s equator, and scientists use the dunes to learn about winds, the sands the dunes are composed of, and topographical changes.
The flyby also allowed Cassini to investigate a mysterious region known as Xanadu Annex.
The main Xanadu region was first imaged in 1994 by the Hubble Space Telescope and was the first surface feature recognized on Titan.
The new Cassini data revealed that the Xanadu Annex was composed of the same type of mountainous terrains observed in Xanadu.
Overall, though, Xanadu and its Annex remain something of a mystery as mountainous terrain elsewhere on Titan appears in small, isolated patches. But Xanadu covers a large area.
“These mountainous areas appear to be the oldest terrains on Titan, probably remnants of the icy crust before it was covered by organic sediments from the atmosphere,” said Rosaly Lopes, a Cassini radar team member at JPL.
Nonetheless, the observation of Xanadu and southern hemisphere terrain during Titan flyby 122 marked the final observation of southern hemisphere targets for Cassini.
In August, when Cassini flew by Titan again, it focused – as will the final three remaining flybys – on the northern hemisphere lake region.
During the August flyby, Cassini discovered deep, steep-sided canyons on Titan flooded with liquid hydrocarbons.
The discovery marked the first direct evidence of the presence of liquid-filled channels on Titan, as well as the first observation of canyons hundreds of meters deep.
Then, when Cassini returned to Titan on 30 November, the flyby was carefully choreographed to gently nudge Cassini into the initial phase of its penultimate orbit.
The nudge from Titan’s gravity altered Cassini’s orbit just enough to send the probe arcing high over and under Saturn’s poles and amazingly close to the outer-most edges of Saturn’s rings.
The new orbit resulted in an orbital period of seven days, and Cassini will perform 20 of these Ring-Grazing Orbits.
As Cassini moved toward completion of the first of these orbits and its first ring-graze, the craft fired its main engine for 6 seconds at 07:09 EST on 4 December – about one hour prior to the first ring-graze.
The engine burn completed Cassini’s orbit alteration to place the craft into the proper position for its penultimate 20-orbit mission.
At 08:09 EST that same day, Cassini grazed through the faint, outer-most F-ring of Saturn.
A few hours after the ring-plane crossing, Cassini began a complete scan across the rings with its radio science experiment to study the rings’ structure in great detail.
“It’s taken years of planning, but now that we’re finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” said Linda Spilker, Cassini project scientist at JPL.
“This is a remarkable time in what’s already been a thrilling journey.”
On 11 December, Cassini completed its second ring-graze dive, returning spectacular, up-close images of what is arguably the most majestic structure in the solar system.
But as amazing and scientifically rich as the images are, they are a reminder that all good things must come to an end.
“This is it, the beginning of the end of our historic exploration of Saturn. Let these images – and those to come – remind you that we’ve lived a bold and daring adventure around the solar system’s most magnificent planet,” said Carolyn Porco, Cassini imaging team lead at the Space Science Institute.
The current series of Ring-Grazing Orbits will set Cassini up for one last flyby of Titan on 22 April 2017 – a flyby that will mark the commencement of Cassini’s Grand Finale.
The final encounter with Titan, designed to cause Cassini’s orbit to jump over Saturn’s rings, will begin the 22 orbit Grand Finale sequence that will take Cassini into the 2,400 km (1,500 mi) gap between Saturn and the inner-most of its rings.
These 22 orbits will culminate on 15 September 2017 at 08:07 EDT (12:07 UTC) when Cassini plunges into Saturn’s atmosphere – transmitting back as much data as it can until Cassini bids its farewell.
(Part 4 – Pluto – of NASASpaceflight.com’s 5-part Year In Review will be published in the coming days)
(Images: NASA and L2 Artist Nathan Koga. The full gallery of Nathan’s L2 images can be *found here*)