Eight and a half years after its launch from Florida, NASA’s Dawn spacecraft celebrated its one year anniversary at the dwarf planet Ceres on Sunday. The craft’s anniversary follows its investigations at the protoplanet Vesta in the asteroid belt and an ongoing series of discoveries regarding Ceres, the closest dwarf planet to Earth.
Dawn – first spacecraft to orbit two celestial bodies:
Launched aboard a Delta II rocket on 27 September 2007 at sunrise, NASA’s Dawn spacecraft was hurtled from Earth toward the asteroid belt on an extremely unique mission for not just the U.S. space agency but for spaceflight in general.
Specifically, Dawn was built to be the first spacecraft to investigate two different celestial bodies in the asteroid belt and become the first spacecraft ever to enter orbit of two gravitationally different bodies.
This unique ability was possible because of an ion propulsion drive that has powered Dawn and allowed for the velocity changes needed to enter orbit of one body, leave orbit of that body, travel the vast distances of the asteroid belt, and then enter orbit of another.
Moreover, just over one year prior to the mission’s launch, the International Astronomical Union officially designated Ceres as one of the known dwarf planets in the solar system at the time.
With the anticipated mission timelines of Dawn and New Horizons, it was quickly realized that, should all go according to plan, Dawn would become the first human object to investigate a dwarf planet at close proximity and, regardless, would become the first spacecraft to perform long duration, in-situ operations around a dwarf planet.
After a near four year journey, Dawn entered orbit of its first target – Vesta – on 16 July 2011.
After over one year of orbital operations at Vesta, Dawn fired its ion engine on 5 September 2012 to begin its historic journey toward Ceres.
Dawn arrives at Ceres:
After a just over two years of cruise operations, Dawn began unofficial proximity operations with Ceres on 1 December 2014 with the imaging of the dwarf planet’s extended disc.
Partial rotation images of Ceres were captured on 13 and 25 January 2015, and on 26 January, Dawn’s cameras exceeded the resolution capabilities of the Hubble Space Telescope in terms of Ceres observations.
On 12 and 19 February, two full rotation observation photographic sessions took place as part of the long-range global mapping campaign for Ceres.
During this campaign, mission scientists were greeted with their first of several unexpected surprises at Ceres – the appearance of a white glimmering area on the surface, which was initially thought to indicate the potential presence of a liquid or other highly reflective material on the dwarf planet’s surface.
Then, at 5:36 a.m. PST (8:36 a.m. EST) on 6 March 2015, mission controllers, who were not in communication with Dawn due to its orientation needs for the orbital insertion burn at Ceres, received the signal from the spacecraft that Dawn was healthy and had entered orbit as planned.
With the signal, Dawn became the first spacecraft to enter orbit of a second gravitationally different body and historically became the first spacecraft to perform proximity operations with and observations of a dwarf planet, entering orbit of Ceres four months prior to the New Horizons probe’s flyby of Pluto.
Speaking one year ago, Chris Russell, principal investigator of the Dawn mission at the University of California, Los Angeles said “We feel exhilarated. We have much to do over the next year and a half, but we are now on station with ample reserves, and a robust plan to obtain our science objectives.”
One year at Ceres:
After being captured into Ceres’ gravity, Dawn continued to fire its ion engine to reduce its orbital altitude and circularize its orbit to one of 13,500 kilometers (8,400 miles) above the surface of the dwarf planet.
This operation was completed on 23 April, at which point Dawn entered its first science orbit and began its intensive primary science campaign at Ceres.
By 13 April, scientists and investigators with Dawn had completed a color map of Ceres’ surface, revealing differences in morphology and colors across the dwarf planet’s surface that indicated that Ceres was once a geologically active body.
In addition to the color map, Dawn’s onboard cameras also revealed that the dwarf planet was far less cratered than expected, also lending evidence to the hypothesis that Ceres was once active.
“This dwarf planet was not just an inert rock throughout its history. It was active, with processes that resulted in different materials in different regions. We are beginning to capture that diversity in our color images,” said Mr. Russell.
Moreover, this color map revealed that the two bright spots captured by Dawn’s cameras during its approach to Ceres were located in cooler regions of the dwarf planet.
On 23 April, Dawn arrived at its 13,500 km circular orbit and began its first primary science phase – a 15-day, one orbit mapping mission around Ceres.
During this mapping phase, on 3 and 4 May, the spacecraft was able to image the two bright reflective spots in the Northern Hemisphere crater of the dwarf planet.
From this altitude, Dawn’s cameras were able to clearly discern that the two bright regions were composed of smaller bright and highly reflective objects, though their exact nature remained elusive.
By 8 May, Dawn completed its first science phase and, on 9 May, began firing its ion engine to perform a month-long orbital descent to its second primary mapping orbit just 4,400 km (2,700 miles) above the surface of Ceres.
This maneuver was completed on 3 June, brought Dawn three times closer to the surface of Ceres then its previous orbit, took the craft’s orbital period from 15 days to 3 days, and marked the commencement of the second primary science phase of the mission.
After arriving in this new orbit, one of the first features Dawn’s cameras were able to capture were the two bright spots that had captivated scientists and the general public for months.
Under the new set of images, the region of bright spots was revealed to be about 90 km (55 miles) across, with each spot consisting of numerous individual bright points of different sizes.
But again, no obvious explanation for these bright spots was understood from the new images returned by Dawn.
“The bright spots in this configuration make Ceres unique from anything we’ve seen before in the solar system,” said Mr. Russell
“The science team is working to understand their source. Reflection from ice is the leading candidate in my mind, but the team continues to consider alternate possibilities, such as salt.
“With closer views from the new orbit and multiple view angles, we soon will be better able to determine the nature of this enigmatic phenomenon.”
This determination is possible with Dawn’s visible and infrared mapping spectrometer, which can identify specific minerals present on Ceres by looking at how light is reflected as each mineral reflects the range of visible and infrared-light wavelengths in a unique way.
In addition to the two bright spots, the first views of Ceres from this new orbit indicated that activity on the surface of the dwarf planet included flows, landslides, and various collapsed structures.
“The surface of Ceres has revealed many interesting and unique features,” said Carol Raymond, deputy principal investigator for the Dawn mission at NASA’s Jet Propulsion Laboratory.
“For example, icy moons in the outer solar system have craters with central pits, but on Ceres, central pits in large craters are much more common. These and other features will allow us to understand the inner structure of Ceres that we cannot sense directly.”
Likewise, Paul Schenk, a geologist at the Lunar and Planetary Institute and member of the Dawn science team, stated that “The craters we find on Ceres, in terms of their depth and diameter, are very similar to what we see on Dione and Tethys, two icy satellites of Saturn that are about the same size and density as Ceres.
“The features are pretty consistent with an ice-rich crust.”
After completing its second primary science phase, Dawn fired its ion engine on 30 June to begin a multi-week maneuver to lower its orbit even closer to Ceres’ surface.
However, shortly after this procedure began, protective software inside the spacecraft detected an anomaly and shut down the engine and placed the vehicle in safe mode.
Across the first two days of July, engineers made configuration changes to Dawn and returned the spacecraft to normal operations.
It was immediately decided that Dawn would remain in its present orbit until engineers could determine precisely what happened and update the craft’s flight plan.
It was later discovered that the cause of the safe mode was a mechanical gimbal system, used to control the spacecraft’s orientation during ion thrusting, for ion engine #3.
However, since Dawn only uses one of its three ion engines at a time, the teams switched to ion engine #2, mounted to a different gimbal, and conducted a series of tests on the 14th and 16th of July.
These tests confirmed that Dawn was ready to perform its orbit lowering maneuver to its third science phase orbit, which began on 17 July.
This five-week burn lowered the spacecraft’s altitude to just 1,450 km (900 miles) – known as the High-Altitude Mapping Orbit (HAMO).
Dawn reached its third science phase orbit in August and began a part of its mission which involved capturing full surface images of Ceres over the course of 11 days and 14 orbits.
The 11-day, 14-orbit cycle was repeated consistently over two months and allowed Dawn to compile six complete surface maps of the dwarf planet at a resolution of 140 m (450 ft) per pixel.
In addition to surface mapping operations, a second high priority aspect of the HAMO phase of the mission was to completely characterize Ceres’ gravity field to help mission planners design the lowest phase of Dawn’s orbital journey.
Also during the HAMO phase, a “surprising bonus,” as described by Dawn’s teams, came from the spacecraft’s gamma ray and neutron spectrometer, which detected three bursts of energetic electrons that may result from the interaction between Ceres and radiation from the sun.
“This is a very unexpected observation for which we are now testing hypotheses,” said Mr. Russell.
By the beginning of October, the third science and HAMO aspect of the mission was complete, and Dawn fired its ion engine again to lower itself to its lowest and final orbit of the mission, just 375 km (230 miles) above the surface of Ceres.
The seven-week burn began on 23 October, and by early December, Dawn was in a 385 km (240 mile) orbit, providing image resolution of 35 m (120 ft) per pixel.
Among the striking first views obtained from this altitude – known as the Low-Altitude Mapping Orbit (LAMO) – were a chain of craters called Gerber Catena, located just west of the large crater Urvara as well as a series of troughs.
While troughs are common on larger planetary bodies and are caused by contraction, impact stresses and the loading of the crust by large mountains, they are not commonly seen on smaller planetary bodies.
Thus, the fracturing found all across Ceres’ surface indicates that similar processes may have occurred on this dwarf planet, despite its small size.
However, just after this orbit lowering maneuver ended, scientists using data gathered in the previous nine months of Dawn’s operations at Ceres revealed a possible reason for the bright spots that pepper the dwarf planet.
In a study that appeared in the journal Nature, study authors, led by Andreas Nathues at Max Planck Institute for Solar System Research, Göttingen, Germany, stated that the bright material was consistent with a type of magnesium sulfate called hexahydrite.
A different type of magnesium sulfate than what is present on Ceres is commonly known on Earth as Epsom salt.
The study suggests that these salt-rich areas were left behind when water-ice sublimated as impacts from asteroids would have unearthed the mixture of ice and salt, which there is currently no evidence of.
Furthermore, “The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” Nathues said.
This seems to link with another aspect of the same study which found a diffuse haze near the surface of Occator crater – where the two now-famous bright spots are located – that fills the floor of the crater.
The study suggests that this may be associated with observations of water vapor at Ceres by the Herschel Space Observatory first detected in 2014.
The haze is present in views during noon, local time, and absent at dawn and dusk, suggesting that the phenomenon resembles activity at the surface of a comet, with water vapor lifting tiny particles of dust and residual ice.
As 2015 drew to a close, Dawn, its mission controllers, and scientists paused to mark the 215th anniversary of the discovery of Ceres on 1 January 1801.
Now firmly in the final aspect of its mission, Dawn is expected to continue operations until at least mid-2016.
At present, there are no plans to crash Dawn onto the surface of Ceres. Instead, it is understood that mission planners will ensure that Dawn is in a perpetually stable orbit of Ceres.
Thus, when Dawn’s mission officially ends sometime this year, the spacecraft will become a permanent artificial satellite of the dwarf planet in the asteroid belt.
(Images: NASA. To join L2, click here: http://www.nasaspaceflight.com/l2/)