NASA's Dawn spacecraft recently marked its one-year anniversary of becoming the first man-made object to enter orbit around a dwarf planet. Join us as we take a look at the spacecraft's many achievements in shedding light on what had previously been a little known member of our solar system, Ceres.
Launched on Sep. 27, 2007, Dawn was tasked with rendezvousing and characterizing the asteroid Vesta and the dwarf planet Ceres, with the primary goal of providing insights into the nature of our solar system and the process by which it was created. The spacecraft utilizes three identical zenon-fuelled ion engines for propulsion, and is sustained by a twin solar array that when deployed gives the spacecraft an impressive 65 ft (19.7 m) wingspan.
These systems, and many others, kept Dawn operational over the course of a 3.1 billion-mile (4.9 billion-km) journey that included a helpful gravity assist from Jupiter, and a historic visit to Vesta, culminating in arrival at its final target and eventual resting place, Ceres.
The successful journey to not one, but two distinct solar system bodies stands as testament to the skill of every engineer and scientist involved in the ambitious program. Since launch, Dawn has been under the care of a team of dedicated mission specialists based on the eighth floor of the Jet Propulsion Laboratory's building 264 in Pasadina, California.
Following the loss of two of the probe's four initial reaction wheels, the team was forced to improvise a fuel saving method of orientating the spacecraft relying heavily on Dawn's hydrazine supply. Despite this unexpected complication, Dawn's mission operators were able to navigate the void of space, manipulating the spacecraft into Ceres' gravitational embrace.
Prior to achieving orbit, Dawn was already making history by providing mankind with views of Ceres far surpassing any previous image snapped by the Hubble Space Telescope. As the spacecraft sped towards the dwarf planet, one feature in particular began to move in to sharper focus.
Previous Hubble observations had highlighted the presence of an unusual bright spot marking the surface of Ceres, but the resolution of the telescope wasn't high enough to determine its nature. As Ceres drew nearer, the phenomenon revealed itself as a series of bright spots located in what is now known as the Occator crater. The feature naturally sparked widespread debate as to the source of the brightness, and of course drew the attention of more than a few conspiracy theorists.
On Mar. 6, 2015, roughly seven and a half years after lift-off, Dawn finally achieved orbit around Ceres. However, whilst captured in orbit, the momentum of the spacecraft approaching the dwarf planet left it in a highly eccentric orbit, requiring the probe to adjust its orbit. Around five and a half weeks of manouvering later, Dawn was sitting in its its first mapping orbit some 8,400 miles (13,500 km) above the surface of the dwarf planet.
Ceres' Occator crater captured from a height of 4,500 miles (7,200 km) captured as Dawn transitioned to its second mapping orbit
Once established in this initial science orbit, the spacecraft set about transmitting data harvested by the probe's sophisticated suite of onboard instruments, allowing NASA scientists to set about compiling the first detailed maps of Ceres' surface. These maps highlighted the barren, cratered surface of the wandering planetoid, hinting at a surprisingly active geological history.
An early analysis of Ceres led scientists to estimate that 25 percent of the mass of the planetoid was comprised of water ice. Furthermore the formation of craters on Ceres in terms of depth and diameter shared many characteristics with the two icy Saturnian satellites, Dione and Tethys.
Meanwhile, scientists continued to pour over increasingly detailed views of the 130-plus white spots that are now known to sporadically mark Ceres' surface. The patches observed in the Occator crater during Dawn's approach remained the brightest examples of the phenomena, with theories on their composition ranging from the presence of cryovolcanoes to vast, sunlight-reflecting salt deposits.
A later study published in the online journal Nature in Dec. 2015 would identify the bright material as type of magnesium sulfate called hexahydrite. The researchers asserted that there could be a global subsurface layer of briny water-ice, which either sublimated leaving behind the bright residue, or was dug up by meteor impacts.
Another paper pubished in the same journal focussed on the detection of ammonia rich clays by Dawn's Visible and Infrared Mapping Spectrometer. The detection of the clays suggested that either Ceres originated outside of the main asteroid belt between Mars and Jupiter (where it is located today), or that the material originated from farther out and subsequently was transferred to Ceres via asteroid strikes.
Over the course of the year Dawn would spiral ever closer to the dwarf planet, first dropping to a height of 2,700 miles (4,400 km), and then into its third mapping orbit 900 miles (1,500 km) above Ceres. Dawn settled into its fourth and final mapping orbit in mid-December last year at a height of 235 miles (380 km), where it remains today.
In mid-June, as Dawn progressed through its second orbit, NASA scientists were taken aback by the discovery of a solitary peak towering over the surrounding terrain. The formation reached an impressive 3 miles (5 km) towards space, and was marked on its sides by an uneven covering of bright material bearing a striking resemblance to the deposits phenomena first discovered in the Occator crater upon Dawn's initial approach to Ceres.
As Dawn moved closer to the surface, it became apparent that the mountain, tentatively named Ahuna Mons, may not be the only example of a peak-like formation. Dawn's science team and planetary scientists across the globe have yet to figure out the geological process responsible for creating Ahuna Mons and its somewhat less defined cousins.
Color-coded elevation map of Ceres' surface, complete with the names of geological features approved by The International Astronomical Union
In its third mapping orbit, Dawn took 11 days and 14 complete orbits to image the entirety of Ceres' surface at a resolution of roughly 450 ft (140 m) per pixel. In its final mapping orbit, also known as the Low Altitude Mapping Orbit (LAMO), the probe was capable of imaging the planetoid with a far superior resolution of 120 ft (35 m) per pixel.
In much the same way that image resolution has increased with proximity, the spacecraft's other instruments, such as Dawn's Visible and Infrared Mapping Spectrometer, have been able to collect more accurate data concerning the distribution of minerals across the surface of the dwarf planet.
Furthermore, since achieving orbit, Dawn has been engaged in continuously observing the quirks of Ceres' electromagnetic field, while images from the spacecraft's framing camera have allowed the science team to create a series of 3D representations and fly-overs of the planetoid's surface.
Whilst there are still many mysteries surrounding Ceres, the data collected by Dawn will undoubtedly succeed in shedding light on the composition and formation of our solar system, and maybe, just maybe we can figure out what created the (at this point annoyingly enigmatic) bright spots.
Composite image of Ahuna Mons captured by Dawn's framing camera with a resolution of 120 ft (35 m) per pixel.
Dawn's primary mission is set to continue up until the end of June this year. The limiting factor to the probe's lifespan lies with its quickly diminishing supply of hydrazine fuel. Once fully expended, the spacecraft will be unable to control its orientation in space. Having run dry, the then inert probe would be incapable of pointing its antenna to Earth in order to receive or relay information, or to angle its solar arrays to collect the nourishing sunlight that has thus far kept it operational for three quarters of a decade.
Once this inevitability occurs, Dawn's batteries will run dry within a matter of hours, leaving the derelict spacecraft to orbit Ceres for a further 50 years before finally smashing into the surface of the dwarf planet.
What will remain is Dawn's undying legacy – a veritable treasure trove of data, and thousands upon thousands of breathtaking images of a strange little world that, before the spacecraft's arrival last year, had been little more than a blur.