Illustration of spacecraft

Artist's concept of Dawn approaching Ceres. Full image here.

10 Things to Know about Dawn

By Zoë Webb-Mack

Feature | July 16, 2018


    July 16 marked the 272nd birthday of Giuseppe Piazzi, who discovered dwarf planet Ceres in 1801. The second planetary discovery of modern times (Uranus was the first), Piazzi’s observation was incredibly exciting and expanded our knowledge of the solar system. But all he saw was a point of light -- little did he know that one day a spacecraft called Dawn would get a closer look! In honor of Piazzi, here’s a list of Dawn’s most exciting findings and accomplishments, over two centuries later.

    Illustration of Dawn approaching Ceres.
    An artist’s concept of Dawn approaching Ceres. Dawn’s ion propulsion system glows bright blue as it expels ionized xenon. Credit: NASA/JPL-Caltech.

    1. Dawn was propelled by a solar-powered ion propulsion system.

    Though ion propulsion was conceived of in the early 1900s and has long been a staple of science fiction, the technology had not been employed for science exploration until the latter half of the century. It works by accelerating ionized xenon between two charged grids with a high voltage before expelling it at very high speed.

    After almost six years of cumulative thrusting during the mission, Dawn’s ion engines thrusted for the last time on June 21, 2018.

    Ion propulsion is far more fuel efficient and much gentler than conventional chemical propulsion systems. It was shown to be viable in 1998 by NASA’s Deep Space 1 (DS1) mission, whose prime objective was to test advanced technologies for future use. Dawn used similar thrusters and eventually surpassed DS1’s record velocity change of 2.7 miles per second (4.3 kilometers per second), achieving an overall velocity change of 6.6 miles per second (10.7 kilometers per second). And because ion propulsion’s thrust is so gentle, the spacecraft is able to maneuver more gracefully while building up speed more gradually than with chemical propulsion.

    After almost six years of cumulative thrusting during the mission, Dawn’s ion engines thrusted for the last time on June 21, 2018.

    Diagram showing Dawn's path to Ceres and Vesta.
    A map of Dawn’s trajectory. Credit: NASA/JPL-Caltech.

    2. Dawn is the only mission to orbit two deep-space destinations.

    Enabled by the exceptional efficiency of ion propulsion, Dawn became the first spacecraft to orbit two deep-space bodies. This is an exciting engineering feat, demonstrating the feasibility of multi-world exploration. And by having a single set of instruments measure properties of both Vesta and Ceres, these properties, and thus the bodies themselves, can be compared in a more meaningful way than if they had been measured by different instruments on multiple spacecraft.

    Black and white image of a hemisphere on Ceres.
    A view of Ceres, the dwarf planet that Dawn explored in the main asteroid belt. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

    3. Dawn is the first mission to visit a dwarf planet.

    The main asteroid belt is a region in our solar system between Mars and Jupiter, containing millions of asteroids. While we may normally think of asteroids as pieces of rocky solar system debris, they’re actually far more interesting. Asteroids can have diameters as small as a boulder or as wide as Los Angeles. Asteroids are also diverse in composition: from metallic to carbon-rich. They can be rubble piles loosely held together by their own gravity, or they can be solid rocks. Some have histories of intense heating, while others have only experienced low-temperature alteration. The asteroid belt also straddles the “snow line,” which roughly separates the inner solar system (which contains mostly rocky bodies with warmer histories) from the outer solar system (mostly gaseous or icy bodies with cooler histories). Exploring bodies in this region can tell us how the solar system evolved.

    Dawn is the first mission to visit a dwarf planet, the class of celestial bodies that also includes Pluto, which the International Astronomical Union introduced in 2006. Dwarf planets tell us a lot about our solar system’s history, and when Dawn arrived at Ceres, it was the first spacecraft to study a dwarf planet up close.

    Artist's concept of asteroids colliding.
    This artist's conception shows how families of asteroids are created. Credit: NASA/JPL-Caltech

    4. The data that Dawn collects provide insight into the history of our solar system.

    The bodies in our solar system formed when the dust and other debris left over after the formation of our Sun came together. Some of these bodies gathered so much mass that they grew to become the major planets of our solar system. Some others remained much smaller, unable to form larger bodies because of the immense gravity of nearby Jupiter shuffling them around. Some of these bodies are more evolved than others. Vesta and Ceres in particular are examples of these “minor planets” because they were on their way to becoming planets when their development was halted. This effectively makes them time capsules from the period in our solar system’s history during which our planets formed. Data from Vesta and Ceres about chemical composition and internal structure help provide a clearer picture about the conditions during this fascinating period, around 4.6 billion years ago.

    Full view of asteroid Vesta.
    A mosaic of Vesta composed of images taken by Dawn’s Framing Camera. Vesta’s northern hemisphere is more densely cratered than its southern hemisphere because a large impact in what is now the south polar region erased the previous cratering record. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

    5. At Vesta, Dawn revealed previously unseen geological features and clarified our understanding of this rocky world.

    Before Dawn visited Vesta and Ceres, the best images we had of these bodies were taken from afar by NASA’s Hubble Space Telescope and telescopes on Earth. While Hubble saw the gigantic Rheasilvia Crater on Vesta, Dawn’s high-resolution imagery revealed much finer details.

    Three meteories
    Dawn’s data confirmed previous telescopic measurements that showed HED meteorites, like those pictured here, came from Vesta. Above are polarized light micrographs of two eucrite meteorites (left and center) and one diogenite meteorite (right). Credit: NASA/University of Tennessee.

    Dawn observed vast canyons near Vesta’s equator up to about 2.5 miles (4 kilometers) deep -- about 2.5 times the depth of the Grand Canyon -- formed by two colossal impacts that sent ripples through Vesta's interior. This is one of several lines of evidence validating that Vesta’s interior is differentiated into layers. Dawn discovered that Vesta’s northern hemisphere is much more densely cratered than its southern hemisphere, likely due to resurfacing resulting from the impact that formed Rheasilvia. Gravity measurements by Dawn confirmed that Vesta has a core and mantle beneath its crust, similar to the inner planets. Dawn also confirmed that howardite, eucrite and diogenite meteorites (HEDs), which together make up about 6 percent of the meteorites found on Earth, originated at Vesta, allowing Vesta’s chemical evolution to be studied in laboratories here on Earth. Likewise, dark carbonaceous material, also seen in the HED meteorites, was likely delivered to Vesta by asteroids or comets from the outer solar system, demonstrating the extent of material migration in the early solar system. Exploring Vesta’s crater walls (some of which are higher than Everest), deep canyons and magmatic history, Dawn illuminated Vesta’s striking geological features and expanded upon our previous understanding of the asteroid.

    Artist's concept cutaway view of Ceres interior.
    A possible model of Ceres’ differentiated interior with a strong crust, icy mantle, and rocky core.

    6. Less was known about Ceres, so Dawn was able to shed light on this icy, possible ocean world.

    Ceres held even more surprises than Vesta. Originally expected to have had a watery history with a clay-like surface and the possibility of water ice within, before Dawn it was unclear whether Ceres had differentiated into compositionally distinct interior layers. Through gravity measurements, Dawn discovered that Ceres is in fact differentiated, with a strong crust rich in ice, a rocky core, and potentially a thin liquid layer, the relic of an early ocean, between the two. What’s more, pre-Dawn images showed a giant bright spot on Ceres, which Dawn’s instruments revealed to be the reflection of sunlight off of salt deposits in a crater mission scientists named Occator, after the Roman agriculture deity of harrowing (a method of leveling soil). In total, Dawn found more than 300 bright spots, called faculae, sprinkled across Ceres’ surface -- many more than had appeared in those initial distant images. In another crater, Ernutet, Dawn found evidence of organics, raising questions about Ceres’ potential for advanced prebiotic chemistry.

    Computer generated view of mountain on Ceres.
    A side view of Ceres’ Ahuna Mons. Elevation exaggerated by a factor of two. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

    7. Dawn’s observations reveal that Ceres may be geologically active.

    Dawn’s observations suggest that Ceres has been geologically active in the recent past, possibly even today. One piece of evidence is Ahuna Mons, a mountain that formed when a mixture of ice, rock and salt from beneath Ceres’ surface pushed through its crust, giving it a domed shape like volcanoes on Earth. Another is Cerealia dome in Cerealia Facula, the brightest area on Ceres, in the center of Occator Crater. The dome likely formed when cold, slushy brine pushed up from beneath the surface. Both of these structures suggest that cold, briny liquid may still be present in the subsurface of Ceres.

    Graphic showing chemistry in a crater on Ceres.
    Carbonates (with highest abundance indicated by red) present in Ceres’ Occator Crater, home to its brightest spot. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/IAPS/INAF.

    8. Dawn raises questions about the murky origins of the two largest objects in the asteroid belt.

    In general, terrestrial bodies originated in the inner solar system and icy bodies mostly in the outer solar system, past the snow line. While Vesta, based on its magmatic composition and internal differentiation, clearly originated in the inner solar system, Ceres’ origin is less clear. Interestingly, both ammonia and carbonates are present on Ceres’ surface. Because ammonia is more abundant in the colder outer solar system, Ceres may have formed there before moving closer to the Sun, or materials from the outer solar system, having drifted inward, could have been incorporated during Ceres’ formation. Furthermore, the presence of carbonates, which are found on ocean worlds in the outer solar system, like Enceladus and possibly Europa, as well as Earth and Mars, requires large amounts of liquid water, carbon dioxide and relatively warm conditions. While Dawn’s data raise many questions about Ceres’ origins, they also bring us closer to finding answers.

    Artist's concept of the Dawn spacecraft above Earth.
    An artist’s concept of Dawn before leaving Earth’s neighborhood. Credit: NASA/JPL-Caltech.

    9. Dawn could provide insights into the history of our own planet.

    The main asteroid belt may have been a source of water for planets in the inner solar system, in particular Earth and Mars. Ceres provides clues about how water, an essential ingredient for life on Earth, may have gotten here. Our planet could have gotten its water from lots of small objects that came from the same source as Ceres – for example, asteroids and dust particles similar in composition. Ceres’ history may also teach us about how similar worlds in the solar system evolved.

    Full disc view of Ceres.
    A mosaic composed of Ceres images taken by Dawn’s framing camera, where Dawn (having arrived at the dwarf planet in 2015) will remain. Color is approximately as it might appear to the eye. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

    10. Dawn is only beginning to tell the story of these fascinating worlds, which beckon further exploration.

    Dawn is a mission of firsts: first science-dedicated mission to use ion propulsion, first to orbit two extraterrestrial bodies and first to visit a dwarf planet. Dawn has not only taught us about Vesta and Ceres but also about the feasibility of ion propulsion and multi-body missions. At Vesta and Ceres, we learned about the diverse worlds of the asteroid belt, providing a window into the formation of our solar system. In a lot of ways, though, Dawn has raised more questions than it alone can answer. These questions may be integral to our understanding of our solar system. When Dawn ends its mission this year, it will continue to orbit Ceres even after it can no longer communicate with Earth. Dawn will remain in a stable orbit at Ceres for decades to come, in order to protect the fascinating, insightful world from potential contamination, thereby preserving the potential for exploration by future missions. While Dawn's journey may be ending soon, its scientific and technical legacy will continue to shape how we view our solar system.