In September 2007, NASA's Dawn spacecraft began its historic journey to the giant asteroid
Vesta and then on to the Earths closest dwarf planet, Ceres. The mission was designed to
study the two largest objects within the main asteroid belt, that can be located between
Mars and Jupiter. Vesta and Ceres are both diverse, intact celestial objects that offer
a scientific snapshot of the solar systems early history, providing an insight into how
planets like ours form. The incredible data that Dawn beamed back to Earth during its
11-year mission gave scientists the chance to compare the two planet-like worlds, both
of which formed very differently and discover more about our solar systems building blocks.
After launching from Cape Canaveral, Florida, the Dawn spacecraft cruised towards Mars,
where it received a gravity assist that boosted its velocity and placed it on the right trajectory
to reach Vesta. For 1.7 billion miles (2.8 billion kilometres) the spacecraft drifted
through the darkness of space until in May 2011, Vesta came into view and Dawn captured
its first image of the mighty asteroid. This stunning image was taken during Dawn's final
approach to Vesta from 750,000 miles away and reveals the asteroid as a small, distant,
white pearl against a speckled, black backdrop. In July 2011 Dawn entered into an orbit around
Vesta, becoming the first spacecraft to orbit an object in the asteroid belt. It captured
hundreds of images and discovered that the bright asteroid is a rough world, that is
covered with deep craters and huge mountains. This high resolution, closeup image exposes
the asteroids Southern Hemisphere for the first time. You can see multiple large and
small craters scattered across its baron surface, but also strange grooves or troughs that stretch
around Vesta's equator. These enormous features are likely the scars of gigantic ancient collisions
and each groove is estimated to be at least 6 miles wide! One of the more striking features
on Vesta can be found in the northern hemisphere and is a set of impact craters that have been
nicknamed the snowman. This true colour image reveals three craters of different sizes and
shapes that are linked at the rim and form an enormous cosmic snowman look alike. The
Dawn spacecraft spent almost a year in orbit around this unusual asteroid and beamed back
hundreds of magnificent images. But it also confirmed that Vesta is unlike other known
asteroids and instead resembles a small planet or even Earths moon. The closeup data collected
revealed that Vesta is indeed a planetary building block, known as a protoplanet, that
is layered with an iron core, a mantle and a rocky crust as previously suspected by scientists.
It is thought that Mercury, Venus, Earth and Mars originally started like Vesta, before
forming into the giant worlds that we see today. After studying Vesta for almost a year,
in September 2012 Dawn departed the mysterious protoplanet and began the next stage of its
mission, the long journey to our closest dwarf planet, Ceres. Around 2 and half years later
and 3.1 billion miles (4.9 billion kilometres) later, Dawn finally arrived at the dwarf world
and began capturing hundreds of stunning closeup images. In March 2015 Dawn became the first
spacecraft to orbit two celestial objects and the first spacecraft to visit a dwarf
planet, just beating the more famous New horizons, that was on a mission to Pluto and arrived
there only a few months later. Dawn snapped this image from a distance of 8,400 miles
away and discovered that Ceres is a cratered, frozen world that is hiding a fascinating
history. Ceres is by far the largest object in the
asteroid belt, with a diameter of 588 miles across (946 kilometres). It's massive size
and roundness means it qualifies as a dwarf planet, but not a main planet, as it failed
to clear out its orbit of smaller objects. The dwarf planet is covered with countless
small, but young craters that seem to smooth out over time due to a layer of ice just below
its surface. One of the more fascinating craters on Ceres
is called the Occator crater and is where one of the brightest spots on the dwarf planet
can be located. This image gives us a birdseye view, directly over the top and reveals a
bright dome in the centre that has been named Cerealia facula.
The bright spots are thought to be either salt deposits from vaporised ice that has
been exposed by ancient impacts or ammonia-rich clay, which according to scientists would
suggest that Ceres originally formed in the outer solar system and migrated in billions
of years ago. However, neither of these theories have been confirmed. A strikingly large feature
on Ceres is a single, lonely mountain that has been named Ahuna Mons. Researchers suspect
that the mountain is actually a dead cryovolcano, that once erupted ice instead of hot magma
like we see on Earth. There is also evidence to suggest that it was once surrounded by
other cryovolcanos, but over time the icy mountains slowly flowed back onto the surface,
leaving Ahuna mons all on its own. Ceres appears as a scarred, baron world that is covered
in fascinating features and discoveries like the flowing ice that formed Ahuna mons, plus
the possible presence of salts within the craters suggest that an ancient, global ocean
also once existed beneath the crust of Ceres and it may still be there today. The amazing
discoveries made by Dawn have revealed that Ceres isn't just a giant icy object dominating
the asteroid belt, but instead a rich, evolving world that can teach us a lot about our solar
systems and how our planet originally formed. In November 2018, after 11 years in space,
NASA announced that Dawn had run out of fuel and that the mission that explored Vesta and
Ceres was now over. Dawn managed to capture thousands of never seen before closeup images
of these mysterious worlds and helped to further answer some of the solar systems most burning
questions. The amount of data that Dawn beamed back will keep scientists busy for decades,
as they analyse the information and no doubt make further incredible discoveries. Although
the spacecraft ran out fuel, Dawn will continue to orbit Ceres in an uncontrolled, but stable
state for another 50 years, before crashing into the surface of a dwarf planet. But until
then it will drift around the icy world as another relic of our magnificent space exploring
history.