DAWN AT VESTA
Before Dawn set sail for Ceres, it spent 14 months orbiting the smaller asteroid Vesta. Vesta measures some 573x557x446km (it is markedly oblate) and it has a high-density, differentiated interior, comparable to the structure of larger terrestrial planets. In fact, some planetary researchers describe Vesta as a protoplanet: if there had been more small planetesimals around, it could have grown into an object with a size more similar to the Moon.
Dawn was at Vesta between July 2011 an September 2012. Its most remarkable findings are related to the huge impact basin at Vesta’s south pole, named Rheasilvia. A dramatic collision with another large body chipped away part of Vesta’s southern hemisphere, leaving this massive circular scar with a large central mountain behind. The impact all but shattered the asteroid: girdling its equator are giant ridges and trenches that formed in the aftermath of the collision. Impact debris – recognisable through its mineralogical composition, which is unique to Vesta – was strewn around the inner Solar System. In fact, five per cent of all meteorites on Earth are actually small fragments of Vesta.
In late 2014, planetary scientists published a detailed geological map, based on Dawn’s data. Thanks this space probe, Vesta is now the best-known asteroid.
the skies with their telescopes in vain, Piazzi stumbled upon the new ‘planet’ by accident. It was the evening of 1 January 1801. He named it Ceres, after the Roman patron goddess of Sicily. A mere 20 years after the discovery of Uranus, the Solar System had gained yet another planet, bringing the number of known planets to eight. However, Ceres didn’t enjoy its planetary status for long: it is now known to be the largest body in the asteroid belt, bearing minor planet number 1.
Despite its great distance, being a few hundred million kilometres away, astronomers have succeeded in calculating Ceres’s diameter. It measures some 975x909km across, while its rotational period is nine hours and 4.5 minutes. Ceres is 80 per cent larger and 3.6 times more massive than the second-largest asteroid, Vesta. In fact, it contains about one-third of the total mass of the asteroid belt. Using the Hubble Space Telescope and the Keck Telescope, scientists have discovered some bright and dark regions on its surface; the largest dark ‘spot’ has unofficially been named Piazzi.
The two-orbit pioneer
The mass of Ceres has been derived from its minute gravitational perturbations on other asteroids. From the object’s known mass and size, its density has been calculated at a pretty low 2.1g per cubic centimetre, suggesting a large fraction of ice – perhaps as much as 25 per cent, which would mean that Ceres contains more water than the Earth’s oceans, albeit in frozen form. Indeed, spectroscopic observations have revealed the existence of hydrated (water-bearing) minerals on the dust-covered surface. And in January 2014,
the European infrared Herschel Space Observatory discovered localised plumes of water vapour – something that has never before been witnessed on any asteroid. Little wonder, then, that planetary scientists are eagerly looking forward to the arrival of the Dawn spacecraft.
Launched on 27 September 2007, Dawn flew past Mars on 17 February 2009 and entered orbit around Vesta on 15 July 2011. After studying this intriguing ‘protoplanet’ for over a year (see ‘Dawn at Vesta’, page 43) it departed, setting off for Ceres. It’s the first time in the history of space exploration that one craft will orbit two separate celestial bodies during its mission – a feat made possible by Dawn’s versatile ion propulsion engine, which uses the minute thrust of electrically accelerated xenon atoms to navigate the craft through interplanetary space.
Tucked between the huge solar arrays – which have a total span of almost 20m – is the Dawn spacecraft itself. It is almost as large as two queen-size beds on top of each other and has a 1.5m highgain antenna for radio communication with Earth. A German-built camera will image Ceres in seven different colour bands, while the Italian visible and infrared mapping spectrometer studies the composition and thermal properties of the dwarf planet’s surface. An American gamma-ray and neutron detector will determine the relative abundances of chemical elements. Finally, using spacecraft telemetry, astronomers will learn about Ceres’s rotational properties.
Ceres the reservoir
On 6 March, Dawn will be captured in Ceres’s gravitational field. But it won’t start orbiting the dwarf planet right away. First, the spacecraft will fly out to a distance of some 75,000km, then turn around using its ion engine and finally manoeuvre itself into a polar orbit around 23 April. In November 2015, the spacecraft will be moved into a low-altitude mapping orbit, at a mere 375km above the surface. The original orbit insertion plan was much simpler, but mission controllers had to redesign the procedure from scratch after an energetic cosmic ray caused temporary havoc to the engine’s electronics on 14 September. Since ion engines work so slowly, the glitch had a huge impact on Dawn’s thrust timeline.
Dawn’s exploration of Ceres will yield valuable insights into the early evolutionary history of the terrestrial planet. Will Piazzi’s planet indeed turn out to be the largest water reservoir in the inner regions of the Solar System? And will the deuterium (heavy hydrogen) abundance of this water match the value in the Earth’s oceans? If so, it would suggest that our planet’s water could have been delivered by asteroids, not by comets – after all, measurements by the European Space Agency’s Rosetta mission have shown that the deuterium abundance in comet 67P/ChuryumovGerasimenko is three times as high as the terrestrial value.
Alternatively, if Ceres also has a high deuterium fraction, this could imply that it was born in the far reaches of the Solar System, way beyond Neptune’s orbit, and that it migrated into the asteroid belt by gravitational interactions of the planets some four billion years ago – a scenario suggested by planetary scientist Bill McKinnon of Washington University. If that turns out to be the case, Ceres would be the first Kuiper Belt Object to be studied up close – a few months before New Horizon’s arrival at Pluto.