The darkest hour is just before the dawn

NASA’s Dawn spacecraft on its way to Vesta and Ceres after lifting off from Cape Canaveral Air Force Station at 7.34 am (EDT) on 27 September 2007

It seems as if unmanned probes are finally getting the media attention they deserve. The long journey home of Hayabusa was, in the truest sense, a space odyssey. The final success of the mission in bringing back material from the asteroid Itokowa was given extensive coverage and rightfully so. More recently, the imminent departure of the Voyager 1 space probe from the Solar System and its entry into interstellar space appears to have brought out the poetic side of the BBC Science Correspondent, David Shukman. And finally, there is Dawn, the NASA probe that refuses to die. On Monday 13 June, as Dawn hurtles towards its epic rendezvous with asteroid 4 Vesta, NASA released a first, albeit fuzzy, video footage of the shattered world that it will be studying in detail for close to a year, starting in mid July.

Of course, Dawn should never have got off the ground. The mission was originally validated by NASA in January 2001 to be the ninth project in its Discovery Programme, only to be cancelled almost three years later in December 2003, and then reinstated in February 2004! In October 2005, work on Dawn was placed in “stand down” mode. Then on 2 March 2006, for the second time, NASA announced that Dawn had been cancelled. Eleven days later, while attending the 37th Lunar and Planetary Science Conference in Houston, Texas, I sat in on the scheduled NASA Headquarters Briefing at 5pm in Crystal Ballroom A. A lady named Mary Cleave, the “Associate Administrator for the Science Mission Directorate”, had been asked to justify the reasons behind the latest Dawn decision, as well as a string of other cuts and postponements. It was a very depressing meeting to say the least. A detailed account of what was discussed on that fateful occasion is given by Emily Lakdawalla in The Planetary Society Blog. Fortunately, a successful appeal against the latest cancellation decision was launched, led in particular by the probes manufacturer, Orbital Sciences Corporation. There was light at the end of the tunnel as, on 27 March 2006, the cancellation was… cancelled! After all these tribulations, Dawn was majestically launched on 27 September 2007 on top of a Delta II rocket from Space Launch Complex 17B at Cape Canaveral Air Force Station.

So, apart from its incredibly laborious start, what is so exciting about Dawn?

Well, lots and lots! First off, it is an extremely ambitious mission. That alone probably explains why it encountered so many problems on the way to the launch pad. Dawn will visit the two largest bodies in the asteroid belt, first 4 Vesta in mid July 2011 (radius ~ 250 km) and then, the only official dwarf planet in the belt, Ceres in 2015 (radius~ 550 km). Rather than just fly-by its targets, as was done in previous multiple target missions such as Voyager 1 and 2, Dawn will actually go into orbit around each and make very detailed observations of their surface features. The propulsion system on Dawn is also very innovative. It is fitted with three ion engines, the first NASA exploration probe to have this configuration.

Vesta is almost certainly the source of an important group of meteorites known as the HEDs (Howardites, Eucrites and Diogenites). By a strange freak of orbital dynamics big chunks of Vesta are sent into Earth-crossing orbits and so end up as samples in our labs. This mechanism is highly efficient, so that there are presently 61 recorded HED meteorite falls compared to only 4 from Mars. Apart from the Moon, and of course Earth, Vesta is the best sampled body in the Solar System. As a result of studying these samples, combined with telescope observations, we think we already know a lot about Vesta. Whether Dawn confirms these ideas we shall just have to wait and see. But happily we won’t have to wait too long.

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