is fantastic!’ This
can only help
to dispel the
habitual myths
which surround
the funding of
space exploration in the public
imagination.
And of course
there are far
greater benefits,
Dr. Marc Rayman, Chief Engineer and Mission Director
as Dr. Rayman
at JPL, with Dawn during assembly. Credit: NASA
explains: ‘We
are lucky to live
in a culture in which we have the resources to invest
in such projects, just as we do in art. These enrich all of
us, help us discover our place in the Universe and open
our eyes to new worlds, both literally and figuratively.’
Ion propulsion
An important part of the return from the mission is
the proving of alternative technologies. The radical
choice of propulsion system has been of particular
importance in enabling Dawn to achieve its ambitious goals. Visiting and orbiting two different bodies
has been made possible thanks to an advanced ion
thrust propulsion system, developed by NASA. Dawn’s
engines are based on the system used successfully on
the Deep Space 1 trial mission for new technologies
between October 1998 and December 2001. A simplified view of how the system works is shown in Figure
5 below. For a detailed description of the ion propulsion technology used for the mission, visit http://
www.nasa.gov/centers/glenn/about/fs21grc.html
Following its launch on a Delta II from Cape Canaveral on 27th September 2007, Dawn has used three
xenon ion thrusters (firing only one at a time) to take
it in a long outward spiral from Earth into the main asteroid belt, via a gravity assist from Mars. The spacecraft’s trajectory and timeline are shown in Figure 6.
The engines have a specific impulse of 3100s and a
thrust of 90 millinewtons (mN), and are capable of acceleration from 0 to 60 mph (97 km/h) in 4 days, firing
continuously. While a chemical rocket on a spacecraft might have a thrust of up to 500 newtons (N = kg
m s-2), Dawn’s much smaller engine can achieve the
same change in trajectory by firing over a much longer period of time, and, above all, using far less fuel.
This is particularly significant when planning rendezvous
and orbital insertion manoeuvres, which is why Dawn is
able to catch up with and orbit two separate bodies.
By October of 2014, as Marc Rayman explains in
his online journal, the spacecraft had thrusted for
1,737 days (68% of its journey), using only 366kg (808
p