for attitude control and orbital insertion. However, on 1st July NASA an-
nounced the outcome of its review of current extended planetary and
lunar science missions, concluding that the Dawn should remain in orbit
around the dwarf planet. One reason for this was felt to be the value of
continued long-term monitoring of Ceres, especially at its approach to
perihelion, the closest point in its orbit to the Sun.
While some may be disappointed by this decision, Marc Rayman’s
feeling is that it was the right one. ‘It was the result of making a careful
choice between two attractive options: remain in orbit around the only
dwarf planet in the inner solar system, or fly by a large asteroid that has
never been visited. Either would be very rewarding. I am very happy
that we were able to give NASA HQ options. Most missions that are in
orbit have to stay in orbit for their extended missions, and spacecraft
that are not already in orbit at the end of the prime mission cannot
enter orbit around something. Dawn has the best of both worlds, so to
speak. NASA, supported by an independent panel of esteemed scien-
tists, concluded that the best use of this interplanetary spaceship was to
carry out further investigations of the first dwarf planet ever discovered.
That is a wonderful outcome!’
And so, Dawn will continue ‘extracting secrets from dwarf planet
Ceres’. Given the presence of organic materials and the possibility of
pre-biotic chemistry, there is no question of allowing the probe to crash
onto and contaminate this world’s surface at the end of its mission.
On the 5th December Dawn completed a month of ion thrusting to
reach a new orbit around Ceres. This sixth Ceres science orbit is ellipti-
cal, ranging in altitude between 7,520 km (4,670 miles) and 9,350 km
(5,810 miles), to begin observing from different angles and gaining new
perspectives. The probe is in good health, and its systems are function-
ing well. It will continue to operate during 2017, then will remain a per-
petual satellite of Ceres when the mission is over, due to its highly stable
orbit. We eagerly await its future observations of this intriguing world!
Acknowledgements:
Thank you again to Dr. Marc Rayman for information and comments.
Visit his ‘Dawn Journal’ at: http://dawn.jpl.nasa.gov/mission/journal.asp
You can see an animation, narrated by Dr. Marc Rayman, showing
some of the highlights of Dawn’s exploration of Ceres so far, including
Occator and Oxo craters, at http://photojournal.jpl.nasa.gov/catalog/
PIA20537
Further reading:
• For mission overview and news: http://dawn.jpl.nasa.gov/
• For information on ion propulsion: http://www.grc.nasa.gov/www/ion/
Glossary
CLATHRATE – a compound in which molecules of one component are
physically trapped within the crystal structure of another. One example
is methane clathrate, or ‘fire ice’, in which a large amount of methane
is trapped within a crystal structure of water, forming a solid similar to
ice. Originally thought to occur only in the outer Solar System, where
temperatures are low and water ice is common, significant methane
clathrate deposits have been found under sediments on the bed of the
Earth’s oceans. There is currently concern that ocean warming due to
climate change may release methane, a greenhouse gas, from these
deposits.
ORGANIC COMPOUND – Any member of a large class of gaseous, liquid,
or solid chemical compounds whose molecules contain carbon.
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