[meteorite-list] Dawn Journal - March 29, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 31 Mar 2009 10:43:39 -0700 (PDT)
Message-ID: <200903311743.KAA28613_at_zagami.jpl.nasa.gov>


Dawn Journal
Dr. Marc Rayman
March 29, 2009

Dear Dawntpanics,

Dawn continues to coast quietly and calmly in its orbit around the Sun,
keeping its main antenna pointed to faraway Earth. The mission control
team has given the spacecraft relatively few assignments in recent
weeks, providing time to prepare for a busier future. To ensure the
distant craft remains healthy and safe, operators transmitted
instructions for conducting routine maintenance, activities that are
familiar to the probe now that it has been on its deep-space journey for
more than 1.5 years. Perhaps what is most noteworthy and satisfying
since the last log is not what Dawn did, but rather what it did not do.
At some unknown time in the past, at some unknown location in space,
under specific conditions and processes we can only speculate about
(albeit under the influence of physical laws that are well understood),
a high energy particle embarked on its own deep-space journey. On March
21 at 1:32 am PDT, its path and Dawn's intersected. The particle
penetrated the spacecraft and reached an electronics unit inside. The
energy that had been given to it elsewhere in time and space was
transferred then and there to a miniature electronic component in a
circuit within that unit.
The circumstances of this event were very much like those that occurred
on January 14, 2008. As a reminder for
the reader who has not memorized the log describing that incident (you
know who you are, and we do as well, but your secret remains safe under
the terms of our readers privacy agreement), the chain of events led to
the spacecraft entering "safe mode."
While safe mode is the desired response to a wide range of unexpected or
problematic conditions, Dawn's engineers recognized that it was not
necessary when space radiation hit that particular device. They
reexamined the extensive analyses that had been conducted before launch
and performed new studies as well, concluding that the mission could be
interrupted by more such events because of the susceptibility of the
component that was hit by the radiation. It was not possible to
establish with high confidence how frequently it could occur (much less
to know specifically when it would occur), but the data suggested that
it might happen often enough that it would hamper the mission,
interfering with too many activities and ultimately consuming too much
of mission controllers' limited time.
The team, therefore, formulated a change to the software in one of the
spacecraft's auxiliary computers that would allow it to accommodate
another radiation hit without the subsequent cascade of events
culminating in safe mode. While changing the software is challenging and
time consuming, the project chose to undertake the work to avoid a
repeat of the incident, judging the certain cost of the change to be
less expensive to Dawn's mission than the possible cost of subsequent
On July 22, 2008, the new software (along
with all the instructions to patch it in) was radioed to the remote
spacecraft. Since then, it has been available should a radiation strike
energize the component as it had 6 months earlier.
The modified part of the software had its first use in flight with the
radiation impact on March 21, when nature repeated itself. The
electronics were unchanged, of course, so the circuit responded the same
way it had 14 months earlier, informing the software of a problem. This
time though, the software responded simply by storing a short message to
transmit along with all the other spacecraft health and status
information during the next scheduled contact with Earth (by
coincidence, later that day). The unnecessarily dramatic activation of
safe mode is no longer part of the outcome, thus allowing the robotic
probe and its human support team to continue their work without
interruption or distraction.
While Dawn happened to be quiescent when this radiation impact occurred,
it might not have been, given the inherent unpredictability of the
timing. It might not be when the next uninvited particle strikes the
same component. Now, with the clear demonstration of the effectiveness
of the software patch, it won't matter. Controllers were gratified to
receive the message on March 21 reporting the spacecraft's nonchalant
response to the radiation, validating their decision to modify the
software last year.
The spacecraft will be busier in the coming weeks than it has been since
it was deflected by Mars last month. The next log,
already available for readers in the future [Note to those readers:
please send a copy to the author in his present to save him the time of
writing it], will cover some of the upcoming special activities and will
recognize another milestone (if not million-milestone) on this journey
of scientific discovery and adventure to distant and unexplored worlds.
Dawn is 327 million kilometers (203 million miles) from Earth, or 875
times as far as the moon and 2.19 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take 36
minutes to make the round trip.
Received on Tue 31 Mar 2009 01:43:39 PM PDT

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