[meteorite-list] Dawn Journal - January 31, 2008

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_1_31_08.asp

Dawn Journal
Dr. Marc Rayman
January 31, 2008

Dear Dawnnoisseurs,

Now in interplanetary cruise, the Dawn spacecraft is following a much
more leisurely pace than the one it maintained during the initial
checkout phase of the mission. While its daily schedule is not
demanding, as it follows (and changes) its orbit around the Sun, it is
separating from Earth at nearly 4 light seconds per day (more than 1.1
million kilometers, or 720 thousand miles, per day). Every 8 hours, the
probe recedes from Earth by a distance equal to that between Earth and
the moon.

The spacecraft has accumulated more than 1000 hours of thrusting with
its ion propulsion system. Although far, far longer than the
overwhelming majority of spacecraft have operated their propulsion
systems, this represents only a small fraction of the total thrusting
required to complete its solar system journey. [Note to editors: This
milestone may be of significance only to human readers. When translated
for those who use different numbering systems or different time systems,
it may not yield an interesting result. (For that matter, 1000 hours is
not a special number when expressed in seconds, days, or millennia.)]

Most of Dawn's time is devoted to thrusting with its ion propulsion
system, but each week the spacecraft stops for a communications session
with controllers on distant Earth, during which it returns detailed data
on the performance of its subsystems throughout the previous week.
Reports of voltages and currents, temperatures and pressures, and myriad
other parameters allow engineers to determine how well the ship has been
doing and how to keep it sailing as smoothly as possible.

On January 14 shortly before 10:00 pm PST, a high energy subatomic
particle, a "cosmic ray," traveled through one of the main panels of the
spacecraft and then penetrated one of the electronics units. The energy
it carried had been imparted to it through an unidentified cosmic
process, and after the particle had traveled across vast distances, that
energy was transferred to a small integrated circuit. Such an event is
not all that uncommon on spacecraft, and Dawn is designed so that most
space radiation does not interfere with its operation. The deposition of
energy in this particular component however triggered the electronics to
inform the software of a problem. To rectify the situation, other
software correctly responded by resetting the computer in that unit.

In the last log, we saw that two master
computers work together to oversee and control activities on the
spacecraft. The computer that was reset in this case was neither of
those; it was one of many auxiliary computers with more limited
responsibilities. In addition to resetting the computer, software
running in the main computer correctly reconfigured systems onboard to
"safe mode." The spacecraft then awaited instructions from engineers on
Earth (or, more accurately, in Dawn mission control on the top floor of
JPL's whimsically named building 264).

A few hours later, when it was time for the weekly communications
session, the Deep Space Network and mission controllers promptly
recognized that the spacecraft was in safe mode. As with the safing in
November, a small team gathered during the
night to begin the diagnosis, and more team members joined after dawn.
It did not take long to reach a conclusive explanation based on the
error code stored by the software and other data downloaded from the
spacecraft. The culprit was a cosmic ray.

By the time the detailed analysis of the safing was concluding, mission
controllers were already commanding the spacecraft step-by-step out of
safe mode, returning it to its normal flight configuration. Within a few
days, Dawn was ready to resume work, and before the end of the week was
thrusting with its ion propulsion system again. (The effect of having
missed some thrusting that week is not significant for the mission.)

While Dawn will thrust during most of its interplanetary cruise, the
flight plan includes some periods of coasting in addition to the normal
weekly communications session. One such period was January 22 - 25. Most
of this time was devoted to updating software in the main computer.

The main software resides in 4 locations (well, 4 locations on the
spacecraft): primary and backup copies in the primary main computer and
primary and backup copies in the backup main computer. Three copies of
flight software 7.0  were installed on
the spacecraft in November. As reported in the last log,
ournal_12_17_07.asp#backup>, the primary main computer was scheduled
to receive its backup copy during this break in thrusting.

While it had been planned before launch to transmit 7.0 to the
spacecraft in November, after the software was finalized, the need for
one additional change was recognized. For technical reasons it was not
necessary to change all the stored copies; rather, it was sufficient to
modify (or "patch") only the version that was running. Making that
change on December 12 advanced the software to 7.0.1.

As Dawn moves farther from its former home, a planet it is not scheduled
ever to visit again, its capability to communicate with Earth
diminishes. The large main antenna is strong enough that no changes are
needed yet, but the smaller, auxiliary antennas do not permit the same
level of performance that they did earlier in the mission. As explained
last month and before launch, when Dawn enters safe mode, it uses 1 of
these auxiliary antennas. Now that the distance to Earth has increased
so much, the weaker signals require lower transmission and reception
rates. Engineers decided several years ago to start the mission with
safe mode programmed to use data rates that were too high to work
throughout the mission but would allow simplified operations during the
first months of the flight. Those months have now passed, so following
that plan established before launch, the software was patched to use
lower data rates whenever safe mode is called. The new values were put
into in the primary and backup locations in the primary and backup main
computers on January 23 and 24.

With these changes, Dawn is now using flight software version 7.0.2. If
you want to update the software running on your spacecraft to this
latest version, with the hottest new features, we have made special
arrangements to provide a copy you can download for free. (Note: this
special offer is not available to customers within 1 decaparsec of Dawn.)

Replacing or updating software on an operating spacecraft is complex,
and there are many opportunities for problems to arise. To allow for
time to understand and resolve any unexpected difficulties, more time
was allocated than was expected to be needed. As it turned out, after
the team's exhaustive preparations, the software uploads went perfectly,
and there was plenty of time to spare. When the work was complete, the
spacecraft simply waited until the scheduled time for the resumption of
thrusting on January 25, and then it returned to powered flight.

Shortly after Dawn launched, we recalled
the 50th anniversary of the launch of humankind's first satellite. As
our spacecraft continues in journey through space, today we remember the
50th anniversary of another launch. The United States' first craft to
reach Earth orbit, Explorer 1, departed from Cape Canaveral's Space
Launch Complex 26A, almost exactly 500 meters (about 0.3 miles)
southwest of complex 17B, from which Dawn began its climb to space. (We
don't include a link here to any previous remarks about Dawn's launch
site, as it was mentioned in more than 63% of the logs posted from April
through September 2007. With so many past references, readers who flow
forward in time are counted upon to remember or not to care about the
launch location.)

Explorer 1's elliptical orbit around Earth carried it about 2550
kilometers (1585 miles) above the surface. On this 50th anniversary of
the beginning of that mission, so important in the history of space
exploration and space science (as well as the Cold War), Dawn is more
than 26 thousand times farther from Earth, in its own orbit around the
Sun. Despite the great difference in distance (as well as other
comparisons in size, mass, capabilities, technological sophistication,
scientific ambition, and more, all of which we spare readers from trying
to absorb), the two projects have much in common. Both are part of
humanity's efforts to help broaden our perspectives beyond our home
planet, to apply both engineering and scientific knowledge as we seek to
gain even more such knowledge, to undertake adventures that include
great challenges but offer great rewards in which everyone can share.
And although it may not feel this way now, with the first mission 50
years in the past and the second just starting, for most of humanity's
future, both will be among the first tentative probes into the cosmic
unknown.

Dawn is 67 million kilometers (42 million miles) from Earth or 175 times
as far as the moon. Radio signals, traveling at the universal limit of
the speed of light, take more than 7.5 minutes to make the round trip.