[meteorite-list] Dawn Journal - September 3, 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 6 Sep 2007 08:32:05 -0700 (PDT)
Message-ID: <200709061532.IAA02446_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_9_03_07.asp

Dawn Journal
Dr. Marc Rayman
September 3, 2007

Dear Dawnbassadors,

NASA is preparing (again) to bring Dawn to the Florida skies as all
systems are gearing up for a September 26 launch. This new date is later
than had been planned just a few months ago; nevertheless, as we shall
see, in the most important sense, this genuinely is not a delay for
Dawn's mission of adventure, discovery, and the search for answers to
exciting and important scientific questions. Earth's next interplanetary
ambassador remains on schedule for its engagements.

Following the decision in July to reschedule the launch, a complex story
described in the previous log and occasionally
told with other scary stories around campfires or microscopic black
holes in elliptical galaxies, the first priority was to move Dawn from
Cape Canaveral's Space Launch Complex 17B to a safe location so NASA's
bold new Mars explorer, Phoenix, could launch from nearby 17A. After the
protective payload fairing (the nose cone) was removed from the Delta II
rocket, the spacecraft and mated third stage were detached from the
second stage on July 22. They were transported back to the Hazardous
Processing Facility clean room at Astrotech Space Operations for safe
storage. Since then, the spacecraft has had a very leisurely summer
vacation, with little to do but allow technicians and engineers to
maintain its readiness while it enthusiastically anticipates the
autumnal beginning of its interplanetary voyage. Meanwhile, the first
and second stages of the rocket remained on the launch pad, where they
had a delightfully close view of the launch of Phoenix on August 4. Soon
Dawn and the third stage will be reunited with the rest of the launch
vehicle.

One of the keys to success in space exploration (and in some other
challenging and complex endeavors of the human species) is careful
planning and preparation for contingencies. In one example of that, as
soon as the crane used to erect Dawn's launch vehicle malfunctioned
on May 30, engineers at JPL, Kennedy Space
Center, and United Launch Alliance began detailed preparations for the
possibility of a launch in September or October. Had the work not begun
as early and as intensively as it did, it is quite likely that it would
not have been possible to complete it in time for launching after
Phoenix. Even with its uniquely capable ion propulsion system, Dawn
cannot conduct its exploration of both Vesta and Ceres if it does not
launch before late October 2007.

In working out the complex strategies for planning launches, engineers
use a great deal of jargon, such as "C3 = 11.4 km2/s2" (C3 equals 11.4
kilometers squared per second squared),
"-2 sigma RLA dispersion for a 95% PCS" (negative two sigma right ascension
of the launch asymptote dispersion for a 95% probability of command
shutdown), or "This project is so cool, I can hardly believe we get paid
to do this" (OK, perhaps this last isn't really jargon, but it is part
of the Dawn parlance). Terms that are particularly pertinent to our
discussion here are the "launch period," the interval of days on which a
launch may occur, and the "launch window," the range of time on any one
day in the launch period during which a launch may take place.

Most interplanetary missions have brief launch periods during which they
must take off or pay the price of a significant change in their
itineraries. As explained in previous logs,
Dawn's travels are unusual because of the extraordinary capability of
the ion propulsion system, which provides a thrust of susurrant
gentleness that is more than compensated for by its virtually tireless
persistence. With so much maneuvering capability, even though its launch
was deferred by months, Dawn's scheduled arrivals at asteroid Vesta and
dwarf planet Ceres are effectively unchanged. The delay in launch does
not necessitate a delay in accomplishment of Dawn's goals, so in many
important ways, from an overall mission perspective, the postponement of
the launch is inconsequential.

Indeed, a complex combination of myriad factors, including the positions
of Earth in its orbit on candidate launch dates in 2007, of Mars in its
orbit in the first few months of 2009, and of Vesta in its orbit late in
2011, makes the new launch more favorable for the mission. Although
still a challenge of astronomical proportion, this will make it slightly
easier for Dawn to complete its assignment. This may be translated to
slightly greater resilience in keeping its alien appointments should it
encounter difficulties on its voyage through the unforgiving and remote
depths of space.

Now Dawn will follow a completely different launch trajectory and take a
different path to Mars. Had it launched in July, the spacecraft would
have used Mars early in April 2009 to boost it along its way to Vesta.
In the new plan, it will swing by the red planet in February 2009. Mars
is one of the easiest destinations in the solar system to reach, and
Dawn could travel there more quickly if that were its sole objective.
Much of the ion thrusting prior to Mars however is designed to aim the
craft so that when it reaches Earth's neighbor, the planet's gravity
slings it in the most effective way to help it in its long flight to
distant Vesta. Leaving Earth in September or October lets Dawn gain
greater benefit from its brief visit to Mars.

Despite remaining with Earth throughout the summer of 2007, Dawn's new
flight profile will allow it to catch up with the old one. The
spacecraft will be in the same place in the solar system in the summer
of 2009 as it would have been had it launched in June or July. The
mission after that will be quite similar to what it would have been with
the earlier launch. The principal difference is that to accomplish the
mission with the later launch, Dawn will consume a little less of its
xenon propellant, so more will be available in case the probe needs to
perform unplanned thrusting.

To account for the new launch date and path to Mars, Dawn's departure
from Earth will be very different from what it would have been in June
or July. Instead of launch windows in the middle of the afternoon, now
Dawn will launch closer to dawn. On September 26, the launch window is
7:25 am EDT to 7:54 am EDT. (To simplify coordination among the many
organizations around the world participating in the launch, liftoff is
scheduled on the whole minute. The capability to round off the time this
way is another benefit of the ion propulsion system's flexibility, and
it should be particularly appreciated by all Dawn enthusiasts on planets
whose clocks don't have second hands, including our newest readers,
members of the Honorable Minority of Antipunctualists in the Horologium
supercluster of galaxies.)

If unfavorable weather or other fortuities prevent launch (possibilities
with which all loyal readers are exceptionally familiar) on September 26,
launch windows during the rest of the launch period are:

Sept. 27: 7:20 - 7:49 am EDT
Sept. 28: 7:14 - 7:43
Sept. 29: 7:09 - 7:38
Sept. 30: 7:03 - 7:32
Oct. 1: 7:12 - 7:31
Oct. 2: 6:55 - 7:24
Oct. 3: 6:49 - 7:17
Oct. 4: 6:44 - 7:13
Oct. 5: 6:41 - 7:10
Oct. 6: 6:38 - 7:07
Oct. 7: 6:35 - 7:12
Oct. 8: 6:34 - 7:12
Oct. 9: 6:33 - 7:11
Oct. 10: 5:43 - 6:23
Oct. 11: 5:42 - 6:22
Oct. 12: 5:13 - 5:54
Oct. 13: 5:13 - 5:57
Oct. 14: 5:16 - 5:58
Oct. 15: 5:18 - 6:00

Mortal readers are encouraged not to waste time trying to discern a
pattern in either the time of the opening of the launch windows or the
window durations. The underlying reasons for these values are manifold
and complicated, and to avoid violating statutes in some spiral galaxies
on publishing dangerously boring text, the explanations will be omitted.
Let's look briefly at just one relatively simple observation: the
19-minute window on October 1 is shorter than all the others. Under
certain circumstances that are unlikely but possible, an earlier launch
window opening on that day would make Dawn pass close enough to the moon
less than 28 hours later that the gravitational deflection of the
spacecraft could only be compensated by significantly more ion thrusting
than planned. Rather than take the small risk of incurring this minor
complication in the mission, the window was shortened. On all other days
in the launch period, as the spacecraft departs Earth in roughly the
same direction, the moon will be elsewhere in its orbit so it will not
cause as much interference in the trajectory. Still, the moon's gravity
is included in all analyses.

Demanding as it is, there is more to replanning the mission than
designing new trajectories for the rocket and the spacecraft. Because
the geometry for the departure from Earth has changed so much, the Dawn
operations team has had to redesign many of the activities scheduled
during the early part of the mission. The location of the spacecraft
relative to Earth and the Sun will be quite different from what had been
planned, so onboard instructions for how to orient in order to achieve
certain objectives must be modified. While Dawn itself has spent an
unusually quiet and leisurely time at Astrotech waiting to be reunited
with its rocket, mission controllers have been very busy indeed
developing new plans and the corresponding sets of commands for the
first few months of the mission.

To maintain proficiency for launch, the team also completed another set
of simulations of the final 16 hours of countdown, launch, and the first
day or so of flight. Most of the week of August 27 was devoted to a
slightly shortened version of the ORTathon
that was conducted early in June. Differences from the first ORTathon
included not only the launch time and trajectory, but also a new set of
fiendish surprises injected by the simulation supervisor and a modified
(and, in this participant's careful analysis, a superior) selection of
snack food in mission control.

One regrettable consequence of the changed launch conditions is that the
timelines presented in the June 23 and July 5
logs will not apply for the new launch period.
This threatens a lucrative deal negotiated with importers of the
stone-engraved versions of these logs on icy asteroids in most irregular
galaxies. Therefore, in the coming weeks, when the relevant analyses are
completed and the new data are available, those logs will be reposted
with the only changes being those that are essential to bring them up to
date for the new launch plan.
Received on Thu 06 Sep 2007 11:32:05 AM PDT