[meteorite-list] Dawn Journal - February 29, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 2 Mar 2008 21:30:39 -0800 (PST)
Message-ID: <200803030530.VAA10714_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_2_29_08.asp

Dawn Journal
Dr. Marc Rayman
February 29, 2008

Dear Phildawnthropists,

Treating intercalary days just as it does most other days in its
interplanetary cruise, today Dawn continues patiently and ever-so-gently
reshaping its orbit around the Sun with the delicate yet persistent push
from its ion propulsion system. The beam of xenon applies a force less
than 8 millionths of what the spacecraft would feel from gravity at the
surface of Earth. The effect of such a bone-rattling 8 micro-g
acceleration is very modest indeed, as discussed in a log written while
Dawn was still on that now-remote planet.

All of the thrusting so far in the mission has changed the spacecraft's
speed by 420 meters per second (940 miles per hour), about 10% of which
was from the tests during the initial checkout phase. This is sizable
for a spacecraft, but it is only a small fraction of what will be needed
on this journey to gain insight into the dawn of the solar system.
During its 10 weeks of accumulated thrust time, the spacecraft has
expended only 19 kilograms (42 pounds), or less than 5%, of its xenon
propellant. Years of thrusting will be required to reach its elusive
scientific targets, asteroid Vesta and dwarf planet Ceres.

Even as Dawn slowly modifies its orbit, the path it takes is already
quite different from Earth's. The Delta rocket, which flew with the
spacecraft only for about an hour in September, gave the probe enough
energy to break the bond of Earth's gravity, so it travels independently
through the solar system, unlike the moon and satellites humans have
launched into Earth orbit. As Dawn and Earth go their separate ways,
today they are moving apart even faster than described in the last log.
During the time it takes Earth to rotate once
(about 24 hours, for those readers not familiar with this planet's
idiosyncrasies), the distance between it and the spacecraft increases by
more than 1.4 million kilometers (nearly 900 thousand miles).

The probe suspends thrusting each week to
turn from the thrust orientation to bring Earth into the sights of its
main antenna. The spacecraft transmits information it has accumulated on
the operation of all subsystems and welcomes any new instructions from
mission control.

During the communications session on February 15, engineers radioed new
data to be used following a reboot of the main computer. These
parameters will be applied by the thermal control system
to manage the temperatures of some components in
the reaction control system (the system of small
conventional thrusters that helps the craft orient itself in the
zero-gravity of spaceflight). As Dawn journeys farther from the Sun, the
ship feels less of the warming rays, so it needs to rely more on its own
heaters, and the new values stored in computer memory help accomplish
that. In a delightful demonstration of flamboyant irreverence, based
solely on this minor change, the software was renamed from 7.0.2
to 7.0.3.

Although the weekly routine is quite adequate to keep the spacecraft
content on its interplanetary voyage, the flight plan included a special
activity on February 21 and 22. The backup science camera
was guided through a set of tests to
demonstrate its functions and capabilities. The instrument received its
initial checkout in December, showing it to be healthy, but this month's
tests were more demanding. The steps were nearly identical to those
executed with the primary science camera,
with the only differences being some of the objects that were observed.
As the spacecraft has progressed in its orbit, a different region of the
celestial sphere was too close to the Sun to be viewed safely by the
camera. Among the targets in both sets of tests were the Carina Nebula
and the star Vega. As expected, they created the same beautiful views
with the backup camera as they did with the primary camera
<http://dawn.jpl.nasa.gov/multimedia/fc_gallery.asp>. The results prove
that both devices function very well and that the new project in Carina
designed to dramatically alter its appearance for observers throughout
the Milky Way (believed by many to be an ostentatious attempt to outdo
Earth's famed artist Christo) did not achieve its stated ambition.
Following the successful completion of the camera tests, the spacecraft
resumed thrusting.

Dawn is 104 million kilometers (65 million miles) from Earth, or more
than 270 times as far as the moon and 70% as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take
almost 12 minutes to make the round trip.
Received on Mon 03 Mar 2008 12:30:39 AM PST