Marc RaymanSep 30, 2008

Dawn Journal: One Year Since Launch

Dear Dawnniversaries,

On the first anniversary of its departure from Earth, Dawn continues with what it has been doing for most of its time in space. With the greatest patience, it is gently reshaping its orbit around the Sun with its ion propulsion system.

In its first year of travels, the spacecraft has thrust for a total of about 253 days, or 69% of the time. Dawn has been in powered flight for 85% of the time since the beginning of its interplanetary cruise phase in December 2007 and about 0.000000005% of the time since the Big Bang. While for most spacecraft, firing a thruster to change course is a special event, it is Dawn's wont. All this thrusting has cost the craft only 67 kilograms (148 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) a year ago.

Dawn lifts off
Dawn lifts off Dawn took off for the asteroid belt atop a Delta II Heavy on September 27, 2007 at 11:34:00 UTC.Image: NASA

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 1.68 kilometers per second (3,760 miles per hour). As the preceding log described, because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Having accomplished only one-eighth of the thrust time planned for its entire mission, Dawn has already exceeded the velocity change required by many spacecraft. (For a comparison with probes that enter orbit around Mars, visit the Red Planet yourself or refer to a previous log.)

Since launch, our readers who have remained on or near Earth have completed exactly one revolution around the Sun. (This log, including the date it is filed, disregards that 2008 is a leap year and that Earth actually takes almost 365.25 days to complete one orbit. Oops -- it isn't being disregarded; in fact, it's right there in the previous sentence, and the longer this parenthetical text goes on, the more attention is being drawn to it. As it makes no significant difference, we request readers do a better job of ignoring it than the writer is doing. Please return to the flow of the log.) Orbiting farther from the Sun than Earth, and moving at a more leisurely pace, Dawn has not traveled even two-thirds of the way around the Sun. Of course, unlike Earth, when it has completed one full circuit (in 2009), it will not be at the same place it started. Earth's orbit is quite repetitive, but the combined effects of the powerful rocket launch, the extensive ion thrusting, and the gravitational deflection from Mars next February will cause the spacecraft to be farther from the Sun at the end of its first revolution than it was at the beginning.

As readers who have followed the Dawn mission during 2008 know, the spacecraft occasionally engages in activities other than routine thrusting as its adventure progresses. On August 26, mission controllers commanded the primary and backup cameras to execute their calibration routines. This not only served to confirm that both units remain healthy, but it also let engineers verify one of the new features in the new software radioed to each camera in April that was not tested at that time.

On September 22, an updated version of a method to establish how much power Dawn's extraordinary solar arrays can generate was tested successfully. The first test was conducted in July, and it yielded only some of the desired information. The revised procedure was very similar to the earlier one, principally differing in the timing of some instructions and values of parameters based on the analysis of that initial run. Because the entire activity, even including the 41-minute round-trip travel time for radio signals, required fewer than three hours in the middle of the afternoon, among the most significant changes that ever-observant mission controllers detected was that no meals were incorporated into the carefully engineered plan.

As in July, the test included rotating both solar array wings 45 degrees, so they did not point directly at the Sun, thus reducing how much light they received and converted to electrical power. The test was carried out during the spacecraft's routine weekly interruption in thrusting to point its main antenna to Earth, but the ion propulsion system was commanded into service when it otherwise would have been idle. Its role then was not to provide propulsion (although it did so); rather, it participated because it is the greatest consumer of power onboard. Dawn's enormous solar arrays, even turned partially away from the Sun and more than 1.66 times farther from the radiant orb than Earth is, were able to provide the 2.5 kilowatts requested by the ion drive at full power. Later in the mission, after all the data have been analyzed thoroughly, the next step in the solar array calibration will be to command the arrays to rotate farther, where they are not expected to be able to deliver all the power requested.

As Dawn begins its second year (as measured back on Earth) of interplanetary flight, the probe steadfastly continues its long journey in the quiet solitude of space, quite isolated from events on or near the distant planet that used to be its home. While no spacecraft has left the vicinity of the Earth-moon system in the year since Dawn's departure, much has happened there, even as the explorer has remained focused on accomplishing its voyage in deep space. From the first circulation of protons at the Large Hadron Collider 100 meters (330 feet) underground, to the beginning of the Fermi Gamma-ray Space Telescope mission 550 kilometers (340 miles) overhead, to the arrival of SELENE (Kaguya) and Chang'e 1 at the Moon, humankind's thrilling work to understand nature has continued. Apparently there have been some other kinds of news as well, from shocking revelations about celebrities, to competitions among athletes and among politicians, to still more shocking revelations about celebrities, but such information is harder to find, given the news media's nearly exclusive focus on myriad science topics. (News coverage may be different on your planet.)

Dawn is 374 million kilometers (232 million miles) from Earth, or 980 times as far as the Moon and 2.49 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 42 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 am PDT September 27, 2008

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