Marc RaymanApr 23, 2008

Dawn Journal: One AU from Earth

Dear Heliodawns,

Dawn is as far from Earth as the Sun. (That seemed a better opening sentence than the next sentence. If you, loyal reader, disagree, please read the next one first.) The Dawn mission is continuing very smoothly, with the spacecraft spending almost all of the time since the last log thrusting with its ion propulsion system.

As the probe and Earth follow their independent orbits around the Sun, not only has their separation grown, but the relative velocity has as well. So, the rate at which the range from the spacecraft to its erstwhile planetary residence increases has itself increased since the previous report. Today the distance is climbing by nearly 1.6 million kilometers (almost 970 thousand miles), or more than five light seconds, per day.

Now, six months after launch, the separation between Earth and Dawn has widened to be equal to the distance between Earth and the Sun. Most readers, particularly those living elsewhere, recognize that the distance between these latter two solar system bodies, about 150 million kilometers (93 million miles) and known to astronomers as an "astronomical unit," is arbitrary. Still, using the Earth-Sun distance as a reference may help put the spacecraft's travel into context, and astronomical units are no more arbitrary than kilometers, miles, or the length scales in common use on over 85% of planets subscribing to these logs.

Dawn’s mission trajectory as of September 2007
Dawn’s mission trajectory as of September 2007 Note: this is likely to be modified following the launch, after the mission team checks out the performance of the ion engines.Image: NASA/JPL-Caltech

That Dawn is as far from Earth as the Sun does not reveal anything, of course, about its direction as viewed from Earth. The Sun and Dawn are 98 degrees apart; Dawn is nearly overhead when the Sun sets. Although the spacecraft is much, much too far from Earth to be visible, even with the most powerful telescopes, readers who find enjoyment, if not inspiration for rich thoughts, in celestial sights may consider gazing in the direction of the emissary Earth dispatched to the cosmos. The craft is in Gemini, about 8 degrees east of Mars, an easily located ruby among the gems of the night sky. Your correspondent (reporting on location from Earth) plans to contemplate his view of the Sun during the day and the sky near Dawn in the evening.

The spacecraft's reliable performance has allowed the operations team to devote time to more than ensuring the spacecraft stays healthy and on course. In addition to an occasional moment of waxing philosophical about another of humankind's robotic marvels being so remote, the team has been preparing for some special activities during the first part of April. Readers not obeying rules of causality already know the outcome, but the rest may look forward to learning the story in the next log.

Dawn is 150 million kilometers (93 million miles) from Earth, or 390 times as far as the moon and as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take almost 17 minutes to make the round trip.

Dr. Marc D. Rayman
8:00 am PDT March 30, 2008

P.S. This log is short so it would qualify for entry in the highly competitive "Best Dawn Log Fewer than 500 Words Written by a Mortal Corporeal Entity" contest. Unfortunately, this postscript causes it to exceed the length limit. That's particularly disappointing, as the first place prize is a small galaxy.

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