Dawn Journal
Dr. Marc Rayman
October 31, 2009
Dear Dawn-o'-lanterns,
Dawn continues to make steady progress on its journey through the solar
system. The spacecraft has devoted another month to thrusting with its
ion propulsion system, ever with its sights set on its rendezvous with
Vesta in July 2011. While it will have other assignments along the way,
propelling itself to the giant protoplanet deep in the main asteroid
belt remains its principal responsibility.
The asteroid belt consists of innumerable objects in orbit around the
Sun between Mars and Jupiter. (Dawn is aiming for the 2 most massive
members of the belt.) Just as with a ball of cotton or a cloud, while
there may appear to be a clear border when viewed from a great distance,
a more careful examination reveals it to be less distinct. There is no
sharp edge to demarcate the boundary. For example, although most
asteroids remain between the two planets, the orbits of some bring them
closer to the Sun than Mars. We can adopt a part of one common
definition in which, to be designated as a resident of the main asteroid
belt, an object's orbit can bring it no closer to the Sun than 1.666
astronomical units (AU). It is not coincidental that this is the greatest
distance that Mars travels from the Sun. (Earth and its inhabitants never
reach more than 1.017 AU from the solar system's gravitational master.)
As with Earth, Mars, and asteroids, Dawn's orbit around the Sun is
elliptical. The principal difference is that the ship is constantly
changing its course by emitting a high velocity beam of xenon ions. (It
has racked up more than 10,000 hours of powered flight, with much more
thrusting ahead.) In a lovely solar system dance in February, Dawn
briefly partnered with Mars for additional assistance on its way.
As we saw in the last log , the spacecraft's orbit grows larger as the
mission progresses, bringing the explorer ever closer to its first
destination. On November 13, it will enter the asteroid belt as its
silent flight takes it past 1.666 AU from the Sun. It will remain in the
belt for the rest of its mission and well beyond. Dawn will become a
permanent inhabitant of that part of the solar system, the first emissary
from Earth to take up residence in the main asteroid belt.
The probe has been here before. On June 30, 2008 it passed the outermost
part of Mars' orbit. But its elliptical path reached its greatest distance
from the Sun of more than 1.68 AU on August 8, 2008, and 40 days after that,
it crossed the orbit of Mars again. On April 17, 2009, then at 1.37 AU from
the Sun, its momentum began carrying it outwards once again. By then it
was in a larger orbit, and thanks to the extensive additional orbital energy
imparted to the spacecraft by its persistent ion thrusting, it will sail
smoothly through 1.68 AU next month and continue deeper into the asteroid belt.
As Dawn continuously enlarges its solar orbit still more, mission
controllers work diligently to ensure the distant craft remains healthy.
They are also preparing to give it some additional tasks before the year
is out, and inside sources reveal that these may be described in an
upcoming log. In the meantime, emitting its eerie bluish glow, the probe
silently streaks toward unexplored worlds, seeking to reveal new secrets
and likely new questions as well.
Dawn is 1.25 AU (187 million kilometers or 116 million miles) from
Earth, or 485 times as far as the moon and 1.26 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of light,
take 21 minutes to make the round trip.
Dr. Marc D. Rayman
11:30 pm PDT October 31, 2009
P.S. Although Dawn works tirelessly in interplanetary space, the team on
Earth is taking a break for Halloween. Observant readers have already
noticed that this correspondent has dawned his costume, and it is a
delightful and impressive disguise indeed. In an act of astonishing
creativity, he is pretending to be someone who can write a (relatively)
short log.
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