[meteorite-list] Dawn Journal - December 31, 2013

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_12_31_13.asp

Dawn Journal
Dr. Marc Rayman
December 31, 2013

Dear Clairvoydawnts,

Now more than halfway through its journey from protoplanet Vesta to dwarf 
planet Ceres, Dawn is continuing to use its advanced ion propulsion system 
to reshape its orbit around the sun. Now that the ship is closer to the 
uncharted shores ahead than the lands it unveiled astern, we will begin 
looking at the plans for exploring another alien world. In seven logs 
from now through August, we will discuss how the veteran adventurer will 
accomplish its exciting mission at Ceres. By the time it arrives early 
in 2015 at the largest object between Mars and Jupiter, readers will be 
ready to share not only in the drama of discovery but also in the thrill 
of an ambitious undertaking far, far from Earth.

Mission planners separate this deep-space expedition into phases. Following 
the "launch phase" was the 80-day "checkout phase." The "interplanetary 
cruise phase" is the longest. It began on December 17, 2007, and continued 
to the "Vesta phase," which extended from May 3, 2011, to Sept. 4, 2012. 
We are back in the interplanetary cruise phase again and will be until 
the "Ceres phase" begins in 2015. (Other phases may occur simultaneously 
with those phases, such as the "oh man, this is so cool phase," the "we 
should devise a clever name for this phase phase," and the "lunch phase.") 
Because the tasks at Vesta and Ceres are so complex and diverse, they 
are further divided into sub-phases. The phases at Ceres will be very 
similar to those at Vesta, even though the two bodies are entirely different.
Dawn thrusting with its ion propulsion system as it approaches Ceres

In this log, we will describe the Ceres "approach phase." The objectives 
of approach are to get the explorer into orbit and to attain a preliminary 
look at the mysterious orb, both to satisfy our eagerness for a glimpse 
of a new and exotic world and to obtain data that will be helpful in refining 
details of the subsequent in-depth investigations. The phase will start 
in January 2015 when Dawn is about 400,000 miles (640,000 kilometers) 
from Ceres. It will conclude in April when the spacecraft has completed 
the ion thrusting necessary to maneuver into the first orbit from which 
it will conduct intensive observations, at an altitude of about 8,400 
miles (13,500 kilometers). For a reason to be revealed below, that orbit 
is known by the catchy cognomen RC3.

(Previews for the Vesta approach phase were presented in March 2010 and 
May 2011, and the accounts of its actual execution are in logs from June, 
July, and August 2011. Future space historians should note that the differing 
phase boundaries at Vesta are no more than a matter of semantics. At Vesta, 
RC3 was described as being part of the approach phase. For Ceres, RC3 
is its own distinct phase. The reasons for the difference in terminology 
are not only unimportant, they aren't even interesting.)

The tremendous maneuverability provided by Dawn's uniquely capable ion 
propulsion system means that the exact dates for events in the approach 
phase likely will change between now and then. So for those of you in 
2015 following a link back to this log to see what the approach plan has 
been, we offer both the reminder that the estimated dates here might shift 
by a week or so and a welcome as you visit us here in the past. We look 
forward to meeting you (or even being you) when we arrive in the future.

Most of the approach phase will be devoted to ion thrusting, making the 
final adjustments to Dawn's orbit around the sun so that Ceres's gravity 
will gently take hold of the emissary from distant Earth. Next month we 
will explain more about the unusual nature of the gradual entry into orbit, 
which will occur on about March 25, 2015.

Starting in early February 2015, Dawn will suspend thrusting occasionally 
to point its camera at Ceres. The first time will be on Feb. 2, when they 
are 260,000 miles (420,000 kilometers) apart. To the camera's eye, designed 
principally for mapping from a close orbit and not for long-range observations, 
Ceres will appear quite small, only about 24 pixels across. But these 
pictures of a fuzzy little patch will be invaluable for our celestial 
navigators. Such "optical navigation" images will show the location of 
Ceres with respect to background stars, thereby helping to pin down where 
it and the approaching robot are relative to each other. This provides 
a powerful enhancement to the navigation, which generally relies on radio 
signals exchanged between Dawn and Earth. Each of the 10 times Dawn observes 
Ceres during the approach phase will help navigators refine the probe's 
course, so they can update the ion thrust profile to pilot the ship smoothly 
to its intended orbit.

Whenever the spacecraft stops to acquire images with the camera, it also 
will train the visible and infrared mapping spectrometer on Ceres. These 
early measurements will be helpful for finalizing the instrument parameters 
to be used for the extensive observations at closer range in subsequent 
mission phases.

Dawn obtained images more often during the Vesta approach phase than it 
will on approach to Ceres, and the reason is simple. It has lost two of 
its four reaction wheels, devices used to help turn or stabilize the craft 
in the zero-gravity, frictionless conditions of spaceflight. (In full 
disclosure, the units aren't actually lost. We know precisely where they 
are. But given that they stopped functioning, they might as well be elsewhere 
in the universe; they don't do Dawn any good.) Dawn's hominin colleagues 
at JPL, along with excellent support from Orbital Sciences Corporation, 
have applied their remarkable creativity, tenacity, and technical acumen 
to devise a plan that should allow all the original objectives of exploring 
Ceres to be met regardless of the health of the wheels. One of the many 
methods that contributed to this surprising resilience was a substantial 
reduction in the number of turns during all remaining phases of the mission, 
thus conserving the precious hydrazine propellant used by the small jets 
of the reaction control system.

When Dawn next peers at Ceres, nine days after the first time, it will 
be around 180,000 miles (290,000 kilometers) away, and the pictures will 
be marginally better than the sharpest views ever captured by the Hubble 
Space Telescope. By the third optical navigation session, on Feb. 21, 
Ceres will show noticeably more detail.

At the end of February, Dawn will take images and spectra throughout a 
complete Ceres rotation of just over nine hours, or one Cerean day. During 
that period, while about 100,000 miles (160,000 kilometers) distant, Dawn's 
position will not change significantly, so it will be almost as if the 
spacecraft hovers in place as the dwarf planet pirouettes beneath its 
watchful eye. Dawn will see most of the surface with a resolution twice 
as good as what has been achieved with Hubble. (At that point in the curving 
approach trajectory, the probe will be south of Ceres' equator, so it 
will not be able to see the high northern latitudes.) This first "rotation 
characterization," or RC1, not only provides the first (near-complete) 
look at the surface, but it may also suggest to insightful readers what 
will occur during the RC3 orbit phase.

There will be six more imaging sessions before the end of the approach 
phase, with Ceres growing larger in the camera's view each time. When 
the second complete rotation characterization, RC2, is conducted on March 
16, the resolution will be four times better than Hubble's pictures. The 
last photos, to be collected on March 24, will reveal features seven times 
smaller than could be discerned with the powerful space observatory.

The approach imaging sessions will be used to accomplish even more than 
navigating, providing initial characterizations of the mysterious world, 
and whetting our appetites for more. Six of the opportunities also will 
include searches for moons of Ceres. Astronomers have not found moons 
of this dwarf planet in previous attempts, but Dawn's unique vantage point 
would allow it to discover smaller ones than would have been detectable 
in previous attempts.

When the approach phase ends, Dawn will be circling its new home, held 
in orbit by the massive body's gravitational grip and ready to begin more 
detailed studies. By then, however, the pictures and other data it will 
have returned will already have taught Earthlings a great deal about that 
enigmatic place. Ceres has been observed from Earth for more than two 
centuries, having first been spotted on January 1, 1801, but it has never 
appeared as much more than an indistinct blob amidst the stars. Soon a 
probe dispatched by the insatiably curious creatures on that faraway planet 
will take up residence there to uncover some of the secrets it has held 
since the dawn of the solar system. We don't have long to wait!

Dawn is 20 million miles (32 million kilometers) from Vesta and 19 million 
miles (31 million kilometers) from Ceres. It is also 2.42 AU (225 million 
miles, or 362 million kilometers) from Earth, or 1,015 times as far as 
the moon and 2.46 times as far as the sun today. Radio signals, traveling 
at the universal limit of the speed of light, take 40 minutes to make 
the round trip.