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

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_05_31_13.asp

Dawn Journal
Dr. Marc Rayman
May 31, 2013

Dear Confidawnts,

Traveling from one alien world to another, Dawn is reliably powering its 
way through the main asteroid belt with its ion propulsion system. Vesta, 
the fascinating and complex protoplanet it explored in 2011 and 2012, 
falls farther and farther behind as the spacecraft gently and patiently 
reshapes its orbit around the sun, aiming for a 2015 rendezvous with dwarf 
planet Ceres.

The stalwart adventurer has recently completed its longest uninterrupted 
ion thrust period yet. As part of the campaign to conserve precious hydrazine 
propellant, Dawn now suspends thrusting once every four weeks to point 
its main antenna to Earth. (In contrast, spacecraft with conventional 
chemical propulsion spend the vast majority of time coasting.) Because 
of details of the mission operations schedule and the schedule for NASA's 
Deep Space Network, the thrust durations can vary by a few days. As a 
result, the spacecraft spent 31.2 days thrusting without a hiatus. This 
exceeds Deep Space 1's longest sustained powered flight of 29.2 days. 
While there currently are no plans to thrust for longer times, the unique 
craft certainly is capable of doing so. The principal limitation is how 
much data it can store on the performance of all subsystems (pressures, 
temperatures, currents, voltages, valve positions, etc.) for subsequent 
reporting to its terrestrial colleagues.

Thanks to the ship's dependability, the operations team has been able 
to devote much of its energies recently to developing and refining the 
complex plans for the exploration of Ceres. You might be among the privileged 
readers who will get a preview when we begin describing the plans later 
this year.

Controllers also have devised some special activities for the spacecraft 
to perform in the near future, accounts of which are predicted to be in 
the next two logs.

In addition, team members have had time to maintain their skills for when 
the spacecraft needs more attention. Earlier this month, they conducted 
an operational readiness test (ORT). One diabolical engineer carefully 
configured the Dawn spacecraft simulator at JPL to behave as if a pebble 
one-half of a centimeter (one-fifth of an inch) in diameter shooting through 
the asteroid belt collided with the probe at well over twice the velocity 
of a high-performance rifle bullet.

When the explorer entered this region of space, we discussed that it was 
not as risky as residents of other parts of the solar system might assume. 
Dawn does not require Han Solo's piloting skills to avoid most of the 
dangerous rocky debris.

The robot could tolerate such a wound, but it would require some help 
from operators to resume normal operations. This exercise presented the 
spacecraft team with an opportunity to spend several days working through 
the diagnosis and performing the steps necessary to continue the mission 
(using some of the ship's backup systems). While the specific problem 
is extremely unlikely to occur, the ORT provided valuable training for 
new members of the project and served to keep others sharp.

One more benefit of the smooth operations is the time that it enables 
your correspondent to write his third shortest log ever. (Feel free to 
do the implied research.) Frequent readers can only hope he strives to 
achieve such a gratifying feat again!

Dawn is 13 million kilometers (7.9 million miles) from Vesta and 54 million 
kilometers (34 million miles) from Ceres. It is also 3.25 AU (486 million 
kilometers or 302 million miles) from Earth, or 1,275 times as far as 
the moon and 3.20 times as far as the sun today. Radio signals, traveling 
at the universal limit of the speed of light, take 54 minutes to make 
the round trip.