Dawn Journal: Matching Paces with Vesta

Dear Adawnherents,

Dawn remains on course and on schedule for its appointments with Vesta and Ceres, colossal protoplanets in the main asteroid belt. Under the gentle and continuous thrust of its ion propulsion system, its journey through the solar system brings it ever closer to its first target. Last month's log included an overview of many of the spacecraft's activities during the final three months before its August 2011 arrival in the first science orbit at Vesta. In this "approach phase," the probe will observe Vesta with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the alien world prior to initiating its in-depth studies. The discussion did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time will be devoted to continuing its ion-powered flight. Let's take a more careful look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Vesta.

Thrusting is not necessary for a spacecraft to remain in orbit, just as the Moon remains in orbit around Earth and Earth and other planets remain in orbit around the Sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. With its extraordinarily efficient ion propulsion system, Dawn's near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the Sun.

The flight profile was carefully designed to send the craft by Mars in February 2009, so our explorer could appropriate some of the planet's orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn's orbit, Dawn lowered Mars's orbit, ensuring the solar system's energy account remained balanced.

While spacecraft have flown past a few asteroids in the main belt (although none as large as the behemoth Vesta nor the still more massive dwarf planet Ceres), no probe has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two solar system targets. Dawn's unique assignment would be quite impossible without ion propulsion. But with its light touch on the accelerator, taking nearly 4 years to travel from Earth past Mars to Vesta and more than 2.5 years from Vesta to Ceres, how will it enter orbit around Vesta, how will it break back out of orbit, and how will it enter orbit around Ceres?

Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its orbit around the Sun. This intriguing challenge was addressed in part in February 2007, as all readers with perfect memory recall. In August 2008, we considered another aspect of what is involved in climbing the solar system's hill, with the Sun at the bottom, Earth partway up, and the asteroid belt even higher. (Readers at that time in the past thoughtfully sent greetings through time to us, which we are now delighted to receive! On behalf of all present readers, we return the kind gesture with our own greetings.) We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the Sun, but then need to turn in order to move along with the body they will orbit.

Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet's gravity can hold it in orbit and carry it around the Sun. These exciting events can range from around 0.6 to 1.5 kilometers per second (1300 to 3400 miles per hour). With ten thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn't have to.

Dawn's method of getting into orbit is quite different, and the key is expressed in an attribute of the ion propulsion system that has been referred to 26 times (trust or verify; it's your choice) before in these logs: it is gentle. Dawn's entire thrust profile for its long interplanetary flight has been devoted largely to the gradual reshaping of its orbit around the Sun so that by the time it is in the vicinity of Vesta, its orbit will be very much like Vesta's. Only a small change will be needed to let the giant asteroid's gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit.

To get into orbit, a spacecraft has to match speed, direction, and location with its target. A mission with conventional propulsion first gets to the location and then, with the planet's gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling out to the orbit of Vesta (and later Ceres), Dawn works on its speed, direction, and location all at the same time, so they all gradually reach the needed values just at the right time.

To think about this facet of the difference between achieving this goal with the different technologies, imagine you want to drive your car along next to another traveling west at 100 kilometers per hour (60 miles per hour). The analogy with the conventional technology would be similar to heading north toward an intersection where you know the other car will be. You arrive there at the same time and execute a whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator, and probably some adrenaline. When you drive an ion propelled car, operating with 10 times the fuel efficiency, you take a different path from the start, one more like a long, curving entrance ramp to a highway. When you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is entirely different from the first method, but the sophisticated techniques of orbital navigation are up to the task.

In late July 2011, as the probe follows its approach trajectory to Vesta, their paths will be so similar they will be moving at nearly the same direction and speed around the Sun (about 20.5 kilometers per second or almost 46 thousand miles per hour). When at a range of about 16 thousand kilometers (9900 miles), the spacecraft will be traveling at less than 50 meters per second (110 miles per hour) relative to its destination. That combination of distance and velocity will allow Vesta to take gentle hold of Dawn. The spacecraft will not even notice the difference, but it will be in orbit around its first celestial target, even as it continues ion thrusting to reach the planned orbit more than 2 weeks later.

With the gradual trajectory changes inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. Dawn is propelling itself along a spiral path around the Sun as it journeys from Earth out to Vesta, the first loop having been completed in June 2009. It will arrive at Vesta before it completes the second revolution. Then its flight profile will be designed to spiral around Vesta as the probe and protoplanet together orbit the Sun. Dawn's first loop around Vesta will be about 10 days, and its second will take 4. It will stop thrusting when it is in "survey orbit," where one revolution takes just under 3 days. After collecting a rich bounty of pictures and other important scientific data from this altitude of about 2700 kilometers (1700 miles), it will resume thrusting, spiraling down to lower and lower orbits, requiring hundreds of revolutions. Dawn's speed will increase as its orbital altitude decreases, so the loops will progressively become shorter.

In 2012, after completing months of close-range scientific observations, it will reverse the spirals, gradually climbing away from the world it has been studying just as it gradually climbed away from the Sun. Vesta's gravitational hold will weaken as Dawn moves farther and faster, its graceful motion ultimately exceeding the strength of the invisible tether that bound it. As gently as it arrived, it will depart. In July of that year, it will once again be on its own in orbit around the Sun, and navigators will instruct it to point its ion thruster to spiral outward more in order to undertake its pursuit of Ceres.

These spiral paths do not occur naturally. Under the predictable and calculable effects of the gravity of the Sun and other bodies (including Vesta or Ceres), Dawn is programmed to orient its thruster in just the right direction at the right time to propel itself on the desired trajectory. A great deal of work was required before launch to devise such a plan. Changes since then have been determined by knowledge gained during the mission, such as an update to the prediction of how much power the solar array will yield.

Engineers have completed work on the approach phase for now. They have reviewed the sequences (the timed instructions the spacecraft will follow) in detail and have tested portions of them in the spacecraft simulator at JPL. The sequences are mature enough that they will be ready for controllers to update and refine as necessary next year before being radioed to the spacecraft. Now the operations team is turning its attention to the subsequent phase of the Vesta mission, survey orbit, where the intensive observations will begin. We will learn more about that in the next log.

Dawn's controllers certainly do not focus all their efforts on preparing for Vesta. (Your correspondent devotes some of his to dancing, but perhaps that's a topic for a future log.) Of course, keeping the spacecraft healthy and on course is essential as well. In addition to commanding it to sustain the needed thrusting, with a weekly hiatus for telecommunications, they perform routine maintenance to ensure the ship remains in top shape. For example, engineers recently adjusted the spacecraft's master clock. Always in the glow of the distant Sun, and never needing to rest or take a break from its duties, the robot has no need to switch to daylight saving time. Nevertheless, a time change was called for because the onboard time had gradually drifted from the correct value. It had last been set on February 27, 2008, and has remained sufficiently accurate for all Dawn's needs. With the gradual nature of this mission, precise timing is generally not necessary, so although they have closely monitored the clock, controllers have allowed it to run without correction. When they commanded the transition from ion thruster #1 to thruster #2 in January in January, they expected the clock to change slightly, and indeed it did. Thruster #2 uses a different power control unit from thrusters #1 and #3. The #2 controller is mounted closer to the electronics assembly that includes Dawn's clock, and now that that device is powered, the heat it dissipates warms the clock a little, so the clock rate is slightly altered. Although much larger values could be accommodated, when the time offset had crept up to 1.37 seconds, operators set it back to the correct time, and they included a change to account for the warmer environment. (Readers may wish to pause for 1.37 seconds to contemplate the difficulties of synchronizing clocks that are farther apart than the Sun.)

An improved version of the test to measure the overlap of the views of the visible and infrared mapping spectrometer (VIR) and the prime science camera was executed successfully. When the measurement was carried out in December, a conflict between commands in the VIR sequence prevented the intended data from being acquired.

As if maintaining the spacecraft's health and powered flight and developing detailed plans for Vesta weren't enough to keep Dawn's engineers happy, they also are continuing work on a new version of the software for the primary computer, scheduled to be transmitted to the spacecraft in June. The mission also will mark three milestones that month, and it may not be a surprise if your correspondent marcs them in the next log.

Dawn is 1.62 AU (243 million kilometers or 151 million miles) from Earth, or 1.61 times as far as the Sun and 650 times as far as the Moon. Radio signals, traveling at the universal limit of the speed of light, take 27 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 am PDT April 28, 2010