As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.
Powering its way through the main asteroid belt between Mars and Jupiter, Dawn continues on course and on schedule for its 2015 appointment with dwarf planet Ceres. After spending more than a year orbiting and scrutinizing Vesta, the second most massive object in the asteroid belt, the robotic explorer has its sights set on the largest object between the sun and Neptune that a spacecraft has not yet visited. This exotic expedition to unveil mysterious alien worlds would be impossible without the probe’s ion propulsion system.
Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that…
We crave power!!
The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft’s huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend from a pitcher’s mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.
To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.
To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.
The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.
In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)
Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.
Under normal circumstances, the spacecraft doesn’t need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.