Dawn Journal: It's Just a Phase
Posted by Marc Rayman
30-03-2010 12:43 CDT
Dawn continues patiently forging through the asteroid belt, its permanent residence, as it climbs away from Earth and the Sun. Having thrust with its ion propulsion system for more than 1.5 years, the spacecraft remains healthy and on target for its rendezvous with alien worlds.
Our interplanetary adventurer still has a great deal of ion thrusting to complete before it can begin its orbital exploration of Vesta next year. Although it will suspend thrusting for a few weeks this summer to conduct some special activities (to follow along, be sure to renew your subscription to these logs the first time our helpfully persistent telemarketers call), it will devote most of the time until early August 2011 in powered flight, continuously reshaping its orbit around the Sun.
In addition to keeping the ship sailing smoothly and on course, Dawn's engineers (who reside and work on distant Earth) are developing the detailed instructions that will guide it into orbit around Vesta and throughout its year of operations there. This process began last month and will continue even as the probe begins executing the first of the commands in May 2011.
Mission controllers compile Dawn's instructions by assigning a time to each individual command. Groups of these timed commands are known as a "sequence." During the current interplanetary cruise phase of the mission, sequences generally extend for five weeks, but some special activities may use sequences as short as a few hours. Usually more than one sequence is executing at a time, but like all the instruments in an orchestra, they are carefully synchronized and coordinated so the overall score accomplishes the composer's artistic intent.
Readers may recall that the mission is separated into phases. Following the "launch phase" was the 80-day "checkout phase." The current "interplanetary cruise phase," which began on December 17, 2007, is the longest. It ends when the "Vesta phase" begins. (Other phases may occur simultaneously with those phases, such as the "oh man, this is so cool phase," the "what clever name are we going to give this phase phase," and the "lunch phase.") Because the mission at Vesta is so complex, it is further divided into sub-phases. The Vesta sequences that are being developed now are for the "approach phase." Approach begins in early May 2011 and concludes three months later, when Dawn will have maneuvered to the first orbit from which it will conduct intensive science observations, known as survey orbit.
Most of the approach phase is dedicated to the final ion thrusting required to slip into orbit around Vesta. All of Dawn's thrusting contributes to rendezvousing with Vesta, but the terminal thrusting will be controlled slightly differently. We will describe the process of using ion propulsion to enter orbit around another solar system body in an upcoming log. For now, however, let's take a look at some of the other activities during the approach phase. While these are being timed in the sequences down to the second, part of the strategy for developing these sequences is to allow the team a means to update the times as the probe closes in on its target. The ion propulsion system provides flexibility in the timing that is different from most missions, and to take advantage of the benefits, the sequences must be correspondingly flexible. All the relative timing within a sequence will be fixed, but the time each sequence is activated can change. So, for example, even though we may change the date the first Vesta approach sequence begins executing by as much as a few days, once that adjustment is made, all the events within the sequence will shift by exactly the same interval. Some small changes other than timing, such as details of the probe's orientation, may be made as well to reflect the latest information available before it is time to transmit the sequences to the spacecraft more than a year from now.
The principal activity other than thrusting during approach is the acquisition of images of Vesta with Dawn's main science camera, primarily for navigation. From the distant vantage point of Earth, astronomers can determine Vesta's location with astonishing accuracy, and the Dawn navigation team achieves extraordinary accuracy in establishing the probe's position, but for the craft to enter orbit, still greater accuracy is required. Therefore, Dawn will observe Vesta's location against the background of stars, and the photographs will be analyzed by celestial navigators to pin down the relative location of the ship and the port of call it is approaching. To distinguish this method from the one by which Dawn is usually navigated, making use of its radio signal, this supplementary technique with pictures is generally known as "optical navigation." There are 24 optical navigation sessions during the three-month approach phase. Many of these will be combined with observations of Vesta designed to help prepare for subsequent scientific measurements.
The positions of the spacecraft and protoplanet will be determined well enough with the current navigation method that engineers will know which stars will appear to be near Vesta from Dawn's perspective. It is the analysis of precisely where Vesta appears relative to those stars that will yield the necessary navigational refinement. When Dawn is closer to Vesta, the giant asteroid will occupy most or all of the camera's view, and stars won't be visible. Then the optical navigation will be based on determining the location of the spacecraft with respect to specific surface features that have been charted in previous images.
For the optical navigation observations, Dawn will halt thrusting and align itself so that Vesta and, when possible, the stars are in view of the camera. It will spend half an hour or more taking images and storing them for transmission at the next scheduled communications session. The information extracted from the images will be used to calculate where the probe is relative to its destination. Engineers then will update the design of the trajectory for the spacecraft to follow to reach its intended orbit and fine-tune the ensuing thrust profile to ensure that Dawn accomplishes the revised flight plan.
The first optical navigation images will be acquired when Dawn is about 1.2 million kilometers (750 thousand miles) from Vesta, or more than 3 times the separation between Earth and the Moon. Dawn's camera is designed for mapping Vesta from orbit. Therefore, instead of a high-power telescope with a narrow field of view, the camera has a relatively low magnification but covers a broad area. The camera achieves the equivalent of a magnification of about 3 compared to unaided human eyes. When these first optical navigation images are taken, distant Vesta will appear to be only about five pixels across. But at that stage, navigators will need to know its location, not its appearance, so the images will be of great value.
For eight of the approach observation periods, in addition to the camera, the visible and infrared mapping spectrometer (VIR) will be trained on Vesta. By taking some early measurements with the camera and VIR, scientists will have the opportunity to make fine adjustments to the instrument parameters in the sequences for later observations.
In one of the optical navigation sessions in July, the camera will acquire many images of the space around Vesta in a search for moons. Astronomers have looked for moons of Vesta before, and will do so again before the explorer reaches its vicinity. Although none has been discovered, Dawn's unique vantage point will provide more data. The existence of moons would be of interest both for science and for mission safety.
When Dawn suspends thrusting to check for moons, it also will collect a series of images as Vesta rotates. Like Earth and all other solar system bodies, Vesta spins. It completes one turn on its axis (one Vestian "day") in about 5 hours, 20 minutes. These measurements will help characterize the alien world still more to aid in navigation and to prepare for subsequent observations with the science instruments. The moon search will be during the second of three observations of a full rotation.
Over the course of the three-month approach, it will be exciting to watch Vesta grow from little more than a tiny smudge in the first optical navigation images until it is too large to fit in the camera's view at the end of the phase. By early June 2011, the images will surpass the best that can be obtained with the Hubble Space Telescope. All succeeding observations will yield better and better views, both rewarding us and tantalizing us as Dawn prepares for its more intensive studies in later Vesta phases.
The spacecraft will glide into a very high orbit in late July and continue thrusting, gently as always, until early August, when it will arrive in its survey orbit at an orbit at an altitude of about 2700 kilometers (1700 miles). The activities to be conducted in the survey phase will be described when mission planners are working on those sequences.
In the meantime, the team is running some of the approach sequences through the Dawn spacecraft simulator at JPL down the hall from mission control. The simulator includes some hardware that is virtually identical to what is on the spacecraft and some software to take the place of other hardware components. The simulator is one of several methods used to check complex sequences before they are approved for transmission to the spacecraft.
It is both unnecessary and impossible to test all sequences. The simulator operates in real-time, so it would take 3 months to run all the approach sequences, and the Dawn team has too many other tests to perform with the simulator to allow that. Because much of the approach phase consists of ion thrusting, an activity which is quite familiar not only to the spacecraft but also to mission controllers (as well as regular readers of these logs), there is no need to test the thrusting periods. Engineers review each sequence to determine which portions would benefit from testing.
While the spacecraft simulator is hard at work at JPL, the actual spacecraft continues its work elsewhere. On February 28, Dawn and the Sun were equidistant from Earth. Now, as the distant explorer continues to propel itself toward its rendezvous with Vesta, it is farther from Earth than the Sun ever is. Moreover, even as the probe and the planet follow their separate paths around the Sun, Dawn will remain farther from Earth than the Sun. The orbits of Mercury, Venus, Mars, and many other members of the solar system family occasionally bring them closer to our planet than the Sun, but Dawn has enlarged its orbit so much that it never will return to the region of the solar system in which it began its ambitious journey of discovery.
Dawn is 1.27 AU (191 million kilometers or 118 million miles) from Earth, or 525 times as far as the Moon and 1.28 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
8:00 pm PDT March 28, 2010