There is only about three quarters of a revolution remaining around the Sun before Dawn leaves Earth to travel on its own to distant worlds. Meanwhile, the project team continues to prepare the spacecraft for its mission. This work has proceeded smoothly despite the chaos of planets apparently coming and going from our solar system.
As readers in other solar systems have no doubt followed with some detached amusement, the definition of “planet” was in the news in this solar system this summer. While much of the focus was on whether Pluto should be considered a planet, Dawn’s second destination, Ceres, also was subjected to this linguistic turmoil. The International Astronomical Union (IAU) adopted a definition of “dwarf planet” that includes Ceres, Pluto, Eris, and perhaps more bodies yet to be characterized sufficiently or even discovered. Ceres is the largest member of the asteroid belt, residing between Mars and Jupiter; the other dwarf planets are part of the Kuiper belt, spending most or all of their time beyond the most distant planet, Neptune.
Resolution 5A passed by the 26th General Assembly of the IAU describes the attributes a body must have to qualify as a dwarf planet. Like a planet, it must orbit the Sun and have sufficient mass for its own gravity to make it nearly spherical. (Vesta, the first stop on Dawn’s interplanetary itinerary, might satisfy the definition of dwarf planet, but not enough is known yet about its gravity and shape.) Unlike planets however, dwarf planets are characterized by not having cleared away other objects from their part of the solar system through the effects of their gravity. This bars any resident of the asteroid belt or the Kuiper belt from membership in the planet club. (Another criterion, that the body not be a satellite, excludes some of the moons of planets from being designated as planets themselves.)
The definition is not widely accepted by the community of planetary scientists, and it remains to be seen how the definition might be changed. Ceres and Vesta were considered planets for half a century following their discoveries in 1801 and 1807 respectively. All scientific evidence indicates that with all the names humans have applied to them, including planets, asteroids, minor planets, protoplanets, and dwarf planets, they have steadfastly remained above the controversy, leading their stately lives without apparent interest.
The Dawn team has never wavered about what to call these bodies; with the utmost clarity and consistency, they have always been known as “Ceres” and “Vesta.” Team members continue to look forward to the wealth of information the spacecraft will return from its orbits around these fascinating places. In continuing to prepare for that, engineers are completing another set of the comprehensive performance tests (as explained in previous logs) to verify that the subsystems on the spacecraft can fulfill the required functions.
Loyal readers will come to be familiar with Dawn’s subsystems as we take it through the rest of its prelaunch preparations and we join it, in spirit if not in person, on its cosmic travels. [Editor’s note: “Loyal readers” is redundant; our recent surveys show 100% of readers in the targeted galaxies are loyal.] As we shall see over the coming years, there is nothing like guiding a spacecraft through the forbidding depths of space to understand how it really works. But now let us have a very very brief introduction to the engineering subsystems that allow Dawn to conduct its mission. In a future log, we will describe the scientific instruments, which will help reveal the natures of Ceres and Vesta.
The command and data handling subsystem includes the main computers that operate the probe along with most of the other electronics. As with most Dawn subsystems, the design includes primary and backup components so that even if a failure occurs far from Earth, the spacecraft can continue to fulfill its scientific mission. This subsystem keeps the spacecraft functioning smoothly as it operates on its own in space. Running in its three primary computers is the master software for the spacecraft, consisting of more than 400,000 lines of C and assembly code. In addition to its own orchestrations of spacecraft activities, it processes commands sent by the mission operations team and issues them when required to other subsystems. It stores the scientific data acquired by the instruments and collects information on the performance of the spacecraft, all to be reported back to Earth. Some engineers would consider this to be the most important subsystem on the spacecraft.
The electrical power subsystem (OK, I know you’re ahead of me on this one) provides the power needed by all electrical components onboard. Its solar arrays convert light from the Sun into electricity, and the subsystem delivers high voltage to the ion propulsion subsystem and lower voltage to all the other subsystems. Because Dawn will need high electrical power for its ion propulsion subsystem even when far from the Sun, the solar arrays are very large for a planetary spacecraft. Each of the two solar array wings is almost 8.3 meters (more than 27 feet) long, and when they are extended shortly after launch, the overall craft will be about 19.7 meters (nearly 65 feet) from wing tip to wing tip. This subsystem includes a powerful battery whose primary purpose is to allow Dawn to operate while on the rocket and during the time immediately after separation when it needs to perform a number of critical functions to deploy its arrays and point them at the Sun. The arrays will generate more than 10 kilowatts at Earth’s distance from the Sun (enough to power 10 average households in the US). This is far more power than Dawn can use, but when it has receded to 3 times Earth’s distance from the Sun, every watt it can yield will be of great value to the spacecraft, with its power-hungry ion propulsion subsystem. Some engineers would consider this to be the most important subsystem on the spacecraft.
The attitude control subsystem (despite the name, this subsystem is as delightful to work with and is as enthusiastic about the mission as all other subsystems) is responsible for controlling the orientation (which engineers refer to as “attitude”) of the craft in the zero-gravity of spaceflight. This subsystem can orient the probe so that it points an ion thruster in the direction required to reach its cosmic destinations, directs an antenna to distant Earth, or aims the camera or other instruments so they may observe their targets. It also will keep the solar arrays pointed at the Sun. To determine its attitude, Dawn uses “star trackers” (again, two are onboard, although only one is needed), cameras that recognize star patterns and thereby reveal the direction they are pointed. (For readers who accompanied Deep Space 1 on its voyage, it was the failure of the sole star tracker during the extended mission that led to the need to conduct the spectacular rescue of the spacecraft. That is described in the logs of 2000, available at http://nmp.jpl.nasa.gov/ds1/archives.html and in late night reruns on most planets not in synchronous rotation around their stars.) The subsystem also carries gyroscopes to improve the accuracy of the pointing. For emergency use, Sun sensors can help the spacecraft establish its approximate attitude when a star tracker is temporarily off-line. Devices known as reaction wheels are electrically spun faster or slower to rotate the spacecraft. Some engineers would consider this to be the most important subsystem on the spacecraft.
For technical reasons, the reaction wheels are not sufficient for all the pointing control Dawn will need during its long mission, so another means is required. In addition to the reaction wheels, which are considered part of the attitude control subsystem, there are two other subsystems that attitude control uses to achieve the orientations it needs. The reaction control subsystem includes 12 small thrusters that use a conventional rocket propellant known as hydrazine; you may not be surprised to know that only 6 thrusters are needed, so even if an entire group of 6 failed, the mission would not be lost. Each brief pulse of a thruster causes the spacecraft to change how fast or in what direction it rotates. This subsystem will be loaded with about 45 kilograms (100 pounds) of hydrazine, although it likely will use much less than that during the mission. Some engineers would consider this to be the most important subsystem on the spacecraft.
Most interplanetary spacecraft use hydrazine-based propulsion not only to turn but also to change their trajectories through space. Dawn is able to undertake its detailed exploration of the most massive bodies in the asteroid belt because it uses a more capable form of propulsion. The ion propulsion subsystem accomplishes this by ionizing xenon gas; that is, it gives it a small positive electrical charge by removing a negatively charged electron from each neutral xenon atom. Once the xenon is ionized, the subsystem can electrically accelerate the ions and emit them at very high speed from any 1 of the 3 ion thrusters. The action of each xenon ion as it is shot from a thruster at up to 40 kilometers per second (89,000 miles per hour) causes a reaction that pushes the spacecraft in the other direction. Dawn will launch with 425 kilograms (937 pounds) of xenon -- more than enough to allow it to travel to and orbit its targets while setting some remarkable records to be described in future logs. Because ion propulsion is so different from conventional propulsion systems, it leads to many differences in the way we design and conduct the mission, and later logs will describe this in more detail (once our attorneys prove their case that the copyright infringement claims by the self-proclaimed Ionic Potentate of Xenon are invalid). In addition to its role in propelling Dawn to Vesta and Ceres, in some cases the ion propulsion subsystem (instead of the reaction wheels or the reaction control subsystem) is used by attitude control to help control the direction the spacecraft points. While this subsystem obviously is important, some engineers would consider the next one to be the most important on the spacecraft.
The thermal control subsystem keeps all of Dawn’s subsystems operating within their required temperature ranges as the craft travels from Earth past Mars to Vesta and then continues on to Ceres, reaching 3 times Earth’s distance from the Sun. The temperatures of delicate electronics, precisely aligned structural elements, sensitive mechanical devices and materials, lubricants, adhesives, hydrazine, xenon, and more all must be controlled. This subsystem must ensure that units stay cool even when they experience direct exposure to the searing Sun while being warmed still more by their own electrical activity and stay warm even when they face the paralyzing cold of darkest space. Louvers on some parts of the spacecraft open or close in response to temperature to let heat radiate away or be trapped on the spacecraft as necessary. Some of the spacecraft panels are embedded with tubes of ammonia to help distribute the heat more uniformly, carrying excess heat from electrically powered devices to others that are powered off or otherwise in need of additional heat. The subsystem also includes more than 140 heaters and is one of the largest consumers of electrical power on the spacecraft. While this subsystem obviously is important, some engineers would consider the ion propulsion subsystem to be the most important on the spacecraft.
The telecommunications subsystem allows Dawn to exchange information with Earth, even at enormous distances. The spacecraft’s main antenna is 1.52 meter (5 feet) in diameter, and 3 smaller antennas allow communications when it is not possible or not convenient to point the large dish at Earth. Dawn will communicate with mission controllers through the 34-meter (112-foot) or 70-meter (230-foot) antennas of NASA’s Deep Space Network (DSN) in California, Spain, and eastern Australia. While Dawn is returning scientific data from Ceres at maximum range, the 100-watt radio signal it transmits, after traversing the vast distance to Earth, will be less than one tenth of one millionth of one billionth of a watt when it is received by a 34-meter antenna. If this energy were collected for the age of the universe, it would be enough to illuminate a refrigerator light bulb for 1 second, yet it is sufficient to carry all the images and other rich scientific data to Earth. Dawn’s receiver, always alert for faint whispers from home, can make sense of a signal weaker than one billionth of one billionth of a watt. Some engineers would consider this to be -- well, you get the message.
After this brief overview of the subsystems, it would be easy to lose sight of what some engineers would consider to be more important than any subsystem: the system. All subsystems have to work together for the spacecraft work. Besides the instruments, some essential parts of that spacecraft are missed in this description of active subsystems, such as the structure upon which everything is built. In addition, to connect the many elements of the subsystems to each other, Dawn includes 9000 wires with a total length of about 25 kilometers (15 miles). The cables and their connectors account for more than 83 kilograms (183 pounds) of the mass that will travel to Vesta and Ceres. When fully assembled and loaded with its propellants, Dawn will be somewhat more than 1200 kilograms (2650 pounds).
Some engineers would consider there to be a larger system, still more important than the entirety of the spacecraft, that is needed to make Dawn a success. Indeed, the full system is not only what flies in space; the complete Dawn system has many elements that remain on Earth, including networks of computers, extensive software, antennas, transmitters, receivers, and a team of dedicated and inquisitive people who recognize their good fortune to participate in this grand adventure.
Now strange as it may seem, there seems to be some evidence that 2 of our readers, despite being loyal, have not yet submitted their names to be carried on the spacecraft. The end of the last log described our plans to include the names of all members of what really is the largest and most important system: the people whose spirits are carried aloft by humankind’s efforts to know the cosmos. Don’t be the last one to add your name to the spacecraft at http://www.dawn-mission.org/DawnCommunity/Sendname2asteroid/nameEntry.asp.
Dr. Marc D. Rayman
September 17, 2006