A fantastic story of adventure, exploration and discovery is reaching its denouement. In the final phase of its long and productive deep-space mission, Dawn is operating flawlessly in orbit around dwarf planet Ceres.
As described in the previous Dawn Journal, every 27 hours the spacecraft swoops as low as about 22 miles (35 kilometers) above the ground, taking stunning pictures and making other unique, valuable measurements with its suite of sophisticated sensors. It then soars up to 2,500 miles (4,000 kilometers) over the alien world before diving down again.
While it is too soon to reach clear conclusions from the wealth of high-resolution data, some of the questions already raised are noteworthy: Are the new pictures totally awesome or are they insane? Are they incredible or are they unbelievable? Are they amazing or are they spectacular? It may take years to resolve such questions. The mission will end long before then, indeed very soon. In this Dawn Journal and the next one (which will be posted in about three Cerean days), we will preview the end.
When Dawn left Earth in 2007, it was outfitted with four reaction wheels, devices that were considered indispensable for controlling its orientation on its long expedition in deep space. Despite the failures of reaction wheels in 2010, 2012 and 2017, the team has accomplished an extremely successful mission, yielding riches at Vesta and at Ceres far beyond what had been anticipated when the interplanetary journey began. But now the rapidly dwindling supply of hydrazine propellant the robot uses in place of the reaction wheels is nearly exhausted.
With no friction to stabilize it, the large ship, with electricity-generating solar arrays stretching 65 feet (19.7 meters) wingtip-to-wingtip, holds its orientation in space by firing hydrazine propellant from the small jets of its reaction control system. The orientation should not be confused with the position. In the zero-gravity of spaceflight, they are quite independent. Unlike an aircraft, a spacecraft's position and the direction it travels are largely unrelated to its orientation. The probe's position is dictated by the principles of orbital motion, whether in orbit around the Sun, Vesta or (now) Ceres, and the ion propulsion system is used to change its trajectory. We are concerned here about orientation.
Dawn can hold its orientation quite stable, but it still lazily oscillates a little bit in pitch, roll and yaw. When the spacecraft points its main antenna to Earth, for example, spending many hours radioing its findings to the Deep Space Network (DSN) as it travels around Ceres, it rotates back and forth, but the angular motion is both tiny and slow. The ship turns about a thousand times slower than the hour hand on a clock. The clock hand continues its steady motion, going all the way around, rotating through a full circle in 12 hours. Dawn needs to keep its antenna pointed at Earth, however. If Dawn were at the center of the clock and Earth were at the 12, it wouldn't let the antenna point any farther away than the hour hand gets from the 12 in about a minute. The tiny angle is only about a tenth of the way from the 12 to the adjacent ticks (both on the left and on the right) that mark one second for the second hand. When Dawn's orientation approaches the maximum allowed angular deviation, the main computer instructs a jet to puff out a little hydrazine to reverse the motion.
When the spacecraft follows its elliptical orbit down to a low altitude, only three times higher than you are when you fly on a commercial jet, it needs to expel hydrazine to keep aiming its camera and spectrometers down as it rushes over the ground. If this isn't clear, try pointing your finger at an object and then circling around it. You are constantly changing the direction you're pointing. For Dawn to do that, especially in its elliptical orbit, requires hydrazine. (If you think Dawn could simply start rotating with hydrazine and then just point without using more, there are some subtleties here we will not describe. It really does require extensive hydrazine.)
Whether pointing at the landscape beneath it or at Earth, it might seem that Dawn could remain perfectly steady, but there are always tiny forces acting on it that would compromise its pointing. One is caused by the difference between Ceres' gravitational pull on the two ends of the solar arrays that occurs when the wings are not perfectly level. (We described this gravity gradient torque when Dawn was orbiting Vesta.) Also, sunlight reflecting in different ways from different components (some with polished, mirror-like surfaces, others with matte finishes) can exert a very small torque. Dawn uses hydrazine to counter these and other slight disturbances in its orientation.
As we have discussed extensively, very soon, the hydrazine will be depleted. Most likely between the middle of September and the middle of October (although possibly earlier or later), the computer will tell a reaction control jet to emit a small burst of hydrazine, as it has myriad times before in the mission, but the jet will not be able to do so. There won't be any usable hydrazine left. It will be like opening the end of a completely deflated balloon. No gas will escape. There will be no action, so there will be no reaction. Dawn's very slow angular motion will not be reversed but rather will continue, and the orientation will slowly move out of the tight bounds the ship normally maintains.
The computer will quickly recognize that the intended effect was not achieved. It will send more signals to the jet to fire, but the result will be no different. On a mission often operating out of radio contact with Earth and always very, very far away, help can never be immediate (after all, radio signals travel at the universal limit of the speed of light), so the robot is programmed to deal with problems on its own. There are several possibilities for what actions Dawn will take, depending on details we will not delve into, but a likely one is to try switching from the primary reaction control jets to the backup reaction control jets. Of course, that won't fix the problem, because the jets will not be at fault. In fact, with no hydrazine available, none of its attempts to correct the problem will succeed.
When Dawn experiences problems it can't resolve on its own, it invokes one of its safe modes, standard responses the craft uses when it encounters conditions its programming and logic cannot accommodate. (We have described the safe modes a number of times before, with perhaps the most exciting time being here.) In this case, the safe mode it will chose will go through many steps to reconfigure the spacecraft and prepare to wait for help from humans on a faraway planet (or anyone else who happens to lend assistance).
One of the first steps will be to temporarily power off the radio transmitter, one of the biggest consumers of electrical power on the ship. Until Dawn can make all of the necessary changes, including turning to point the solar panels at the Sun, it will not want to devote precious energy to unnecessary systems. Electrical power is vital. Without it, the spacecraft will be completely inoperative, just as your car, computer, smartphone or lights do nothing at all when they are deprived of power.
Dawn will try to do all its work using only the energy stored in its battery (which it keeps charged, using excess power from the solar arrays). It knows that later, once the arrays are in sunlight, it will have plenty of power, but in the meantime, it needs to be parsimonious. The computer, heaters, motors to rotate the solar arrays, and some other devices are essential to getting into safe mode. The radio is needed only after the spacecraft has completed other steps.
The spacecraft will not complete those other steps. One of them is to turn to point at the Sun, ensuring that the large solar arrays are fully illuminated. But without hydrazine, it will have no means to accomplish the necessary turn.
So, Dawn will not be able to achieve the planned orientation with the solar arrays generating electrical power. The computer will stubbornly refuse to turn on the radio, instead continuing to try to turn so the Sun will light up the arrays and infuse the robot with its electrical lifeblood.
Dawn will continue to try as long as it has power, whether flowing from partially lit solar arrays or from the battery. All the while, the spacecraft will continue to rotate at the same leisurely speed it did when it had hydrazine. But instead of gently oscillating back and forth, it will simply keep going in the same direction, like a clock's hour hand slowed down to measure months instead of hours.
Some of the time, the solar arrays will face away from the Sun and the battery will drain. Some of the time the solar arrays will point at (or near) the Sun just by luck. But Dawn doesn't rely on luck. Until it has a stable orientation with the arrays reliably on the Sun, the computer will insist that power not be devoted to the radio. First things first: first achieve a condition that can be safe for days, weeks, or even months, and then radio Earth for help. The programming did not anticipate being completely unable to control orientation.
Engineers have analyzed what will happen and observed many examples of it in the spacecraft simulator at JPL. Eventually, the computer may make some other attempts. But Dawn's struggle will be brief, lasting only hours before the battery is exhausted. The seasoned adventurer will sink into unconsciousness. At some later time, as its stately rotation brings the solar arrays back into the light, it may well begin to revive, but the cycle will repeat. The newly awakened Dawn will try to point at the Sun and hold that position, taking advantage of the power from the fortuitously illuminated solar arrays. But soon its continuing rotation will point the arrays into the dark of space again. It might seem that half the time the arrays would receive light and so it should be able to operate at half power, but it doesn't work that way. At Dawn's distance from the Sun, a little bit of that faint light on the solar arrays is not sufficient.
After an extraordinary extraterrestrial expedition, more than a decade of interplanetary travels, unveiling two of the last uncharted worlds in the inner solar system, performing unique and complex maneuvers, encountering and overcoming a host of unanticipated problems, Dawn will be on the losing end of a battle with the cold, hard reality of operation in deep space. Its mission will be over.
The spacecraft will be well over a million times farther from Earth than the International Space Station. How will we know when it has run out of hydrazine if its radio is off? (The reaction control system is expected to operate normally as long as there is usable hydrazine, so there will be no prior indication that its exhaustion is imminent.)
Even as it goes about trying to fix or recover from problems, the computer issues some brief status reports. (They often are more informative than the dialog boxes that pop up on your computer, and Dawn never asks you to click on something to proceed.) If the loss of hydrazine happens to occur while Dawn is communicating with Earth, one of those concise reports may be received before the computer turns off the transmitter. The short message would be like a farewell tweet that Dawn is ending its mission.
Most of the time, however, the probe does not point its main antenna at Earth. When it zips down to low altitude, it aims its sensors at the ground, so the antenna is pointed in an arbitrary direction. Dawn transmits a very broad radio signal through one of its auxiliary antennas so scientists and engineers can follow its motion very precisely. (We have explained before that this allows them to determine the interior structure of the dwarf planet.) That radio connection is too weak for anything else, so Dawn won't be able to tweet its news. If the last of the hydrazine is spent when Dawn's orbital motion is being tracked, the radio signal will simply disappear.
In its elliptical orbit, Dawn spends far less time traveling fast at low altitude than it does traveling slowly at high altitude, much as the girl on a swing we encountered in April. And when it is high up, we generally do not have radio contact at all. So it is more likely that the hydrazine will be depleted when Dawn is out of touch than when the DSN is recording its radio transmissions, through either the main antenna or an auxiliary antenna. Then the next time one of the antennas of the DSN aims at Dawn's location in the sky, it will strain to hear the faint radio whisper of the faraway probe, but all will be silent.
Dawn controllers and the DSN will work together to be sure the inability to detect the spacecraft isn't some other problem, perhaps in mission control or in the tremendously complex DSN. Over the course of a few days, they will use more than one antenna and will take a few other measures. After all, there could be other reasons for a temporary loss of signal, and engineers will work through the possibilities. But given Dawn's resilience and sophistication, if it remains uncommunicative during that time, the conclusion will not be in doubt. Even without a tweet, it will be clear Dawn has run out of hydrazine and is at the end of its operational life.
After conducting a systematic investigation, when the Dawn project is confident of the situation, we will announce the result. In the next Dawn Journal, we will consider a more personal side of this story.
But what of Dawn's long-term fate? Remember, its orientation in space is largely independent of its orbital motion. The spacecraft's inability to point where it wants, to power its systems, and to communicate with its human handlers will have virtually no effect on where it goes.
Dawn doesn't need propulsion to stay in orbit around Ceres, just as the Moon doesn't need it to stay in orbit around Earth and Earth doesn't need it to stay in orbit around the Sun. And that's important. We do not want Dawn to come into contact any time soon with the dwarf planet it orbits.
Ceres is subject to planetary protection, a set of standards designed to ensure the integrity of possible future "biological exploration" of the alien world. That terminology does not mean there is biology on Ceres but rather that that exotic world is of interest in the field of astrobiology. Ceres was once covered with an ocean and today harbors a vast inventory of water (mostly as ice but perhaps with some liquid still present underground). It also has a supply of heat (retained even now, long after radioactive elements decayed and warmed the interior), organics and a rich variety of other chemicals. With all these ingredients, Ceres could experience some of the chemistry related to the development of life. Scientists do not want to contaminate that pristine environment with Dawn's terrestrial materials.
Not all solar system bodies need such protection. The Moon, Mercury and Venus, for example, have not been of interest for searches for life or for prebiotic chemistry. For that reason, spacecraft are allowed to land or crash on those worlds because there is no expectation of subsequent biological exploration. Also exempt from such rules are tiny asteroids, including two that are being explored this year, Ryugu and Bennu. They are entirely unlike giant Ceres. They are often mistakenly thought of as being similar because of the oversimplified notion that all are asteroids. We will provide an illustration of the dramatic difference in the next Dawn Journal.
The planetary protection rules for Ceres specify that Dawn not be allowed to contact it for at least 20 years. There is a common misconception that the time is needed to allow the spacecraft to be sterilized by the radiation, vacuum and temperature extremes of spaceflight. That's not the case. Many terrestrial microbes are impressively hardy, and there is good reason to believe that some that have taken an unplanned interplanetary cruise with Dawn would remain viable after much longer than 20 years.
The requirement for 20 years is intended to allow enough time for a follow-up mission, if deemed of sufficiently high priority given the many goals NASA has for exploring the solar system. Two decades should be long enough to mount a mission that builds on Dawn's many discoveries. We would not want such a hypothetical mission to be misled by finding microorganisms or non-biological organic chemicals that were deposited by our spacecraft. As we'll see below, the deadline for another mission to get there before Dawn contaminates Ceres is likely to be significantly more relaxed even than that.
Earlier this year, when the team was figuring out how to fly to and operate in an orbit like the one Dawn is in now, much of their work was guided by this planetary protection requirement. We did not want to enter an orbit that would not meet the 20-year lifetime. We could not take the chance of going to an orbit with a shorter lifetime and plan for subsequent maneuvers to increase the duration. We were not sufficiently confident Dawn would have enough hydrazine to remain operable long enough to make its observations and still be able to change its orbit.
The team studied elliptical orbits with different minimum altitudes. Trajectory experts investigated the long-term behavior of each orbit as Ceres' irregular gravity field tugs on the spacecraft revolution after revolution, year after year. Like Earth, Ceres has some regions of higher density and some of lower density. As Dawn orbits over these different regions, they gradually distort the orbit. The analyses also accounted for the slight pressure of sunlight, which not only can rotate the spacecraft but also can push it in its orbit. An orbit with a minimum of 22 miles (35 kilometers) was the lowest that the team was confident would comply with planetary protection, and that's why Dawn is now in just such an orbit.
And after 20 years? Calculations show that even over 50 years, the orbital perturbations are overwhelmingly likely to be too small to cause Dawn to crash. In fact, there is less than a one percent chance of the orbit being distorted enough that Dawn would hit Ceres. In other words, our analysis gives us more than 99 percent confidence that even in half a century, Dawn will still be revolving around Ceres, the largest object between Mars and Jupiter, the only dwarf planet in the inner solar system and the first dwarf planet discovered (129 years before Pluto).
Leaving the remarkable craft in orbit around the distant colossus will be a fitting and honorable conclusion to its historic journey of discovery at Vesta and Ceres. Dawn's scientific legacy is secure, having revealed myriad fascinating and exciting insights into two quite dissimilar and mysterious alien worlds. This interplanetary ambassador from Earth will be an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.
Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 3.46 AU (321 million miles, or 517 million kilometers) from Earth, or 1,270 times as far as the Moon and 3.42 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 pm PDT August 21, 2018