This blog entry from John Grotzinger, the project scientist for NASA's Curiosity Mars rover, was originally prepared for use by the Planetary Society and explains the importance of some of the rover's findings.
It was fun for me to catch up with Emily Lakdawalla of the Planetary Society at the American Geophysical Union meeting, and to discuss our new Curiosity mission results. They focus on the discovery of an ancient habitable environment; we are now transitioning to the focused search for organic carbon. What's great about Emily's blog is that with her strong science background she is able to take complex mission results and translate these into something that can reach a broader and more diverse audience. I'll try to do the same here.
Since we first reported our results on March 12, 2013, from drilling in Yellowknife Bay it has been my experience that lots of people ask questions about how the Curiosity mission, and future missions, will forge ahead to begin with looking for evidence of past life on Mars. There is nothing simple or straightforward about looking for life, so I was pleased to have the chance to address some of the questions and challenges that we find ourselves most frequently discussing with friends and colleagues. The Planetary Society's blog is an ideal place to take the time to delve into this.
I also need to state at the outset that what you'll read below is my opinion, as Curiosity science team member and Earth geobiologist, and not necessarily as its Project Scientist. And I have only worked on Mars science for a decade. However, I can say that many other members of the Curiosity team share this opinion, generated from their own experiences similar to mine, and it was easy for us to adopt these ideas to apply to our future mission. To a large extent, this opinion is shaped by our experience of having spent decades trying to explore the early record of life on Earth. As veterans of the Mars Exploration Rover and Curiosity missions, we have learned that while Mars has significant differences from Earth, it also has some surprising similarities that could be important in the search for evidence of ancient Martian life - a "paleobiosphere," if you will. The bottom line is that even for Earth, a planet that teems with life, the search for ancient life is always difficult and often frustrating. It takes a while to succeed. I'll try to explain why later on.
So here goes....
The Dec. 9, 2013, publication of the Curiosity team's six papers in Science provides the basis for understanding a potentially habitable environment on ancient Mars. The search for habitable environments motivated building the rover, and to that end the Curiosity mission has accomplished its principal objective. This naturally leads to the questions of what's next, and how we go about exploring for organic carbon?
To better understand where we're coming from, it helps to break down these questions and analyze them separately. With future advocacy of missions to Mars so uncertain, and with difficult-to-grasp mission objectives located between "the search for water" (everyone got that) and "the search for life" (everyone wants NASA to get on with it), the "search for habitability" and the "search for carbon" are important intermediate steps. By focusing on them scientists can identify specific materials to study with more sophisticated future missions and instruments, or to select for sample return, or to be the target of life detection experiments.
Note: You can get access to all six of these Science papers here or here. The latter site also has the papers we published back in September. Science has a policy that allows us to post a "referrer link" to our home websites. This redirects the query to AAAS, where the paper can be downloaded without cost.
Let's start with "habitability." We reported the discovery of an ancient lake, and one that formed clay minerals. The presence of clays represents more benign environmental conditions than the acid sulfates found by Spirit and Opportunity. However, clays are not the only thing needed to demonstrate habitability. The bar is high: In brief, a mission needs to demonstrate the presence of water, key elements regarded as the building blocks of life (including carbon), and a source of energy. And you need to find them all together, and at the same instant in geologic time. In turn, each one of these must be characterized further to qualify an environment as having been habitable. Finally, it's never black and white; understanding habitability is part of a broad continuum of environmental assessment, which is why orbiters and earlier rovers and landers are important assets in this process as well.
It is also important to define what group of organisms is being imagined to have inhabited the environments - their requirements will vary. Single-celled microorganisms are a great place to start based on our understanding of the early evolution of life on Earth, which was dominated by microbes for at least the first two billion years of the planet's history. More specifically, the Curiosity team has been focusing on the conditions of habitability relevant to "chemolithotrophs," a group of microbes that feeds on chemical energy available in rocks.
The water of a habitable environment should be relatively fresh, or at least not contain so much salt that the relative abundance of water is so low (what chemists call "water activity") that the osmotic pressure on cells would cause them to collapse. My favorite analog here is honey. Yes, it's an aqueous environment but no, it's not habitable: The sugar content is so high that microbes can't live in it. This is why honey doesn't spoil when not refrigerated. Salt serves the same role as sugar; too much salt inhibits life. Acidity is also important, although microbes have been shown to tolerate an extraordinary range of pH, including the very lowest values encountered in natural environments on Earth. However, more moderate pH favors a greater diversity of microorganisms, and thus more options to explore for emerging life forms. Finally, the water needs to last a long time on the surface; the longer, the better. A flow of water emerging on the surface of Mars from an underground source and boiling off in the presence of Mars' modern low atmospheric pressure is not a good scenario for life. A stable source, such as a very ancient lake, with associated streams, and water flowing through the ground beneath it, is much better. We envision for the lake/stream/groundwater system that Curiosity discovered at Yellowknife Bay that the water could have existed for millions of years potentially. But even shorter periods are viable - the qualitative point here is that the rocks at Yellowknife Bay record more than a one-time event.
Key building blocks of life.
A conventional list of key elements for life will include "CHNOPS" - carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Previous orbiter and landed missions have provided ample evidence for H, O, and S via observations of sulfate and clay minerals, and P was measured by earlier rovers and landers. Curiosity has done the same. The tricky stuff is N and C and, along with P, they must all be "bioavailable," which means to say they cannot be bound tightly within mineral structures that water and microbial chemical processes could not unlock. Ideally, we are looking for concentrated nitrogen- and phosphorous-bearing sedimentary rocks that would prove these elements were actually dissolved in the past water at some point, and therefore could have been available to enable microorganism metabolism. But in the interim Curiosity has been able to measure N as a volatile compound via pyrolysis (heating up rock powder in the SAM instrument), and P is observed in APXS data. We feel confident that N was available in the ancient environment, however we must infer that P was as well. Two of the Science papers, Grotzinger et al. and Ming et al., discuss this further.
Carbon is the elephant in the room. We'll discuss organic carbon further below, but here it's important to make one very important point: Organic carbon in rocks is not a hard-line requirement for habitability, since chemoautotrophs can make the organics they need to build cellular structures from metabolizing carbon dioxide (CO2). These organisms take up inorganic carbon as CO2 dissolved in water to build cellular structures. Organic carbon could serve as fuel if it was first oxidized to CO2, or could be used directly for biomass, or could be part of waste products. As applied to Mars it is therefore attractive to appeal directly to CO2, presumed to have been abundant in its early atmosphere. Curiosity does indeed see substantial carbon generated from the ancient lake deposits we drilled. The CO2 that was measured is consistent with some small amount of mineral carbon present in those lake mudstones. These minerals would represent CO2 in the ancient aqueous environment. Furthermore, it is possible that Martian organic sources have been mixed with inorganic sources of carbon in the mudstone; however, any organic contributions from the mudstone would be mixed with Earth-derived sources during analysis (see Ming et al. paper).
All organisms also require fuel to live and reproduce. Here it is essential to know which kind of microorganism we're talking about, since there are myriad ways for them to harvest energy from the environment. Chemolithotrophs derive energy from chemical reactions, for example by oxidizing reduced chemical species like hydrogen sulfide or ferrous iron. That's why Curiosity's discovery of pyrite, pyrrhotite, and magnetite are so important (see Vaniman et al. and Ming et al. papers). They are all more chemically reduced than their counterparts discovered during earlier missions to Mars (for example, sulfate and hematite). Chemolithotrophic microbes, if they had been present on Mars at the time of this ancient environment, would have been able to tap the energy in these reduced chemicals (such as hydrogen sulfide, or reduced iron) to fuel their metabolism. If you are interested in more detail regarding these kinds of microbial processes I can strongly recommend Nealson and Conrad (2000) for a very readable summary of the subject.
The next section describes where I think we're headed in the future. We'll continue to explore for aqueous, habitable environments at Mt. Sharp, and along the way to Mt. Sharp. And if we discover any, they will serve as the starting point for seeing if any organic carbon is preserved and, if so, how it became preserved.
Now there's a ten-dollar word. Taphonomy is the term paleontologists use to describe how organisms become fossilized. It deals with the processes of preservation. Investigations of organic compounds fit neatly in that category. We do not have to presume that organic compounds are of biologic origin. In fact, in studies of the Earth's early record of life, we must also presume that any organic materials we find may be of inorganic origin - they may have nothing to do with biology. Scientific research will aim to demonstrate as conclusively as possible that the materials of interest were biogenic in origin. For Earth rocks that are billions of years old, it's rare to find a truly compelling claim of ancient biogenic carbon. Here's why.
On a planet that teems with life, one would presume these discoveries would be ordinary. But they aren't, and that's why fossils of almost any type, including organic compounds (so-called "chemofossils"), are so cool - it's because they are rare. That's also why taphonomy emerged as an important field of study. We need to understand how biologic materials become recorded in Earth's rock record. It's important in understanding modes of organism decomposition, to interpret ancient environmental conditions, and in reconstructing ancient ecosystems. But there also is one other reason that is particularly relevant for early Earth, and even more so for Mars: If you want to find something significant, you have to know where to look.
To explore for organics on Mars, three things have to go right. First, you need to have an enrichment of organics in the primary environment where organic molecules accumulate, which is large enough so that your instrument could detect them. Second, the organics have to survive the degrading effects associated with the conversion of sediment to rock. Third, they must survive further degradation caused by exposure of rock to cosmic radiation at Mars' surface. Even if organics were once present in Martian sediment, conversion to rock and exposure to cosmic radiation may degrade the organics to the point where they can't be detected.
Organics degrade in two main ways. The first is that during the conversion of sediment to rock, organics may be chemically altered. This generally happens when layers of sediment are deposited one on top of the other, burying earlier-deposited layers. As this happens, the buried sediment is exposed to fluids that drive lithification - the process that converts sediment to rock. Sediments get turned into rocks when water circulates through their pores, precipitating minerals along the linings of the pores. After a while the sediment will no longer feel squishy and it becomes rigid - lithified.
During the process of lithification, a large amount of water may circulate through the rock. It can amount to hundreds, if not thousands, of times the volume of the pore space within the rock. With so much water passing through, often carrying other chemicals with it, any organics that come into contact with the water may be broken down. Chemically, this occurs because organics are reduced substances and many chemicals dissolved in water are oxidizing. Those two chemical states don't sit well together, and this tends to drive chemical reactions. Simply put, organics could be broken down to the point where the originally organic carbon is converted into inorganic carbon dioxide, a gas that can easily escape the lithifying sediment. Water on Mars may be a good thing for habitability but it can, paradoxically, negatively affect the preservation of organics.
Now, if any organics manage to escape this first step in degradation, then they are still subject to further degradation when the rock is exhumed and exposed to the surface of Mars. There it will be bombarded by cosmic radiation. I won't go into the details here, but that is also bad news for organics because the radiation tends to break apart organic molecules through a process called ionization. The upper few meters of a rock unit is the most susceptible; below that the radiation effect rapidly dies away. Given enough time the organics could be significantly degraded.
The Hassler et al. paper just published in Science reports that the surface radiation dose measured by Curiosity could, in 650 million years, reduce the concentration of small organic molecules, such as amino acids, by a factor of 1000, all other factors being equal. That's a big effect - and that's why we were so excited as a team when we figured out how to measure the cosmogenic exposure age of rocks we drilled (see Emily's blog and the Farley et al. paper). This gives us a dependable way to preferentially explore for those rocks that have been exposed for the shortest period of time. Furthermore, it is unlikely that organics would be completely eliminated due to radiation effects and the proof of this is that a certain class of meteorites - the carbonaceous chondrites - have been exposed to radiation in space for billions of years and yet still retain complex organics. This provides hope that at least some types of organics should be preserved on Mars.
Being able to account for the radiation history of rocks that Curiosity might drill is a very big step forward for us in the search for organic molecules. It is a big step forward in learning how to explore for past life on Mars (if it ever existed there). Now we have the right tools to guide the search for rocks that might make the best targets for drilling. Coupled with our other instruments that measure the chemistry and mineralogy of the rocks, to help select those that might have seen the least alteration of organics during burial, we have a pretty good sense of what we need to do next. That's because we have been through this before on Earth.
Over the years Emily has written many blogs dedicated to the discovery of interesting minerals on Mars. There are many reasons for this, but I'll suggest one more that may grow in importance in years to come.
Believe it or not, the story starts with none other than Charles Darwin. In pondering the seemingly instantaneous appearance of fossils representing complex and highly differentiated organisms in Cambrian-age rocks (about 500 million years ago), Darwin recognized this as a major challenge to his view of evolution. He explained the sudden appearance of fossils in the record by postulating that Cambrian organisms with no known antecedents could be explained by "record failure" - for some unknown reason, older rocks simply didn't record the emergence and evolution of life's beginning. Conditions weren't suitable to preserve organisms as fossils.
Most of that story goes on in the direction of evolutionary biology, and we'll skip that, rather focusing instead on learning more about taphonomy. What is important for Mars was the discovery of minerals that could preserve evidence of early microorganisms on Earth. (For a good read on Precambrian paleobiology, try Andy Knoll's "Life on a Young Planet: The First Three Billion Years of Evolution on Earth.")
We now know that pre-Cambrian time represents about 4 billion years of Earth's history, compared to the 540 million years represented by Cambrian and younger rocks that Darwin had studied. (See Emily's blog on the Geologic time scale.) We also know now that the oldest fossil microbes on Earth are about 3.5 billion years old, and that in between there is a compelling, but very sparse record of the fossil organisms that Darwin had anticipated. However, what's even more remarkable is that it took 100 years to prove this. And this was with hundreds, maybe thousands, of geologists scouring the far corners of the Earth looking for evidence.
The big breakthrough came in 1954 with the discovery of the "Gunflint microbiota" along the shores of Lake Superior in southern Canada. A University of Wisconsin economic geologist, Stanley Tyler, discovered microscopic threads of what we now understand to be fossil bacteria in a kind of rock called "chert". Chert is a microcrystalline material formed of the mineral quartz, or silicon dioxide, which precipitates very early in waters that contain microbial colonies. It forms so early that it turns the sediment almost instantly into rock, and any microbes become entombed in a mineral so stable it resists all subsequent exposure to water, and the oxidizing chemicals dissolved in water, for billions of years.
As it turned out, this was the Rosetta stone that helped decipher the code to the field of pre-Cambrian paleontology. It took almost 10 years for the discovery to be fully appreciated (the initial report in Science was viewed with much skepticism), but once it was confirmed, in the mid-1960s, the field exploded. Once geologists and paleontologists knew what to search for, they were off to the races. Since that initial discovery, other magic minerals have been found that preserve ancient microbes, sometimes with spectacular fidelity. But chert is still the mineral of choice, and I never pass by it in the field without collecting some.
We don't know yet what magic minerals exist on Mars that could have trapped and preserved organics. Clays and sulfates hold promise, and that's why we're so interested in them. Silica, perhaps similar to terrestrial chert, has been observed from orbit at a few places on Mars, and in Spirit rover data from Gusev crater. The great thing about Gale crater as a landing site is that we have so many choices in this trial-and-error game of locating a mineral that can preserve organic carbon.
The figure below provides some sense of the impact of this discovery. It is modified from a similar figure published in a very nice summary by Bill Schopf, a Professor of Paleontology at UCLA. Bill also was a very early participant in this race for discovery and has made a number of very significant contributions to the field.
In studying Mars, the importance of this lesson in the search for life preserved in the ancient rock record of Earth cannot be overstated. Curiosity's discovery of a very Earth-like ancient habitable environment underscores this point. With only one or two rovers every decade, we need to have a search paradigm: something to guide our exploration, something to explain our inevitable failures. If life ever evolved on Mars, we need to have a strategy to find it. That strategy begins with the search for organics, and regardless of their origin - abiotic or biotic, indigenous to Mars or not - they are important tracers for something more significant. Curiosity cannot see microfossils, but it can detect organic compounds. And just as with microfossils on Earth, we first have to learn where organics on Mars might be preserved. So that's what we're going to try and do.
In August 1964, this test fixture was used by the Spacecraft Design Section at NASA's Jet Propulsion Laboratory in Pasadena, Calif., to study spin stabilization of spacecraft - in this case, Rangers 8 and 9 (part of the Ranger Block 3 design). Many spacecraft had used spin stability for attitude control during acceleration or thrust, and it was found that a slower spin provided better stability for the coasting phase.
One method of decreasing the spin of a spacecraft, or de-spinning, was the deployment of yo-yo devices. Weights were attached to rigid or stretch cords, then released while the fixture was spinning. The cords would unwind, like the arms of a figure skater extending to slow a spin, and then the cords were released. In this photo, the cables and weights can be seen, attached to the outside of the white circle. The test fixture is surrounded by what appear to be bales of paper and trash to absorb the impact of the weights when they were released from the spinning test fixture.
Gliding smoothly through the main asteroid belt between Mars and Jupiter, Dawn continues to make good progress on its ambitious mission of exploration. It is patiently but persistently pursuing Ceres, the second destination on its interplanetary itinerary.
Protoplanets Ceres and Vesta, the two most massive residents of the asteroid belt, were discovered at the beginning of the 19th century, and they have tantalized astronomers and others curious about the nature of the universe ever since. (Indeed, Ceres was the first dwarf planet discovered, having been found 129 years before Pluto.) They have waited patiently for a visitor from Earth since the dawn of the solar system. Dawn's objective is to turn these uncharted orbs from tiny smudges of light amidst the stars into richly detailed places. It succeeded spectacularly at Vesta in 2011 - 2012, and it remains on course and on schedule for doing so at Ceres in 2015.
Next month, the adventurer will pass an invisible milestone on its celestial journey. On Dec. 27, it will be equidistant from these behemoths of the asteroid belt as all three follow their own independent heliocentric paths. The spacecraft will be 0.21 AU (19.4 million miles, or 31.3 million kilometers) away from each world, the one already visited and the one yet to be reached. And as the indefatigable ship sails on the cosmic seas with its sights set on Ceres, our anticipation for glimpsing the alien landscape ahead grows and grows, while the now-familiar scenery of Vesta shrinks into the distance, fading over the horizon.
The next day, Dawn will be equidistant from two other solar system bodies, both of which have been known (to our human readers, at least) for somewhat longer than Ceres and Vesta have. On Dec. 28, our celestial ambassador will be 2.46 AU (229 million miles, or 368 million kilometers) from Earth and the sun. (We cannot specify in which century either of them was discovered.)
Its complex route through the solar system has already taken the spacecraft farther from each of these bodies before. In the current phase of the mission, it is receding from the sun again, climbing the solar system hill from Vesta to Ceres. (It approached the sun in late 2012 and 2013 as part of the strategy for arriving at Ceres's orbit when Ceres itself was there.) Having attained its greatest distance from Earth for the year in August, the spacecraft is temporarily getting closer to its planet of origin. (More precisely, as we discussed then, Earth is currently moving toward Dawn, because Earth travels faster in its solar orbit than Dawn does in its much more remote orbit.)
Dawn will reach two more impressive milestones in December, although neither pertains to its location. Soon the craft will surpass four years of ion thrust. While most spacecraft rely on conventional propulsion and hence coast most of the time (just as planets, moons, and asteroids do), Dawn's mission would be impossible if it did that. In order to orbit and explore two distant destinations, the only terrestrial probe ever to attempt such a feat, it must accomplish a great deal of maneuvering. It spends the majority of its time using its uniquely efficient and capable ion propulsion system, constantly putting a gentle pressure on its trajectory to gradually reshape it. Although the spacecraft has already accumulated far more time in powered flight than any other mission, it still has a great deal more ahead.
And in December, that thrusting will push the craft's speedometer past an extraordinary 20,000 mph (8.94 kilometers per second). (As we have seen in many previous logs, such as this one, this measure of the speed does not represent the actual spacecraft velocity. Nevertheless, it is a useful metric that avoids the complicating effects of orbital mechanics.) That is more than twice the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.
Dawn spends most of its time emitting a lovely blue-green beam of high-velocity xenon ions to propel itself. As foretold in the prophecy commonly known as the October log, however, we are now in one of just two periods of the long mission in which coasting is better for the trajectory than thrusting. Mission controllers took advantage of this time to instruct the robot to perform some special activities that would have been less convenient during routine ion thrusting. The reliable ship completed all of them flawlessly.
For more than 27 hours on Nov. 12 and 13, Dawn operated in a mode that had not even been conceived of when it was designed and built. It controlled its orientation in the frictionless, zero-gravity conditions of spaceflight using a scheme that was developed long after it left Earth. This "hybrid" control method operated perfectly, validating the extensive work engineers have invested in it and verifying its readiness for use at Ceres.
When it embarked on its bold journey more than six years ago, the ship was outfitted with four reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. It generally used three at a time, with a fourth kept in reserve. For such a long and complex expedition, extending to well over one million times farther from Earth than the International Space Station, backup systems are essential.
One of the wheels experienced increased friction in June 2010, but the mission continued with the other three. A second met the same fate in August 2012, as Dawn was climbing away from Vesta. Other spacecraft have encountered similar issues with their reaction wheels as well, and the consequences can be dire.
We have described the operations team's swift and productive responses to the regrettable behavior of the reaction wheels in a number of logs (see, as one example, here). As soon as the first wheel faltered, JPL and Orbital Sciences Corporation began working on a method to operate with fewer than three in case another one had difficulty. They developed software to operate in a hybrid mode of two wheels plus the small hydrazine-powered jets of the reaction control system and installed it in the craft's main computer in April 2011 so it would be available at Vesta if needed.
Given the problems with reaction wheels on Dawn and other spacecraft, engineers do not have high confidence that the two remaining units will operate for long (although it certainly is possible they will). Thanks to their remarkable ingenuity and resourcefulness, the team has devised a detailed plan that should allow Dawn to complete its extraordinary mission using only the hydrazine thrusters, achieving all of its objectives in exploring Ceres regardless of the condition of its wheels. (Note that it is not even obvious that doing so is possible, but then again, it isn't obvious that sending probes so far from our home planet is possible either. Part of the thrill of a solar system adventure is overcoming the extremely daunting challenges.) So now, hybrid control would provide an enhancement, extending the supply of precious hydrazine propellant and giving the spacecraft the opportunity to operate even longer at Ceres than it would without the two functioning wheels. When the hydrazine is exhausted, the mission will conclude.
Dawn will use hybrid control only in its lowest altitude orbits at Ceres, the final phase of the mission. (Beginning in December and continuing in 2014, we will describe all phases of the Ceres plan in detail.) Hybrid control will be called upon to perform three kinds of tasks for the spacecraft: train the suite of sophisticated sensors at the mysterious world beneath it, point the main antenna to distant Earth to transmit its findings and receive updated instructions, and rotate from one orientation to another. The innovative system has now unerringly demonstrated its capability to accomplish all three by executing exactly those functions earlier this month.
The confirmation that hybrid control works as intended is not the only task Dawn is carrying out during this coasting period. All of its scientific instruments (including even the backup camera) are being powered on and given thorough health checks, verifying that they remain fully functional and ready to reveal Ceres's secrets. Engineers also conducted some tests with the ion engine that has operated the longest of the three to confirm expectations of how it will perform at Ceres.
On Dec. 9, Dawn's four-week coast period will end. Once again it will turn to point an ion engine in the direction needed to push forward to its rendezvous with the distant and exotic world ahead. As the probe nears and then passes the halfway point on its remarkable journey from Vesta to Ceres, it is pulled by forces even more powerful than ion propulsion: the attraction of discovery, the lure of the unknown, and the draw of tremendous new insights and profound new understandings to be gained in a daring adventure far, far from home.
Dawn is 18 million miles (29 million kilometers) from Vesta and 22 million miles (35 million kilometers) from Ceres. It is also 2.78 AU (258 million miles, or 415 million kilometers) from Earth, or 1,125 times as far as the moon and 2.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 p.m. PST November 30, 2013
This photo was taken in November 1960 to show the lightweight balsa wood impact limiter that was to be used in the NASA Jet Propulsion Laboratory's Ranger Block II spacecraft design (Rangers 3, 4, and 5). The woman holding the sphere is Systems Design secretary Pat McKibben. The sphere was 65 cm in diameter, and it surrounded a transmitter and a seismometer instrument that was designed by the Caltech Seismological Laboratory. The sphere would separate from the spacecraft shortly before impact and survive the rough landing on the moon. The capsule was also vacuum-filled with a protective fluid to reduce movement during impact. After landing, the instrument was to float to an upright position, then the fluid would be drained out so it could settle and switch on.
Due to a series of malfunctions in 1962, these three Ranger spacecraft either crashed without returning data or missed the moon. In July 1964, the first successful Ranger spacecraft, Ranger 7, reached the moon and transmitted more than 4,000 images to Earth.
Dear All Hallows' Dawns,
Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.
The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn's heliocentric orbit was the same as the rocky behemoth's. Now they are very far apart, and by early 2015, the robotic explorer's path will be close enough to Ceres's that they will become locked in a gravitational embrace.
Without ion propulsion, Dawn's unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA's Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.
During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.
We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.
Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).
The first interval that interplanetary trajectory designers designated as "optimal coast" was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars's orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)
The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month's log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.
While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.
In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.
Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT October 31, 2013
P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year's simple (yet outlandish) costume, this year's will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.
On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind's only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.
On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.
Most of the ship's first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.
For those who would like to track the probe's progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.
In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn's wont. All this thrusting has cost the craft only 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)
Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta's speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn's launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth's orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body's orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of the journey is changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts' websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on September 27, 2007, its orbit around the sun was exactly Earth's orbit. After launch, it was in its own solar orbit.
|Minimum distance from the Sun (AU)||Maximum distance from the Sun (AU)||Inclination|
|Dawn's orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn's orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn's orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn's orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn's orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn's orbit on Sept. 27, 2011||2.15||2.57||7.1°|
|Dawn's orbit on Sept. 27, 2012||2.17||2.57||7.3°|
|Dawn's orbit on Sept. 27, 2013||2.44||2.98||8.7°|
For readers who are not overwhelmed by the number of numbers, investing the effort in studying the table may help to demonstrate how Dawn has patiently transformed its orbit during the course of its mission. Note that two years ago, the spacecraft's path around the sun was exactly the same as Vesta's. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the extraordinary capability of the ion propulsion system. Without that technology, NASA's Discovery Program would not have been able to afford a mission to explore it in such detail. But now, Dawn has gone even beyond that. Having discovered so many of Vesta's secrets, the stalwart adventurer left the protoplanet behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. A true interplanetary spaceship, Dawn is enlarging, reshaping and tilting its orbit again so that in 2015, it will be identical to Ceres's. A mission to both Vesta and Ceres would have been impossible without ion propulsion.
One way to chart Dawn's progress is with numbers. Another is to look inside ourselves and feel the awe at an extraordinary enterprise undertaken on our behalf. This robotic emissary from Earth to the cosmos has journeyed far, swooping by Mars even as its sights were set on distant, uncharted lands beyond. Powering its way through the solar system with a blue-green beam of xenon ions, the ambitious explorer is introducing humankind to ancient worlds, giant remnants from the dawn of the solar system. With each passing year, we travel farther and see more thanks to Dawn. It is amazing that creatures humble yet bold, who are physically confined to the vicinity of their planet, not only take on such daunting challenges but actually succeed in reaching so far and attaining so much. As Dawn begins the seventh year of its interplanetary odyssey, we can marvel at all that it has accomplished so far and look forward with eager anticipation to more rewards that lie ahead: new knowledge, new insights, new perspectives, and new fuel for those who feel the burning passion to venture still farther.
Dawn is 23 million kilometers (14 million miles) from Vesta and 44 million kilometers (27 million miles) from Ceres. It is also 3.32 AU (496 million kilometers or 308 million miles) from Earth, or 1,230 times as far as the moon and 3.31 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2013
Because the data return rate from Mariner 4 was very low, the Mariner 4 Television Experiment Team spent hours waiting for each new image to appear. In this photo they are waiting for the first picture from Mars. Mariner eventually returned 22 images. From left to right: Robert Nathan (NASA's Jet Propulsion Laboratory), Bruce Murray (associate professor of planetary science), Robert Sharp (Caltech), Robert Leighton (principal investigator), and Clayton La Baw (JPL).
Murray had been a member of the Caltech faculty for about five years when this photo was taken in July 1965. He went on to replace William Pickering as Director of JPL in 1976, retired from that position in 1982, and returned to Caltech.
In today's universe, it seems unimaginable that a planetary spacecraft would leave the comfort of its terrestrial perch without some kind of imaging system on board. But in the early 1960s, as NASA's Jet Propulsion Laboratory was reveling in the success of its first planetary mission to Venus and setting its sights on Mars -- a destination whose challenges would unfurl themselves much more readily than they had with Venus -- for some scientists, the question of camera or none was still just that, a question.
Bud Schurmeier, project manager for NASA's Ranger missions, a few years ago recalled, "There were a lot of scientists who said, 'Pictures, that's not science. That's just public information.' Over the years, that attitude has changed so markedly, and so much information has been obtained just from the photographs."
The recent passing of former JPL Director and career-long planetary imaging advocate Bruce C. Murray, 81, is a reminder of how different our understanding of the planets -- and our appreciation of them -- would be without space-based cameras.
This truth was evident as early as 1965, when NASA's Mariner 4, carrying an imaging system designed by a young Murray and his colleagues, arrived at Mars. It marked the world's first encounter with the Red Planet, a remarkable achievement in itself. But for an anxious press, public and mission team, the Holy Grail lay in catching that first glimpse of Mars up-close.
It was a waiting game that was too much for some. For everyone, in fact:
What resulted became known as "The first image of Mars." And in many ways it symbolizes -- more than any of the actual 22 photographs captured by Mariner 4 -- how significant this opportunity to truly "see" Mars had been.
Now, nearly 50 years after Mariner 4's arrival at Mars, imaging systems are an integral piece of our quest to understand the planets and the universe beyond, playing key roles in scientific investigations, spacecraft navigation and public support for missions. It's because of that first image that we can now look at that red dot in the night sky and picture what has become our new reality of Mars:
Traveling confidently and alone, Dawn continues to make its way through the silent depths of the main asteroid belt. The only spacecraft ever to have orbited a resident of the vast territory between Mars and Jupiter, Dawn conducted a spectacular exploration of gigantic Vesta, revealing a complex place that resembles the terrestrial planets more than typical asteroids. Now the interplanetary adventurer is on its long journey to the uncharted dwarf planet Ceres, by far the largest of all asteroids (975 kilometers, or more than 600 miles, in equatorial diameter). In 2015, the mysterious world of rock and ice will begin to give up its ancient secrets to the immigrant from distant Earth.
Earth, Vesta, Dawn, and Ceres are following their own separate paths around the sun. The spacecraft is patiently reshaping its orbit, using its uniquely efficient ion propulsion system to accomplish a deep-space expedition that would be impossible with conventional propulsion.
As we have seen in many previous logs (including, for example, here), the higher an object's orbit, the slower it needs to move in order to balance the gravitational pull, which diminishes with distance. Blistering Mercury orbits the sun faster than Venus, Venus goes faster than Earth, Earth goes faster than Mars, and Mars goes faster than the residents of the asteroid belt and the cold planets of the outer solar system. In the same way, satellites that orbit close to Earth, including the International Space Station, move faster than those at greater altitudes, and the moon travels even more slowly in its very high orbit.
Dawn is now a permanent inhabitant of the main asteroid belt. Therefore, the massive sun, the gravitational master of the solar system, has a weaker grip on it than on Earth. So as Dawn maneuvers from Vesta to Ceres, Earth revolves more rapidly around the sun. This month, their independent motions have taken them to their greatest separation of the year, as they are on opposite sides of the sun. How truly remarkable that humankind can accomplish such a feat!
On August 4, the planet and its robotic ambassador to the cosmos were an extraordinary 3.47 AU (519 million kilometers, or 322 million miles) apart. (To recapture the feeling of your position in the universe then, it may be helpful to know that the maximum range was attained at 4:16 a.m. PDT.) From the perspective of terrestrial observers, had they possessed the superhuman (and even supertelescopic) vision needed to descry the tiny ship far beyond the blindingly bright star, Dawn would have appeared to be very close to the sun but not directly behind it. To rendezvous with Vesta and then with Ceres, the spacecraft has tilted the plane of its solar orbit. Some of the time it is north of Earth's orbital plane, sometimes it is south. August 4 was during the northern segment, so Dawn would have been a little north of the sun.
It's time to refer to one of those novel clocks available in the Dawn gift shop on your planet (although if you already have such a clock, it probably doesn't tell you that it's time -- we stand by our policy of full refunds within 24 hours, as measured by our Dawn clocks). With the sun at the center of the clock, Earth's motion would be like that of a short minute hand. Dawn, both farther from the sun and moving more slowly, would be following the path of a longer hour hand. If we ignore the effect of the ion thrust, which is constantly changing the orbit, and the slight misalignment of the hour hand representing Dawn's being in a different plane, the conditions on August 4 were like those at 6:00.
As time progresses and Earth continues circling the sun, it will come closer to Dawn until April 2014 (like 12:00). Even then, however, they will be over 1.55 AU (232 million kilometers, or 144 million miles) apart, and they will never be that close again. The spacecraft will continue climbing higher and higher from the sun toward Ceres, so by the time Earth loops around once more, Dawn will be even farther from it. In the meantime, when next the arrangement is like 6:00, in December 2014, the separation will be more than 3.78 AU (565 million kilometers, or 351 million miles), even greater than the remarkable range a few weeks ago.
The tremendous distance this month between the spacecraft and the humans it represents provides a convenient occasion to reflect on the extraordinary nature of this ambitious mission of discovery, and regular readers know that we rarely eschew such an opportunity. Indeed, Dawn and Earth have been on opposite sides of the sun three times before (albeit not at the same distance), in November 2008, November 2010, and March 2012. In each case, we explained more about the nature of the alignment and contemplated the profundity of such an epic adventure.
Well beyond Mars, fewer than a dozen probes have ever operated as far from Earth as Dawn. Those interested in the history of space exploration (such as your correspondent) will enumerate them, but what should be more rewarding is marveling at the extent of humanity's reach. At this astounding range, the deep-space ship was well in excess of one million times farther from Earth than the International Space Station and Tiangong-1.
Some readers may have heard tell that in days of yore, a few fortunate humans traveled far from Earth. That was long, long ago, but tales pass from generation to generation, and even some dim memories survive. Apollo astronauts physically ventured to the moon, and many, many more people were along for the ride. Yet even in those ancient times of amazing escapades, no person reached farther than the moon does. But Dawn, built and operated by humans to take them where they themselves cannot (yet) go, was 1,300 times farther than the greatest distance attained by those primitive explorers. And as with Apollo, anyone can participate in this grand undertaking. This is a journey for everyone.
The scope is staggering and may not be possible to grasp fully. We evolved to have a clear understanding of distances that mattered to terrestrial creatures who had neither means nor need to comprehend much beyond their local environment. But now, thanks to the brilliance, persistence, meticulousness, creativity, and curiosity of our species, our reach extends to thousands of times farther than any individual has traveled. We do more than only aspire to extend ourselves from Earth. With our spacecraft, we create the means to transport ourselves throughout the solar system. Our handiwork visits places and shows us sights far, far, far beyond what could even have been imagined until recently. Powered by the noble spirit of adventure and the yearning for knowledge, our celestial emissaries carry us with them. Through them, as we behold alien landscapes, sometimes eerie in their familiarity yet more often eerie in their exoticism, but always beautiful and spellbinding, everyone who hungers for insights into the universe is nourished. Unbound by the planet on (or very near which) we reside, we take flight through space and time, and we are witness to spectacles of cosmic proportions dating back to the dawn of the solar system. With Dawn, we are able to reach beyond our humble home, indeed beyond ourselves. We learn much as we do, but we grow wiser even just for the effort itself.
Dawn is 21 million kilometers (13 million miles) from Vesta and 47 million kilometers (29 million miles) from Ceres. It is also 3.43 AU (513 million kilometers or 319 million miles) from Earth, or 1,270 times as far as the moon and 3.40 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.
Dr. Marc D. Rayman
1:00 p.m. PDT August 30, 2013
In 1961, mathematicians from NASA's Jet Propulsion Laboratory and Caltech worked together to construct a Hadamard Matrix containing 92 rows and columns, with combinations of positive and negative signs. In a Hadamard Matrix, if you placed all the potential rows or columns next to each other, half of the adjacent cells would be the same sign, and half would be the opposite sign. This mathematical problem had been studied since about 1893, but the solution to the 92 by 92 matrix was unproven until 1961 because it required extensive computation.
From left to right, holding a framed representation of the matrix, are Solomon Golomb, assistant chief of the Communications Systems Research Section; Leonard Baumert, a postdoc student at Caltech; and Marshall Hall, Jr., a Caltech mathematics professor. In a JPL press release, Sol Golomb pointed out the possible significance of the discovery in creating codes for communicating with spacecraft.
The team used JPL’s IBM 7090 computer, programmed by Baumert, to perform the computations.