Welcome to the first Space Week article of 2011. Fifty-four years ago today (I am publishing this on the 4th, but just barely) the first artificial satellite Sputnik I was launched into orbit, marking the beginning of humanity's exploration of the Universe beyond our planet and its atmosphere. Today's topic has to do with the Dawn spacecraft, and I admit I was seriously considering "Breaking Dawn" for the title, if only to get some stray "Twilight" fan-girl traffic directed here, where they would then read something of value for a change. Maybe just having the phrase in the previous sentence will be enough, but in any case let us delve into the matter at hand.
Those of you who have been Space Week readers for many years may recall mentions of the Dawn spacecraft and its mission in previous editions. The mission is of great interest to me for three major reasons. First, it is the first spacecraft to orbit and study any object in the main asteroid belt. Other missions have made brief flybys of such objects en route to their intended destinations, but this time the main belt is the destination. Second, it will be the first spacecraft to orbit and study one object, and then break orbit and travel to and study a second distinct object. Dawn has already reached its first target, the main belt asteroid Vesta, and is currently in the middle of its year-long investigation of this fascinating not-quite-planet. Next summer it will leave Vesta and travel to Ceres, a dwarf planet and the largest object in the asteroid belt, arriving in February 2015 for a stay of at least six months. Third, Dawn is the first full-scale mission to employ the ion engine as its primary means of propulsion.
So why the asteroid belt? Well, it is quite an interesting place. There are, however, a few myths about it that should be addressed before proceeding further. The most prominent of them is that it is the Solar System's equivalent of a shooting gallery. While it is true that there are a lot of asteroids that orbit in the main belt (otherwise it wouldn't be the main belt, would it?) the distances between them are so large that one could easily pass through the whole thing without running into a single one, and in fact many spacecraft have. Though the boundaries are somewhat diffuse, the main belt is usually considered to begin at around 2 AU (astronomical units, a measurement defined as the average distance between the Sun and the Earth, around 93 million miles, and useful when dealing with distances within the Solar System) and end around 3.25 AU. This is from slightly beyond the orbit of Mars to about half of the distance between that planet and Jupiter. That is quite a large amount of space, so even if it were populated by millions of asteroids, as some have theorized, there is more than enough room to both accommodate them and ensure that anything traversing the belt does not get pulverized (assuming that is its goal).
Also, the asteroid belt is not some sort of time capsule, filled with leftover material from the formation of the Solar System that has remained there until the present relatively unchanged. Though there are some objects in the main belt that fit this description, the belt itself and its residents have changed over time, mostly due to the presence of the planet beyond its outer edge. I have written before of the dominant role Jupiter has played in the evolution of the Solar System, so suffice it to say here that its gravitational influence has worked both to keep the material in the belt from initially forming into a planet and to subsequently mess with the orbits of everything that happens to be there. Thus many asteroids that are in the main belt now did not start there, and many that did are for various reasons no longer around. And while it most definitely is not a shooting gallery, there are collisions between asteroids that alter their compositions. Finally, some of the larger ones may be subject to geologic forces similar to those found on Earth, pushing them even further from their initial state.
That being said, one of Dawn's primary purposes is to shed light on the formation and evolution of planets. Its two targets, Vesta and Ceres, are the largest objects in the asteroid belt (#2 and #1, respectively), and have stayed more or less where they were to start out. However, they are very different from each other, and studying them will help to solve important questions about how various processes play out over time. Vesta, following Ceres' reclassification as a dwarf planet, is the most massive of the asteroids, having an irregular shape but a differentiated interior (meaning various types of rocks have congregated together under internal gravity, rather than remaining randomly distributed). One question Vesta may help to answer is how and when magnetic fields arise on planets. Apparently Vesta's rocks are more strongly magnetized than those of Mars, possibly as the result of a massive collision it suffered around a billion years ago, so data from Dawn may help us understand how and why Mars lost its magnetic field, and how our own may change in the future. Ceres is by far the largest object in the asteroid belt, and since it is spherical it is now considered a dwarf planet. Ceres has a great deal of ice on its surface, could have seasonal polar caps, and may even harbor an ocean below its icy shell, all of which make it a place of great interest. One question Ceres could help answer is how the oceans formed on Earth. There are a few theories out there, and one of them involves a number of objects similar in composition to Ceres, though not as large, being thrown at us by Jupiter during the early period of intense and heavy bombardment the Earth sustained before things calmed down a little. So data from Dawn may help us understand whether or not this theory is plausible, as well as give us another example of how water and ice work to shape a planet over time.
The thing that will enable Dawn to do all of this is its ion engine. First tested by the Deep Space 1 probe in the late 90's, the ion engine has the potential to open up many new regions of the Solar System to exploration. Much more compact, more versatile, and longer lasting than traditional chemical rockets, the ion engine uses the thrust generated by charged particles (in Dawn's case Xenon gas) to move a spacecraft. It starts out slow, but since an ion engine can operate continuously over periods of several years, it can build up speeds similar to those rockets can provide while being more efficient, less bulky, and more reliable. It is also what will enable Dawn to leave Vesta and travel to Ceres for the second leg of its mission, something that would be extremely difficult for a traditional rocket powered spacecraft. Of course, Dawn did require a rocket to leave Earth's gravity well, but after that it has been clear ion sailing. The other feature of ion engines is that they take a long time to get you where you want to go (Dawn took a little less than four years to reach Vesta, and will take another 2.5 to get to Ceres) but when you consider that an ion engine needs a lot less fuel to get there, and that traditional rockets and their fuel is expensive (usually to the tune of around 50% of any mission's total cost), it is well worth the wait. Thus I hope that the ion engine will soon become the method of choice for robotic exploration, and that Dawn's success will inspire many more ambitious missions that will unlock the secrets of our Solar System.
There are, of course, many online resources for those who wish to learn more about Dawn, the asteroid belt, and ion propulsion. The best place to start is the Dawn mission home page, and to follow your curiosity after that. Next up we will discuss some recent news that has the potential to really shake up physics as we have understood it for the past century.