There used to be a missing planet. It had long been realized that there was an empty gap in the Solar System, between Mars and Jupiter. The two were just too far apart. The distribution of the planets was described well by a relation proposed by Johann Titius and Johann Bode, and this relation predicted a planet in the empty zone beyond Mars. But none could be found. The discovery of Ceres in 1801 seemed to have filled this gap, and for a while Ceres was considered along the other planets. But by the time Vesta was found in 1807, the situation had become more confusing. Vesta was the fourth object to be discovered here, and although a little smaller than Ceres, and orbiting a bit closer to us, Vesta was clearly very similar to Ceres. So now we had two (or even four) planets in the gap. Far from being a gap, the area was becoming overcrowded with too many planets.
But Ceres, Vesta, and the others were just too small (much smaller than the Moon), and more and more such objects were found in this region of space. So a new expression was coined: Ceres became the first of the so-called minor planets. By 1850, the objects between Mars and Jupiter had acquired a second name, and were called ‘asteroids’, but the name minor planet (which includes objects elsewhere in the solar system – but not comets) remained in use.
History continued in the usual way. First a new kind of object is discovered, something special. Next, a second one is found, and now there is a new class of objects. And then someone finds a third, fourth and fifth one. But in a group of several, normally some are alike and one or two are different, exceptional. And invariably, the exception turns out to be the first one discovered. So it was here, in the gap. Most asteroids are rubble-like, piles of loosely bound rocks, with all kinds of strange shapes. A Japanese spacecraft has just arrived at one shaped like a diamond. (It is not for sale, in case you wondered, but they do intent to bring some material back to Earth.) But Ceres was a bit more like a mini-planet, larger and well rounded. So it changed class again, and became a member of the new class of dwarf planets, which so far has five members but it is estimated there may be as many 200 dwarf planets in the Solar System. The re-grading of Ceres left the asteroids without a clear leader, turning rubble into rabble.
NASA decided to have a closer look, and sent a spacecraft to investigate both the dwarf Ceres and the asteroid Vesta. When spending a lot of government money, an evocative name is essential: the spacecraft was therefore called Dawn. As an aside, Dawn became the first spacecraft to orbit two different celestial bodies (not including Earth). It found that Vesta and Ceres are very different from each other, and both are different from the dwarf planets and plutinos of the outer solar system (Pluto and its relatives). In the class of the dwarf planets, the first one known again has become the outcast, the exception. Déjà vu; history repeats. And there was another exceptional find. Ceres has volcanoes.
The asteroid belt
The asteroid belt fills the region between Mars and Jupiter. ‘Fills’ is a relative term, but the sheer number of objects is impressive. There are more than 200 asteroids over 100 km in diameter, and the number larger than 1 km is estimated at a staggering 1 million. The main belt is between 2.2 and 3.2 astronomical units (AU) from the Sun. (Earth is by definition at 1 AU from the Sun, Mars is at 1.5 AU and Jupiter at 5 AU.) There are some gaps in the belt, where the orbital period would be in resonance with Jupiter, a bad place to be as eventually Jupiter pulls them out of their orbit. The region out to 2.5 AU from the Sun is called the inner belt, and the region further out forms the outer belt.
There are three main types of asteroids. The C-type are carbonaceous, S-type mainly consist of silicates, and M-type are metallic with a lot of iron and nickel. Different types are presumed to have different origins. Similarity between orbits and composition suggests that many asteroids are related. Five main families have been defined, and it is thought that they have formed in collisions which broke up the five parent bodies. A recent suggestion is that almost all (more than 85%) asteroids of the inner belt come from these five parents, left-over building blocks from the formation of the Solar System. Don’t expect too much: each parent would have been much smaller than Ceres.
The asteroid belt has a dodgy reputation. It is a neighbourhood to avoid, especially at night, where the lawless asteroids are waiting in the shadows to get you, take your rocks and metals and smash you to bits. The first spacecraft to cross it, Pioneer 10, flew slightly out of the plane to reduce the risk of collisions. That turned out to be unnecessary. Even though there are a lot of objects, there is also a lot of space, so it is almost as empty as the rest of the Solar System. And they don’t add up to much. All asteroids together account for only 4% of the mass of the Moon. And a quarter of that is accounted for by just Ceres.
The asteroids don’t always stay in the asteroid belt. Sometimes Jupiter’s gravity dislodges one and it strays into the inner Solar System. The two small moons of Mars, Phobos and Deimos, are captured asteroids. Other asteroids are now on orbits that takes them inside the Earth’s orbit, and therefore at potential risk of collision with us. The Chelyabinsk meteor, which injured some 1000 people in 2013, was such an asteroid, if a very small one. The damage was not from the actual meteor, but from the shockwave as it entered the atmosphere. It was a wake-up call: the asteroids have become a roving rabble, crossing into the civilized part of the Solar System and endangering our lives and property.
The engine of Dawn
Traditionally, space travel uses chemical rockets. They are powerful, very loud, and have the reputation of being so difficult to operate that they have given us the most incomprehensible field of science known to mankind, encompassed in the famous expression ‘it is not rocket science’. But think a bit harder, and rockets don’t sound nearly so brilliant. As so often, volume hides the fact that ego exceeds ability. A rocket works by pushing hot gas out of the back, and the push forward comes from the recoil. If you were to propel a car in this way, you’d keep it moving by throwing things out of the back window. Basic physics tells you that this only works well if the speed of the projectiles is much higher than the speed of the car. (Absence of any friction is also helpful.) But in the case of rockets, the ‘projectile’ is the gas heated by burning it, and the speed of the gas (set by the temperature) is limited to ‘just’ a few kilometres per second. It sounds impressive, but the rocket needs more than 7 km/s just to get into orbit, and 11 km/s to leave Earth behind, so the exhaust is too slow. The basic requirement from physics is not met. Rockets are a poor solution to the problem of space travel.
Because of this too-low exhaust speed, the fuel in a chemical rocket weighs more than the useful part of the rocket. And as you have to carry the fuel with you until the point where it is burned and thrown out of the back window, much of the fuel is used to accelerate the rest of the fuel, which weighs more than the rocket so requires more fuel – and so on and so on. Very soon, 90% or more of the rocket becomes fuel. A casual look at a rocket reveals the problem: the rocket is huge whilst the capsule on top is tiny. The rocket is the fuel tank – the capsule is the passenger compartment. It quickly gets silly. Rockets are designed for missiles, which only need to travel at a few times the speed of sound. For space travel, such stone age missile technology is holding us back. To call something rocket science is NOT a compliment.
Nowadays, even our cars are changing, towards becoming electric. We are also working on our rockets. The goal of rocket science is to make the exhaust go faster than the rocket – without slowing down the rocket. The first attempt for this was Project Orion, driving the spacecraft by exploding nuclear bombs right behind it. This was typical 1950’s thinking, using the ultimate brute force technique, but it could have worked (mostly) given 50 years of research to develop materials able to withstand a nuclear explosion every second. Project Orion could have put a cruise liner into space and reached Saturn within months. The nuclear test ban halted the program in its tracks, just when they first applied to carry out test explosions. Electrical rockets were 1960’s thinking, applying flower power to space travel. It covered the opposite, soft power side of the spectrum. The next idea is to ionize gas, and use an electric field to accelerate the resulting ions. The ions can reach speeds of 40 km/s, which is perfect for space travel (at least within the Solar System – interstellar travel is a bit more demanding). There is a drawback: you are limited by the available electrical power, and so you can only push out a small amount at any one time. It is efficient, but the acceleration is painfully slow. You couldn’t launch anything this way from the ground – gravity would laugh at you. But it works very well once safely in space. For instance, you could use a chemical rocket to get into low-earth orbit (which ‘only’ requires 7 km/s) and switch to an ion drive to go to infinity – and beyond – very slowly. Dawn did use a chemical rocket to get into space and close to its desired orbit, but used an ion drive after this.
In an electrical rocket, the fuel does not need to explosively burn. In fact, you quite specifically do not want it to explode. It is therefore normal to use an inert (noble) gas for fuel. Dawn used xenon: such a heavy element is good as the thrust increases as the square root of the mass of the atom (for the same power).
The first step is to ionize the xenon. It is not stored in that form, but it is ionized just before being used. This is done by bombarding it with high energy electrons, from a cathode tube. Next, the xenon is accelerated. Dawn uses a gridded ion engine for this, which basically uses a very high voltage difference between two grids: the xenon ions are pushed away from the positively charged grid and towards the negative charged grid. The speed of the ions depends on the applied voltage, and is limited mainly by the available electrical power. Dawn is powered by 36 square meter of solar panels which provide a respectable 10 kW. The accelerated ions fly out of the back end of the engine, and the recoil provides he thrust, pushing the space probe forward. It is important to eject the liberated electrons as well, since otherwise the craft would end up with an ever-increasing negative charge. (In space there is no way to ground the craft!). This is done by ejecting the electrons in a separate (oppositely charged) cathode called the neutraliser.
Dawn has three separate ion engines, which are used in turn. While one of the engines is on (which was most of the time during the orbit) it uses about 10 grams of xenon per hour. It was launched with 385 kg of xenon, enough for 4 years of full thrust! Here, ‘full thrust’ should be taken with some care: the Top Gear TV program would give it a rating of zero, as from 0 to 100 km/h would take four days. On the other hand, by the end it would have done 5000 km using just 1.5 liter (1 kg) of xenon fuel. A Toyota Prius would need 300 kg of fuel to travel the same distance. And Dawn would still be accelerating. Typically, a major change of orbit in space requires a velocity change of 2-3 km/s. That would take Dawn a year of full thrust. The Top Gear presenters would feel nothing but frustration – but in space, no one can hear them scream.
Ion engines have been in development for a long time. The first one was build in 1959 (using mercury as propellant, operator safety not being seen as important in those days) and testing in orbit was first done in 1970.
Dawn was launched in the usual way, using chemical propulsion to get off the ground. There had been a slight hick-up: the mission had been cancelled after cost overruns and technical issues, re-instated, suspended, re-cancelled, and finally re-re-instated in 2006 when cancelation turned out to be almost as expensive as finishing the project. The total cost approached half a billion dollars. (Nowadays you could hardly launch a car for that price.) (Not quite true: a launch costs of the order of 100 million dollars.) The launch vehicle was a Delta II rocket with three stages and nine boosters. Launch was in the early morning of Sept 27, 2007. The Delta rocket was almost 40 meters tall; Dawn itself is just under 2.5 meters. The difference in size is a good illustration of the problem of chemical rockets: the fuel tank was 15 times longer than the rest of the vehicle! (The Delta II rocket is operated by the United Launch Alliance, a joint venture of Lockheed Martin and Boeing which until 2012 held a government-sanctioned monopoly on satellite launches in the US. ULA obtained a reputation of high reliability at great cost.)
The launch itself had some issues too. Originally, it was scheduled for June 20. A number of different delays occurred, caused by part-delivery issues, a broken crane, an accident with the solar array, poor weather, conflict with the launch of Phoenix, more poor weather, and finally a ship entering the off-shore exclusion zone. In the end, it was launched three months late. Even my local train company, known for its extreme willingness to sell tickets at increased price but severe reluctance to actually run the trains, would be impressed.
The launch put Dawn on a trajectory towards, but not reaching, Mars. The ion engine was used to make up the missing speed, and 18 months later it flew past Mars. It picked up some extra speed from the Mars gravity assist, and used this to reach the asteroid belt. Vesta was approached in July 2011, with careful use of the ion thruster to match the velocity and hydrazine thrusters to go into orbit. Dawn stayed at Vesta for a year. July 2012 was the time to move on. The ion engine was used to slowly (very slowly) increase speed. The encounter with Ceres was at the other side of the Solar System and it took a while to get there – close to three years. In March 2015, Dawn entered an orbit around Ceres. The original schedule had it remain there for a year, at which time the mission would be ended. An application for funding to visit a third body was rejected: NASA found that more science could be done by studying Ceres for longer. Two extensions later, the mission was funded until the hydrazine thrusters (used for for orbit insertion manoeuvring, and nowadays for pointing the spacecraft which ion thrusters can’t do) would run out. This is expected to happen in September this year.
Dawn arrived at Ceres in less than perfect shape. The reaction wheels had developed problems; these are used to point the spacecraft and now it had to be done using the limited supply of hydrazine. The imaging cameras during the approach require pointing operations (they are ‘point and click’ and the pointing was the problem). So few images were taken of Ceres during this phase. But once in orbit, the science could start.
Dawn has shown us a new way of exploring the Solar System. Was it worth its half a billion dollar? No doubt about that! This was the first exploration of two new worlds – and the first BOGOF mission (buy-one-get-one-free, in case you wondered). What did it buy us? What did we learn? And how about that volcano? Stay tuned.
Albert, July 2018