Dawn over Ceres: the journey

Ceres, dwarf ruler of the asteroid belt

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.

Ryugu, about 1 km across and shaped like a diamond. Image taken by the Hayabusa 2 spacecraft Credit: JAXA

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.

Design of the ion engine

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.


Launch of Dawn

The Delta-II rocket

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.

Click on image for full resolution

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

26 thoughts on “Dawn over Ceres: the journey

  1. Is the gps not working anymore? There is a sizable gap between the last blue circle and the edge of the page which I dont think was there before but im not sure. It also looks like the general summit subsidence has stopped entirely and now all the deflation is through the ring faults.

    • Both CRIM and UWEV are still working fine at USGS page:

      I dont think that graph was very usefull anyways, the vertical component would be a more direct way to measure deflation. It seems to be still going on at the same rate at Pu’u’o’o, Mauna Ulu and at the summit GPS, with the exception of UWEV which is dropping faster and CRIM that has slowed.

    • Nishinoshoma is a very interesting volcano, it is the newest volcanic island that will actually become a bigger island in the future.

      Surtsey will erode to its central plug eventually and it might be a while before a new eruption happens near there again, and most new islands in general don’t even get that far and erode away within months. Nishinoshima is different, it has already erupted twice in the short time since its initial big eruption, so eventually it will grow to become a stratovolcano like the other older volcanoes to the north of it, and the way it is going now that might not take very long at all.
      It is a rare chance at seeing the early life of an island arc volcano from its very beginning.

    • Öræfajökull is the most interesting Iceland volcano, for the reason of its effect, in an instant.
      It is on the SE edge of Iceland Like the current activity at Hawaii, nothing but coincidence here though in the real world. Effect nasty, in the Pacific, in Atlantic, only translation is from Valhalla and that ain’t good either… grim with a bang.

  2. youtube.com/watch?v=OfYuf_BnODI

    change in composition of ash?

    • No that is just what reticulite looks like. There hasn’t been a change in magma composition since May 18 when new fresh magma started erupting as opposed to stored magma from 1960 or before.

  3. Everyone thought loihi would be Hawaii’s newest island… lol

    This is a tumulus formed on the submarine part of the lava flow that was going towards cape kumukahi last week. Evidently there is lava flowing quite well underwater and while the main channel now far bypasses this area there are probably some lava tubes that still feed a bit of the lava into Kapoho.
    It is actually a lot smaller than it looks too, only a few meters offshore and about 9 meters wide.

  4. A picture of the huge landslide earlier this week at Fagraskógarfjall in Iceland, probably caused by the very wet weather. At almost 2 km2 in area it is the largest landslide in Iceland since Askja 2014 (a smaller area but much deeper flow which caused 50 meter high waves).

    • Wow, I wonder if they will name that lake if it turns out to be a permanent feature. It looks like there could have been other big landslides off that mountain before too.

      A polar bear was also apparently sighted in the northern plain of Melrakkaslétta this past week too, which I think is a very rare event for summer when there is no sea ice.

  5. https://www.youtube.com/watch?v=5ormRfPahzw&ab_channel=R%C3%A9unionExtr%C3%AAme

    A good video showing the recent eruption on piton de la fournaise.
    It only lasted about a day but it did happen along quite a long fissure and occurred on the north side of the volcano which is quite unusual, the last time that happened was over 250 years ago when the formica leo cone formed. It might be a possible start of an extra-caldera eruption, which would be a first for historical time if it happens in that direction.

    I would have considered this eruption to be impressive at any other point, but considering the current eruption at kilauea might have erupted more lava than piton has in its entire 400 year recorded history it is sort of like saying a garden sprinkler is impressive after seeing a broken fire hydrant… 😉
    The two have many similarities even to the point where they are sometimes considered twins, but in terms of eruption volume there is almost no comparison at all…

  6. After the quake a few hours ago, I was looking to see whether the lava river would respond. It hasn’t overflown (yet) but is looking more ragged (crusted0 so perhaps bits of the cone collapsed into it. There is lava around fissure 8 in other directions, which also could perhaps be due to a bit of collapse. Overall, the lava river has not been as vigorous as in recent days but it is still flowing strong, just a tad lower,

    • It looks like lava has overflowed that small lava lake near pu’u 8. Some of the lava is going towards the south too, probably as a pahoehoe flow. If it has done this then maybe the lava river is starting to close up slightly and extra lava is forced to overflow the vent itself instead of going into the river.

  7. https://www.facebook.com/phil.hawaii/videos/10215652201120102/?hc_ref=ARTjp7vDfwznHoKMdxQAvq5EgoHGixMSo2wIQbhP4hVl4EYRZPZRRHVJ-D6AhGtFl8w

    This is the most recent live update from Philip Ong. About an hour in he talks about what he thinks might happen in the future and it seems like he has a similar idea of what will happen as I do. I would disagree with him on the idea of this being best compared to 1924 as things have gone far beyond that now but it is encouraging when someone with actual experience at HVO has the same sort of conclusion as you independently 🙂

    It is probably somewhat necessary for him to do this given that he has some position of authority and people really like to take things out of context, but I think he is maybe being a little bit too cautions with his ideas of eruptions being small in size for a while though. While it is true that scenario did happen after 1924, earlier collapses that were much more easily comparable to the current one had no such period of inactivity and in at least one case that was very similar to the current activity there was a significant increase after a major collapse.
    In case anyone didn’t see a previous comment I wrote on the last page, I saw that in the recent radar images, the outermost fault appears to be a deep vertical ring fault rather than a slump scarp like the more inner collapses. The part of the fault near HVO as well as another part near keanakako’i appear to be under slight extension. In my opinion these areas are as likely places for new eruptions as halemaumau is, and if the fault is under extension it could allow eruptions to be entirely aseismic in a way that is worryingly reminiscent of hekla, as well as the not unlikely and equally unpredictable complication of these eruptions happening without the magma chamber inflating beforehand… Should the continued earthquake swarm far under Pahala be the sign of a new surge of magma, it would not actually be unreasonable to expect a similar sort of eruption to hekla too if it can escape immediately. As far as I know there have never been historically recorded instances of true ring faults appearing in the caldera so there is no realistic or well observed comparison to this activity.

      • 1.5 km wide and at least 400 meter deep hole in the ground that mostly wasn’t there 2 months ago… Really only geology does stuff like that in a human lifetime 😉

        Looking at it from this angle the bottom of it actually might already be below the water table. Where 3 months ago there was a lava lake there might be a water lake by the end of the year, and then probably another lava lake this time next year… 😉

  8. https://pubs.usgs.gov/pp/pp1613/pp1613.pdf

    A very interesting paper about the 1974 summit eruptions. Written in 2000 but it actually backs up a lot of ideas that we have had in the discussions.

    Pretty much the idea of a large ring fault eruption is something like this:

    Except at least 10 times bigger, no exaggeration…
    (This happened in 1971 in one of the areas I noted actually)

  9. HV046 has been lost. It disappeared after the most recent quake. Kilauea is eating these things.

    • It seems to me GPS monitoring stations as BYRL, CRIM, OUTL, MALU etc. do not update anymore either. No new blue dots are appearing in the graphs past days? Maybe there is some kind of communication problem.

      • Now that you mention it at least DEVL clearly hasnt updated in several days (I have been watching that same point at the end for more than a week I think and I hadnt even noticed) it must be the same for the other GPS.

        • If they all link through the HVO building them that could be why. Maybe the building is too damaged and part or all of it collapsed, or the power cable got cut by a recent quake?

  10. Pingback: Vesta’s volcanoes: Dawn’s blast in the past | VolcanoCafe

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