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Dawn: Beginning Approach to Ceres

Speaking of spacecraft that do remarkable things, as we did yesterday in looking at the ingenious methods being used to lengthen the Messenger mission, I might also mention what is happening with Dawn. When the probe enters orbit around Ceres — now considered a ‘dwarf planet’ rather than an asteroid — in 2015, it will mark the first time the same spacecraft has ever orbited two targets in the Solar System. Dawn’s Vesta visit lasted for 14 months in 2011-2012.

We have the supple ion propulsion system of Dawn to thank for the dual nature of the mission. In the Dawn version of the technology, xenon gas is bombarded by an electron beam. The resulting xenon ions are accelerated through charged metal grids out of the thruster. JPL’s Marc Rayman, chief engineer and mission director for the mission, explained thruster design in one of the earliest of his Dawn Journal entries:

Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second… Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion’s extraordinary efficacy.

Slow but steady wins the race. For the same amount of propellant, a craft equipped with an ion propulsion system can achieve ten times the speed of a probe boosted by today’s conventional rocketry, says Rayman, but on the other hand, an ion-powered spacecraft can manage to carry far less propellant to accomplish the same job, which is how missions like Dawn can be executed. It’s also true that one of Dawn’s thrusters pushes on the spacecraft with about the force of a piece of paper pushing on a human hand on Earth. Dawn isn’t exactly the spacecraft equivalent of a Ferrari — at full power, the vehicle would go from 0 to 60 miles per hour in a stately four days.

Fortunately, space is a zero-g environment without friction, so the minuscule thrust has a chance to build up. ‘Acceleration with patience’ is Rayman’s term. In addition to enhancing maneuverability, ion thrusters are also durable. Dawn’s three thrusters have completed five years of accumulated thrust time, more than any other spacecraft. If all goes well at Ceres, we can’t rule out an extended mission that might include other asteroid targets, just as we hope for a Kuiper Belt object encounter for New Horizons after its 2015 flyby of Pluto/Charon.

dawn_ceres

Image: An artist’s concept shows NASA’s Dawn spacecraft heading toward the dwarf planet Ceres. Dawn spent nearly 14 months orbiting Vesta, the second most massive object in the main asteroid belt between Mars and Jupiter, from 2011 to 2012. It is heading towards Ceres, the largest member of the asteroid belt. When Dawn arrives, it will be the first spacecraft to go into orbit around two destinations in our Solar System beyond Earth. Credit: NASA/JPL-Caltech.

Keep an eye on Rayman’s Dawn Journal as the Ceres encounter approaches. His latest entry goes through the historical background on the dwarf planet’s discovery, and includes the fact that the Dawn team has been working with the International Astronomical Union (IAU) to formalize a plan for names on Ceres that builds upon the name given to it by its discoverer. Astronomer Giuseppe Piazzi found Ceres in 1801 and named it after the Roman goddess of agriculture. The plan going forward is for surface detail like craters to be named after gods and goddesses of agriculture and vegetation, drawing on worldwide sources of mythology.

Deep space has been yielding unexpected results since the earliest days of our exploration, and with Dawn approaching Ceres it’s instructive to recall some of the discoveries the Voyagers made as they moved into Jupiter space, starting with the surprisingly frequent volcanic activity on Io. Ceres will doubtless yield data just as intriguing, says Christopher Russell (UCLA), principal investigator for the Dawn mission:

“Ceres is almost a complete mystery to us. Ceres, unlike Vesta, has no meteorites linked to it to help reveal its secrets. All we can predict with confidence is that we will be surprised.”

Not quite twice as large as Vesta, Ceres (diameter 950 kilometers) is the largest object in the asteroid belt, and unlike Vesta, it apparently has a cooler interior, one that may even include an ocean beneath a crust of surface ice. We’ll know more soon, for Dawn has emerged from solar conjunction and is communicating with Earth controllers, who have programmed the maneuvers for the next stage of operations, which includes the Ceres approach phase. At present, the spacecraft is 640,000 kilometers from the dwarf world, approaching it at 725 kilometers per hour.

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Comments on this entry are closed.

  • Andrew LePage December 31, 2014, 13:16

    This is going to be an even more exciting encounter than Vesta was since seven less is known about Ceres. Thanks to ion propulsion technology, Dawn has been able to visit two asteroids and will allow other ambitious missions to fly throughout the solar system in the future (e.g. the upcoming ESA mission to Mercury). This is the culmination of decades of research into making ion propulsion practical with the first test flight in space taking place 50 years

    http://www.drewexmachina.com/2014/07/20/50-years-ago-today-the-first-ion-engine-test-in-space/

  • Ashley Baldwin December 31, 2014, 14:02

    The propulsion of the future ,ion thrusters. ‘NEXT’ is the new step up system from Dawn’s ion drive , and will be available for NASA’s future missions . Work is also complete on the VASIMR system which is several orders of magnitude more powerful again and has had test “burns” of five and a half years with no evidence of wear and tear on the system. As Paul says, the key is in the build up of power and speed over time although newer more powerful systems like VASIMR will build up speed much quicker . On average NEXT and VASIMR use a seventh of the propellant volume versus the equivalent chemical rocket. VASIMR is due to be installed on the ISS to perform its yearly orbit maintenance burn . As it is in a lowish orbit , this decays and needs a top up by a short rocket burn on an annual basis. Fifteen minutes of VASIMR is enough per year to achieve this. The cost of the ion system is one twentieth of the $220 million cost of a chemical rocket ,which is some saving.
    The draw back of ion systems is that they need power , electrical power, to charge the reaction plate and ionise the propellant that creates the propulsive force by pushing against it. It is also needed to create the strong magnetic fields necessary to protect the engine from the high temperature ions ( a million kelvin) and direct them out of the back of the engine in a focused stream. Several hundred kilowatts. Well beyond solar panels or even a radio-isotope thermoelectric generator. The ISS doesn’t possess that much so it will use a “trickle charge” battery for the brief yearly burn. Long term VASIMR and its descendants will need a nuclear reactor to provide the hundreds of kilowatts of power long term. NASA were developing such a system, Project Prometheus, till the financial crisis loomed and it was scrapped. Sure we’ll see it reappear one day and perhaps the goal of “Mars in 90 days” will be realised. That’s how good ion thrusters are.

  • ljk December 31, 2014, 14:03

    Dr. Rayman gave a very informative and amusing talk on Ceres and Dawn early this month, which can be viewed in its entirety here:

    https://www.youtube.com/watch?v=Qa5fQ0dkOW0

  • Joy December 31, 2014, 15:34

    Extended mission possible? Wow. First I have heard of that, assumed the xenon would be spent after the Ceres mission. Picking a 3rd asteroid would be hard, so many choices.

  • Christopher L. Bennett December 31, 2014, 15:37

    I like to think that Ceres being named for a goddess of agriculture is appropriate. If it does have as much water as we think, it could be a primary source of water and organic compounds for space habitats and colonies in the future. If I’m not mistaken, it actually takes less energy (or a comparable amount, anyway) to ship a mass from Ceres to Earth orbital space than it would take to launch it out of Earth’s gravity well.

  • Alex Tolley December 31, 2014, 17:20

    @Chris Bennett – I think the delta v’s for Ceres to Earth orbit, and Earth to LEO are comparable. The real benefit is that Earth to LEO requires high thrust chemical rockets in expensive boosters consuming a lot of fuel per kg orbited, plus the expended launcher. Water transport from Ceres can use relatively high Isp low thrust engines, even solar sails, allowing for relatively inexpensive, and reusable transfer orbit vehicles. If water is the propellant, Ceres provides both the payload and the propellant.

  • Michael December 31, 2014, 18:00

    Ceres offers a great jump off point to the outer solar system, it has all of the components needed for a great base. There is indication of minerals on the surface, a lot of water, a fast rotation period of only 9 hours and if solar concentrators are used plenty of light for photosynthesis and power. I and I believe most others can’t wait for the encounter with this enigmatic little world which will teach us much about our solar systems formation and ultimately play a pivotal role in humanities space future.

    I wish Paul and all the readers of Centuari Dreams a Happy and prosperous New year.

    ‘May the light of humanity reach all the Stars before our gift of enlightenment fades into the dark.’

  • Hop David December 31, 2014, 20:25

    Christopher, from Ceres’ surface it’s take about 5 km/s for injection to a trans earth Hohmann orbit. If the Hohmann window didn’t occur near an ascending or descending node, it’s likely a substantial mid flight plane change would be needed, Ceres’ orbit is inclined about 10º. At earth arrival, it’d take another 5 km/s to park in Low Earth Orbit (LEO), though much of that 5 could be shed with aerobraking.

    Escape from earth’s surface is about 11.2 km/s and you also have an additional ascent penalty to get above earth’s atmosphere — about 1.5 km/s. So you’re correct, Ceres to the earth moon neighborhood can take less delta V than escaping from earth.

    With it’s high angular velocity and shallow gravity well, a Ceres elevator would be a piece of cake compared to an earth or even a Martian elevator. A Ceres elevator could help provide insertion delta V for transfer orbits to other destinations. Also ion engines could dock with a Ceres elevator. Ceres gravity is too strong for an ion rocket to leave or soft land on Ceres.

  • Brett Bellmore January 1, 2015, 10:42

    Quite possibly the best colonization candidate in the Solar system: Easy to get to, water, spinning fast enough to use a skyhook for launches to other destinations, and probably even has hydrothermal ores.

    Likely not enough gravity for health, but you could bury centrifuge habitats under the surface.

  • Andrew LePage January 1, 2015, 11:05

    @Joy December 31, 2014 at 15:34

    A couple of years ago there had been some talk about sending Dawn on a flyby of Pallas after leaving Ceres (with it highly inclined orbit, Pallas is moving too fast for Dawn to enter orbit with the fuel it has left). But I think that plan has been shelved due to a lack of fuel and time. Still, there are lots of other smaller targets to consider in the asteroid belt!

  • Wojciech J January 1, 2015, 12:21

    Hop David
    “With it’s high angular velocity and shallow gravity well, a Ceres elevator would be a piece of cake compared to an earth or even a Martian elevator. ”
    I always liked the idea of an elevator being used on Ceres. The ground would be used as mining site for extraction of resources including water, while the elevator would transfer the materials to orbital colonies around Ceres.
    We will see soon how this wonderful place looks like. Perhaps it is the best suited planet in our Solar System for colonization, even better than Mars? We will see.

  • Wojciech J January 1, 2015, 12:24

    Brett Bellmore
    “Likely not enough gravity for health, but you could bury centrifuge habitats under the surface.”
    My proposal would be to have tethered O’Neil type colonies around it, with the tethers serving at the same time as transport system of resources from the ground. I don’t know if this is doable from engineering point of view, but it would be interesting to see if any proposal like that was studied or entertained as a concept by expert or experts.

  • Wojciech J January 1, 2015, 13:10

    Andrew, would it be possible for Dawn to explore Psyche as a follow up mission or is too far away?
    http://en.wikipedia.org/wiki/16_Psyche

    It is one of the most fascinating objects in the asteroid belt, being a suspected metallic core, its surface and traits are probably something we haven’t seen before.

  • Brett Bellmore January 1, 2015, 14:06

    You’d likely do both, as there’d be mining AND space operations, and no sense in commuting either direction. At 3% of Earth gravity, designing a buried O’Neil colony likely wouldn’t be that difficult.

    Synchronous orbit around Ceres is only 782km above the equator, which combined with Ceres’ size and small tilt implies that a solar power satellite stationed there would be eclipsed a good deal of the time, so you’d want two or three of them, and a power distribution system between.

    I’d frankly prefer a manned mission to Ceres to the one to Venus that Nasa is talking about. The astronauts would actually have things to do besides cloud gazing. It’s probably the first place we should have a permanent presence beyond the Earth/Moon system.

  • Michael January 1, 2015, 16:27

    I would love to see a rover on this world and potentially a sample return mission, if there was any place in the solar system that could harbour life this world is the place! At around 500 m/s escape velocity launching anything from the surface or from place-to-place around this world would be done with ease. Now if there is a sub-surface ocean we could send a thermal driller there long before we get to Europa and it would be much easier to do, it could even teach us a thing or two about the process if we decide to go to Europa.

    Here are two articles that you may find interesting about Ceres and its potential to aid humanities expansion into the cosmos.

    http://www.lpi.usra.edu/decadal/sbag/topical_wp/AndrewSRivkin-ceres.pdf

    http://eltamiz.com/files/ceres3.pdf

  • ljk January 1, 2015, 20:15

    https://www.youtube.com/watch?v=5OFgJwdZxRc

    Crazy Engineering: Ion Propulsion and the Dawn Mission

    Published on Dec 29, 2014

    Ion propulsion isn’t something found only in science fiction. JPL engineer Mike Meacham looks at how ion engines are being used to drive NASA’s Dawn spacecraft through the solar system. Dawn is approaching dwarf planet Ceres in the main asteroid belt with arrival expected in March 2015.

    Previously, Dawn orbited Vesta, the second-largest body in the asteroid belt. Learn how ion propulsion works and why it’s the reason Dawn will be the first spacecraft ever to orbit two solar system bodies.

    More about Dawn at:

    http://dawn.jpl.nasa.gov/

  • Hop David January 1, 2015, 23:47

    Ceres isn’t easy to get to from earth. From LEO to Ceres surface takes about 10 km/s. And that’s not considering plane change expense that Ceres’ ~10º inclination imposes.

    It’s true Ceres has a shallower gravity well. But it lies in a more distant heliocentric orbit. It takes more delta V for trans Ceres injection and also more delta V to rendezvous with Ceres at the transfer orbit’s aphelion. Unlike Mars or earth, Ceres doesn’t have an substantial atmosphere that can lend a helpful hand via aerobraking. Hohmann trip time is about 1.3 years.

    In my view our moon, Phobos, Deimos and the near earth asteroids are much lower hanging fruit. The nature of the frozen volatiles in the lunar cold traps is still unknown. But if some of the more optimistic estimates are correct, the moon could well have an important role to play in the development of space.

  • Eniac January 2, 2015, 0:07

    I love the idea of a space elevator on Ceres. Keep in mind, though, that on small, airless bodies you do not need to rely on the body’s rotation. Mount the “elevator” (more aptly: “sling”) on a rotating platform at one of the poles (Or anywhere, if the body’s rotation is sufficiently slow), and operate it at any rotation rate you like.

    Realistically, with such a mechanical sling, you can impart about 2 km/s of delta-v to departing payloads, very efficiently from electrical power and without any fuel or reaction mass. This should open up a lot of asteroids for nearly free transport of materials to Earth. Unfortunately, Ceres is not one of them, there you’d have to add a substantial engine or a sail, if the 5 km/s number Hop David provides above is correct.

  • Joy January 2, 2015, 0:30

    as per Hop David

    “With it’s high angular velocity and shallow gravity well, a Ceres elevator would be a piece of cake compared to an earth or even a Martian elevator. A Ceres elevator could help provide insertion delta V for transfer orbits to other destinations. Also ion engines could dock with a Ceres elevator. Ceres gravity is too strong for an ion rocket to leave or soft land on Ceres.”

    Correct. The Clarke orbital altitude for Ceres is just 782 km. With the low Ceres surface gravity of 0.028 g, an untapered elevator cable could be made of conventional materials including even steel. (Steel would have a breaking length of 925 km at Ceres surface gravity, and an orbital tether experiences reduced gravity with altitude, so no problems). However since the first cable would have to be fabricated on Earth, better to use a lighter and stronger material such as Dyneema or Kevlar to reduce shipping costs. Ion drive craft could gently dock at the Clarke orbit station. Using an extended cable as a counterweight would indeed be the way to go for giving departing craft a push start on their way. Unfortunately, the tangential velocities at release from a Ceres cable are not very high. Even extending the cable to 5000 km only gives a tangential velocity of 1 km/sec, but that is better than nothing.

    Ceres is a much better site for colonization than Mars. The dust and atmosphere of Mars are significant negatives, as is the deep gravity well. The toxic perchlorates in the Martian regolith make it far from good soil for farming. As pressurized and radiation shielded habitats are needed anyway, better to be digging caverns in a 0.028 g object than a 0.38 g object. Underground, the photoperiod could be normalized to 24 hrs. The main uncertainty regarding Ceres is whether adequate nitrogen is present in the regolith. If not, colonists would have to import N2 at considerable expense. As noted above, there could be an economy based on the export of water to other colonies.

  • Lionel January 2, 2015, 7:51

    Fantastic post Paul, and Happy New Year to everyone

    @Ashley Baldwin, I’m curious as to whether we might try and use VASIMR with solar power very close to the sun. Mercury at its periapsis has 10.6x intensity of sunlight as Earth does and at apoapsis 4.6x. Perhaps solar powered VASIMR propulsion could open up Mercury to us for larger payloads, such as delivered a landed or rover to explore icy polar craters

  • Lionel January 2, 2015, 7:54

    @Hop David

    Thanks for the orbital mechanics info!

    Presumably the delta V for deliveries from Ceres to LEO could be reduced further by passing by Mars for gravity assists?

  • Harry R Ray January 2, 2015, 10:28

    This being my first comment in The Year Of The Dwarf Planet (Ceres AND Pluto. Last year was The Year Of The Comet, with the Rosetta mission AND the close encounter with Siding Spring at Mars) I just want to point out a rather ironic parallel in the two mutual events: the vast difference in the SPEED of the encounters! Last year, Rosetta was approaching it’s target comet at an agonizingly slow pace, whereas Siding Spring was approaching Mars at a breakneck speed. Conversely, this year, Dawn is currently less than TWICE the Earth-Moon distance from Ceres, whereas New Horizons is STILL almos one hundred million miles from Pluto. But; the REAL irony of this to me, is; images taken AT THIS TIME of both targets would look VERY SIMILAR due to the EXTREMELY HIGH MAGNIFICATION of the New Hporizons Cameras (and the ability of these cameras totake extremely long exposure images) If my calaulations are correct, as early as this time next week, images of Ceres will start to show some detail, and thus, surpass any Pluto images until late June. What an interesting year this is gonna be!

  • Lionel January 2, 2015, 13:37

    I tend to agree with Hop David that The Moon, NEAs, and Mars’ moons are lower hanging fruit for industrialisation and colonisation…

    in my view Ceres hangs rather higher up, though looks to be so juicy when we finally have a platform to reach it with

  • Hop David January 2, 2015, 14:06

    Lionel,

    Personally I believe the moon, NEOs and Phobos and Deimos would probably be developed before we progress to the Main Belt. Given an elevator from Phobos, the main belt becomes more accessible. I talk about a Phobos elevator as well as Vesta and Ceres elevators at http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevators-clarke-towers.html

  • Ashley Baldwin January 2, 2015, 16:09

    Dear Lionel. I don’t think ion thruster engines would work well for this task. The problem with the inner solar system and especially for Mercury is slowing down rather than speeding up. We are all use to gravitational assists speeding up spacecraft like Cassini and New Horizons to save time getting to the outer system . The gravitational assists to Mercury are used to slow down the spacecraft enough to get captured by Mercury’s Gravity ,as it falls into the steep gravity well near the Sun . Even then it will require assistance from chemical rocket to succeed. I’m no orbital engineer and its complex stuff which is why it takes so long getting into orbit around Mercury. BepiColombo is next up and will take 8 years with a number of fly bys of Mercury itself first before 1-2 year mission with two separate orbiters. In terms of a direct flight I would guess it would need a lot of thrust to slow down. Ion thrusters have low thrust, compared to a conventional chemical rocket , their strength is in their high efficiency in building up speed ( specific impulse-sort of miles per gallon in space) over the long times their reliable engines can fire . They use an eighth as much propellant as chemical rockets performing some tasks. That said VASIMR is unusual amongst ion thrusters in having a relatively high thrust of 5 N, approaching some chemical rockets. The problem is whether solar cell technology would be up to turning all that extra solar energy into electricity necessary to power the final gravity capture. It might be but I suspect the gravity assist/chemical rocket method is more efficient at present. VASIMR will come into its own when we have small nuclear reactors to power it . Hardly any moving parts it can work on almost anything but is best with cheap and safe argon and can work for decades I suspect. Reactors are orders of magnitude more powerful than the best solar cells and RTGs available. It because of this that the outer solar system is where these systems will be most effective.

  • Michael January 2, 2015, 17:58

    @Joy January 2, 2015 at 0:30

    ‘The main uncertainty regarding Ceres is whether adequate nitrogen is present in the regolith. If not, colonists would have to import N2 at considerable expense. ‘

    Ceres has an planer tilt of around 10 degrees and a rotational tilt of about 3 degrees so there is the possibility of (nitrogen) ammonia collecting in any deep shadowed craters of the poles like on our moon from comets. Ammonia also just loves water and could potentially been dissolved into any sub-ocean.

  • Lionel January 2, 2015, 19:44

    @Hop David
    A copy of your Phobos beanstalk schematic showing all the orbit options has been saved in a folder on my computer called “Favorite Space Images” for a year or so

    So perhaps economical future cargo or robotic convoys headed to the outer solar system will travel via both the Phobos then the Ceres elevators for remarkably low delta v passage

  • Lionel January 2, 2015, 19:59

    @Ashley Baldwin

    Many thanks for your reply – the specifics of capture into Mercury’s orbit are very interesting and I guess probably resembles the situation if say we wanted to put a probe into orbit around Titan or one of the Jovian moons

    Indeed heading closer towards the sun does involve slowing down, relative to earth – both to enter the transfer orbit of where we want to go (say Venus first for gravity assist) and then to become captured by the destination body. I’m also not an orbital mechanics expert or even novice, but I thought that slowing down just meant that the thruster point in the opposite direction. As for the gravity capture side, I hadnt thought there was a lower limit on the amount of thrust the engines would require – I just thought it’d take much longer with lower thrust

    Thanks again for your reply – I’ll try and do some reading about what’s involved at a later point; thanks also for the info about BepiColombo

    Maybe Hop David has some comments on whether capture into Mercury’s gravity is possible with some kind of ion engine

  • Hop David January 2, 2015, 22:11

    Lionel, you are correct that slowing down or speeding up would depend on which direction the thrusters are pointed.

    However, ion thruster are poorly suited for the inner solar system (in my opinion). Ion thruster burns are sloooow. They can take days, weeks or even months to achieve a given delta V.

    In low earth orbit, the pace is fast and furious. A circuit (360 degrees) about the earth takes 90 minutes so an object is moving 4 degrees/minute. Time spent near a near earth periapsis would be on the order of tens of minutes, not enough time to do a long ion engine burn.

    Earth’s heliocentric orbit moves at a much more leisurely angular velocity — about a degree per day (360 degrees/365 days). So something coming near earth’s neighborhood in our solar system would be around days or weeks.

    Mercury moves about 4 degrees a day, a faster pace than earth. It’s also moving faster in terms of kilometers per second: about 48 km/s vs earth’s 30 km/s.

    Ceres moves a degree every 4 or 5 days. So something drifting by Ceres’ neighborhood can be around for weeks or months. Ceres is moving about 18 km/s with regard to the sun. The more leisurely pace of the outer solar system is more amenable to an ion engine’s slow burns.

  • Michael January 3, 2015, 6:28

    On the thoughts of a cable elevator why not use a tube instead, gases could easily move up and down the inside of the tube allowing a lot of transport of volatiles to and from orbit. Transports could use the outside of the tube if they wanted to, they could also use the inside of the tube say using hydrogen gas as a pressure control medium. I was once looking at a design such as this using S-glass and then using a laser down the centre to project energy to the external transports that used the outside of the tube by the use of a magnetically controlled reflector device inside the tube.

    Also if Ceres is found to have a iron core we could potential use superconductor coils at each pole -magnetically linked- to provide a large global protective field, a large iron core would help immensely.

    @Lionel

    You have to dump orbital momentum, so if you had enough propellant and time going to mercury by ion drive is feasible. Using fuel from the moon including xenon which could be trapped under ice caps at the lunar poles I see it as an engineering issue only.

  • ljk January 3, 2015, 15:44

    NASA’s Revolutionary Dawn Spacecraft Begins Final Approach Phase to Dwarf Planet Ceres

    By Ken Kremer

    After a seven year trek and at the dawn of a new year NASA’s revolutionary Dawn spacecraft has begun the final approach phase to dwarf planet Ceres for an unprecedented examination of this mysterious and never before visited world.

    Coincidentally, Ceres was discovered on New Years’ Day, Jan. 1, 1801 by Giuseppe Piazzi of Italy.

    Ceres is the largest and most massive object in the main Asteroid Belt, which lies between Mars and Jupiter. It measures about the size of Texas with a diameter of approximately 590 miles (950 kilometers).

    Scientists are keenly interested in Ceres as it may harbor an ocean of liquid water as large in volume as the oceans of Earth below a thick icy mantle despite its small size – and thus could be a potential abode for life.

    But no one knows for sure and unlocking the secrets to the most fundamental science questions of life and the solar system’s formation is what Dawn’s objectives are all about.

    And with the recent findings from ESA’s Rosetta mission that asteroids may have delivered more water to the early Earth than comets, Dawn’s arrival at watery Ceres couldn’t be more prescient.

    Full article here:

    http://www.americaspace.com/?p=74304

  • Michael January 3, 2015, 18:13

    Forgot to add this little clip of Ceres rotating

    http://dawn.jpl.nasa.gov/multimedia/images/anim_ceres.gif

  • Lionel January 4, 2015, 13:29

    @Hop David

    Thanks for your reply – I’ve been thinking about this business and will share my thoughts here – basically (1) I very much agree with your reasoning in the context of chemical rockets but (2) contend that the dynamics with ion drive probes are very different – and that the delta-v change absolutely does not need to be made whilst in the gravitational influence of the target (e.g. a comet). Going into more detail about these two points:

    (1) with chemical rockets, it makes intuitive sense to me that orbital capture around a close orbiting body (e.g. Mercury) is much more demanding than capture around a far orbiting body (e.g. Saturn).

    Going to a body closer to the sun requires slowing down at the periapsis of e.g. a Hohmann transfer orbit. The target body is moving quickly, and the probe quicker still. Travelling to a further out body in the solar system means speeding up the probe at apoapsis of a transfer orbit. The planet is moving slowly, and the probe even slower.

    Being completely handwavey, I suggest the *percentage* velocity differences between the probes and the target planets are comparable in both cases. In the former case, the delta-v between the probe and planet is some difference of two very high velocities. In the latter case, the delta-v is perhaps a comparable percentage difference, but of two far slower velocities. So it seems to me that chemical rockets will have to shed a lot more delta-v more quickly when visiting an inner solar system planet versus and outer solar system planet, which is what you and Ashley Baldwin have said above.

    (2) Now, here is where I differ on the details about ion drive probe operation:

    During the duration of their thrusting, ion drives do not follow orbits, but gently thrust through inward or outward spirals or “windings”. Orbits are what probes follow in-between the burns (whether driven by chemical or ionic propulsion). Mathematically, a winding is a continuum of infinitesimal orbits, a bit like how a curve is an infinitesimal of straight lines. Each xenon ion thrusted out causes our probe to move onto a minutely different orbit.

    The way that these windings are manoeuvred means that the ion drive probes,
    (a) have, over months and years, already almost matched the solar orbit of their targets by the time they reach them,
    (b) therefore upon meeting have extremely similar angular speeds around the sun as their targets (= tiny delta-v between them) such that the ion-drive probe passes through latter’s orbit far far more gradually then a chemical rocket would. (I think that to the extent that an ion drive probe can be precisely controlled, it can sidle up to it’s target with arbitrarily small delta-v. That is to say that angular speed around the sun or larger body doesn’t matter, it’s the relative delta-v’s that matter, a bit like how gentle docking can be achieved between spacecraft the ISS is achieved despite both bodies orbiting the Earth at 4 degrees a minute)

    Now, I’m not sure how things change moving into the inner solar system vs. moving out, but I agree with Michael that with enough time to dump the orbital momentum it’s doable. It’s a different scheme than with chemical rockets – which dump all their momentum whilst in the gravitational influence of the target.
    With ion engines the orbital momentum dump (or build up) happens over months and I imagine something like 99.9% is already dumped by the time the probe passes into the region of gravitational influence of the target.

    So it’s my intuition that given long enough an ion drive probe can wind closer and closer into the sun such that it approaches Mercury – if the winding took years then the probe may make ten or more orbits around the sun before reaching Mercury – but when it finally does it could approach with small enough delta-v and similar enough orbit to be captured by the latter in a very gentle fashion. (this would then be followed by the probe having to thrust for many weeks or months more to wind into a low altitude over Mercury for the science)

  • Rob Henry January 4, 2015, 18:33

    The time for these ‘evolutionary’ speculations is after Dawn has finished its encounter – this should be the hour for revolution. Let’s start with the fringe speculation that Ceres is the home of life on Earth/Sol. This is based on it being the only body during the late heavy bombardment that doesn’t seem to have been hit and thus had its surface sterilised to several thousand degrees. We have already covered that on these pages.
    https://www.centauri-dreams.org/?p=6529

    Note that this is no Europa with average temperature 50K and steel hard ice crust, but averages 168K. To me this would seem to allow plants to live under a few meters of transparent ice, warmed further by light absorption. The current regional temperature highs look like 235K (Wikipedia), only twenty degrees from the freezing point of frozen brine. So my humble prediction – Dawn will discover the distinctive absorption of chlorophyll in the warmest most sunlight spots there.

  • Hop David January 5, 2015, 13:45

    Lionel,

    An ion probe could indeed make a gradual spiral to a destination and thus avoid the high thrust impulsive burns usually called for at the end of a Hohmann transfer orbit.

    But these slow spirals take more delta V than a Hohmann ellipse. A general rule for these spirals is to take the difference in speeds between the departure and destinations orbits.

    First an ion drive would need to spiral out of earth’s gravity well, this would take about 7 km/s. Then going from earth’s 30 km/s to Mercury’s 48 km/s orbit would take another 18 km/s. Then going from the edge of Mercury’s sphere of influence to Low Mercury Orbit takes another 3 km/s. A total of about 28 km/s from LEO to LMO.

    Using high thrust, impulsive burns it’s about 11 km/s from LEO to LMO.

    Still, the ion drives would have a better mass fraction even with the higher delta V budget. Exhaust velocity can be 20 to 30 km/s for an ion drive vs around 4 km/s for chemical. And 28/30 or even 28/20 makes for a smaller exponent in the rocket equation than 11/4.

    But another problem would be the punishing environment that close to the sun. And a slow spiral would mandate years of exposure to that harsh environment before arrival.

  • Michael January 5, 2015, 15:32

    @Rob Henry January 4, 2015 at 18:33

    ‘The current regional temperature highs look like 235K (Wikipedia), only twenty degrees from the freezing point of frozen brine. So my humble prediction – Dawn will discover the distinctive absorption of chlorophyll in the warmest most sunlight spots there.’

    Here is a theoretical thermal map of Ceres and it appears somewhat colder than that ~ 177 K at the equator.

    http://www.lpi.usra.edu/meetings/lpsc2013/eposter/1655.pdf

  • Lionel January 5, 2015, 16:25

    @Hop David

    very nice numbers and information!

    I’be just seen that next year’s ESA/JAXA mission to Mercury, BepiColombo, which @Ashley Baldwin mentioned about, uses an ion thruster stage to get over to Mercury (it takes 8 years) and chemical rockets for escaping Earth’s / capturing into Mercury’s gravity. From Wikipedia:

    “The Transfer Module is equipped with two propulsion systems: a standard chemical propulsion system (CPS) which is bipropellant using MMH/MON3. The CPS will be used for Earth escape and then it will be pyrotechnically isolated and function in blowdown mode for the cruise.[clarification needed] The MTM supplies electrical power for the two hibernating orbiters, as well as for its electric propulsion system.[10]

    The spacecraft will be propelled in cruise by a form of ion drive dubbed solar electric propulsion, which has a very high specific impulse and very low thrust. Unlike a chemical rocket which fires for several seconds, it will keep propelling the craft for years, building up far more speed per mass of fuel as the years go by. This will be ESA’s first mission outside the Earth–Moon system using such a form of propulsion. This drive will actually push against the direction of travel, instead of with it; the spacecraft will be falling toward the Sun, accelerated by its gravity, and will have to slow down enough to eventually enter Mercury’s orbit. Moments before Mercury orbit insertion, the MTM will be jettisoned from the spacecraft stack.[10]”

  • Lionel January 5, 2015, 16:26

    @Hop David

    nice numbers and information!

    I’ve just seen that next year’s ESA/JAXA mission to Mercury, BepiColombo, which @Ashley Baldwin mentioned above, uses an ion thruster stage to get over to Mercury (it takes 8 years) and chemical rockets for escaping Earth’s / capturing into Mercury’s gravity. From Wikipedia:

    “The Transfer Module is equipped with two propulsion systems: a standard chemical propulsion system (CPS) which is bipropellant using MMH/MON3. The CPS will be used for Earth escape and then it will be pyrotechnically isolated and function in blowdown mode for the cruise.[clarification needed] The MTM supplies electrical power for the two hibernating orbiters, as well as for its electric propulsion system.[10]

    The spacecraft will be propelled in cruise by a form of ion drive dubbed solar electric propulsion, which has a very high specific impulse and very low thrust. Unlike a chemical rocket which fires for several seconds, it will keep propelling the craft for years, building up far more speed per mass of fuel as the years go by. This will be ESA’s first mission outside the Earth–Moon system using such a form of propulsion. This drive will actually push against the direction of travel, instead of with it; the spacecraft will be falling toward the Sun, accelerated by its gravity, and will have to slow down enough to eventually enter Mercury’s orbit. Moments before Mercury orbit insertion, the MTM will be jettisoned from the spacecraft stack.[10]”

  • Lionel January 5, 2015, 16:51

    @Michael

    I wonder what effect periodic tidal heating from Jupiter might have – if I rember correctly tidal effects are the main exlanation for activity on Enceladus even though the latter has an extemely circular orbit – far more so than Earth.

    Closest approach between Ceres and Jupiter looks like 2.25AU to 3-ishAU and I guess the former overtakes the latter every 12-ish years

    Not sure if Jupiter’s influence was mentioned on the poster you shared (it loads in a resolution too low to read – maybe just because I’m on a smartphone whilst riding a train)

  • Michael January 6, 2015, 4:09

    @Lionel January 5, 2015 at 16:51

    ‘I wonder what effect periodic tidal heating from Jupiter might have – if I rember correctly tidal effects are the main exlanation for activity on Enceladus even though the latter has an extemely circular orbit – far more so than Earth.

    Closest approach between Ceres and Jupiter looks like 2.25AU to 3-ishAU and I guess the former overtakes the latter every 12-ish years’

    Jupiters influence is what prevented a sizeable planet forming in the current asteriod belt so it would have had some effect but the orbit time periods would have made any heating effects minimal I would think. The moons around Jupiter and Saturn have low orbital periods which increases the heating effects.

  • Rob Henry January 6, 2015, 15:49

    Yes Michael, that diurnally averaged temperature is too low, but then plants can’t grow at night anyway. A H2O-H2O2 eutectic mix as the fluid for life will get us down even further (<220K). Houtkooper made a good argument for that being a better fluid for life at Martian temperatures.
    http://arxiv.org/ftp/physics/papers/0610/0610093.pdf

    I repeat, Ceres is no Europa. Estimates of the maximum available power to a Europan ecosystem if the high energy materials generated on its surface are to be delivered to the interior at the implied resurfacing rates place it at around 0.1-1 watt per square kilometre. Here, if 10% of Ceres is unfrozen 10% of the day, and then captures light at sugar cane efficiencies, we are talking 60,000 W/sqkm.

  • Lionel January 6, 2015, 17:42

    @Michael

    I see what you mean – Enceladus’s orbital period is a mere 1.3 days.
    I have no idea what the total range of gravitational pulls of Jupiter upon Ceres is, but I guess it’s the rate of change that matters (also, presumably the variation of the sun’s pull on Ceres thoughtout its elliptical orbit would be a much bigger tidal effect anyway)

    Changing the subject, one interesting number I found: Ceres is 30x as massive as Enceladus

  • Rob Henry January 7, 2015, 19:01

    Lionel and Michael, why not use the effect of our own moon as a basis for ballpark figures. Tidal force is proportional to mass x the inverse of the cube of distance, call that force 1. Now…
    Sun on Earth = 0.4
    Jupiter on Io = 20,000
    Jupiter on Europa = 5,000
    Saturn on Enceladus = 30,000
    Sun on Ceres = 0.1
    Jupiter on Ceres = zero

    So forget pumping up its eccentricity or any other tidal mechanisms. Put simply, it has nothing to ‘push’ against anyway.

  • Rob Henry January 7, 2015, 19:14

    To further the above, tidal force is directly related to the cube of visual angle times density so simply look from your planet or moon and unless you see something Moon sized or bigger in the sky above you won’t have to worry about tidal problems.

  • Michael January 8, 2015, 3:43

    Thanks Rob, I knew it was very low. On another point is the pressure inside this world, it is very low indeed. So if life made it into the interior it would have all the nutrients, protection, warmth and pressure it needed to survive and grow. I get a centre core pressure of only 3000 bar. At around 200 km below the surface of Ceres the pressure is similar to that of the Mariana trench on Earth where life can survive just fine.

  • Lionel January 8, 2015, 6:50

    @Rob Henry

    Thanks Rob, very good to see those figures and know the relation and the “look in the Sky” rule of thumb makes things super straightforward

    One question though – how does axial rotation/tidal locking affects things?
    I presume if the Earth was tidally locked with the moon, or perhaps rotating close to following the moon such that there were roughly 13 days a year, then lunar tidal effects upon earth would be much reduced. Is this right?

    Or does tidal locking itself generate heat within the bodies? Pluto and Charon’s mutual effect comes to mind

  • Rob Henry January 8, 2015, 15:42

    Yes Lionel, the effect should tend toward zero as tidal lock (including zero eccentricity) approaches, so these are maximum figures. Unfortunately, working out any figures gets complicated, but here is an example
    http://en.wikipedia.org/wiki/Tidal_locking#Timescale
    Note how you could estimate the relative expected power output from that equation if the body is still rotating independently.

    Here are some figures comparing Earth with tidally locked but Laplace resonance effected Io (remember, it is suffering 20,000 times the tides)
    Earth 47 TW activity, 0.087 W/sqm
    Io 130 TW, 3 W/sqm
    Sure, Earth probably has proportionately more radiogenic heating, and certainly more heat left over from its formation. Io only has a 2 TW contribution from its Flux Tube. All these are lumped together from heat balance calculations so, using a fudge factor of 2 to adjust for these, Io is 5 times as active as Earth, 70 times by surface area, and 350 times per unit mass.

    Also note that if Io broke free of the Laplace resonance, yet still had Europa and Castillo to perturb it, its activity would drop (by my understanding, by an order of magnitude or two).

  • Lionel January 8, 2015, 17:59

    Thanks Rob all very interesting

    Enceladus is also tidally locked but in a resonance with Dione

    Just rediscovered this “Enceladus is a powerhouse” newspiece which quotes a 2007 paper that gave tidal power generation as 1.1GW, meanwhile there’s heat been observed corresponding to 4.7GW , leaving the vast majority unaccounted for
    http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20110307.html

  • Rob Henry January 9, 2015, 20:18

    Lionel, I am not absolutely certain that there is a problem with Enceladus, as its resonance is not locked in as with Io/Europa /Ganymede. I have seen estimates that when Ganymede FIRST joined that resonance, it would have suffered, briefly, 200 times more tidal activity that now. Here is one paper on it
    http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf