Dawn’s chief engineer reviews the mission

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In a long and very detailed post, the chief engineer and mission director of Dawn gives us a very detailed update on the successful state of the spacecraft’s mission.

Not only does he describe what has been gathered at Ceres since the spacecraft arrived a year ago, he gives us this crucial information about the state of this paradigm-shattering ion engine spacecraft, the first to travel to two different objects in the solar system:

Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship’s remaining lifetime is very limited.

A year ago, the team couldn’t count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.

NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.

Dawn has proven the value of ion engines. I would expect to see them used many more times in the future, especially missions heading to low gravity environments.



  • Steve Earle

    Amazing mission. Growing up I was fascinated by Pioneer 10, and Voyagers 1 and 2. It’s incredible how well these and other spacecraft have performed above expectations.

    Having said that, it appears that Dawn’s Ion engine is still working, too bad they can’t use that to put it into a Solar Orbit where it wouldn’t be necessary to use up the hydrazine remaining as quickly. Although the faulty reaction wheels would make that futile as well?

    Just thinking out loud. Shame that the engine, electronics, and solar power are all in good shape otherwise….

  • Tom Billings

    This mission is an excellent example of what directions future ion engine propulsion should go towards. First is using ion engines for attitude control, as is starting to be done for GEO comsats. This will save strongly on propellant mass taken up by Hydrazine in Dawn’s case. Second is making shorter missions, by increasing maximum thrust for inside gravity wells that must be climbed out of. A future ion engine mission should have the structure of the engine built in orbit, where its structure can be 1/10th as massive for the same thrust. This would be possible because the engine no longer has to survive 6 gees boost through the atmosphere, and worse acceleration by vibrations. For the same mass, it would allow 10 times the thrust, because a far larger structure for the engine will allow space-charge limitations of ion engines to be ameliorated through greater engine volume.

    This will also make for lighter attitude control ion thrusters, that can placed farther from the center of mass, increasing their torque on the spacecraft, because they are built in orbit and added to the vehicle before it leaves. Finally, we should take advantage of these high volume thrusters by beaming power from either the Earth or from Earth orbit to the spacecraft as it leaves the gravity well. It has long been noted that the best returns for use of early and small solar power satellites are to power things *in*Space*, rather than competing with power sources already established here on Earth. This strategy will enable exits from the Earth/Moon gravity well at far higher rates, with less mission time and with a far higher percentage of the mass that current and near-term launchers can deliver to LEO devoted to the mission, instead of to climbing the gravity well. Once outside the Earth/Moon gravity well the option would be available to shed the engine mass that is not needed when the vehicle is only powered by its own solar cells, or its own nuke power plant.

    Whether the scheme is the “Tethers Unlimited” concept of “SpiderFab”, or the “Made in Space” concept of “Archinaut”, or some other, matters less than that the building experience start being gathered ASAP.

  • Steve Earle

    Interesting concepts. Another way to avoid high launch G’s would be a Space Plane. Not as gentle as an orbital tether but still better than a rocket launch.

    Building on orbit would be the ideal. We need to start getting a lot more experience in space construction if we are ever to make the step past lab-experiments-for-show in a tinker toy space station…..

  • Edward

    I see a great advantage to on-orbit manufacture and construction. There would still need to be verification that the fabrication was done right. For example, antennas and reflectors would need some form of anechoic chamber ( https://en.wikipedia.org/wiki/Anechoic_chamber ) to make sure that they broadcast and receive as they should.

    There would also need to be an ability to fix or correct anything that did not work as it was expected to, which would allow for repairs of items that fail on orbit, perhaps extending their lifetimes and reducing insurance costs.

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