Posted on 09/27/2007 5:03:30 AM PDT by Aristotelian
Nasa's Dawn mission to the asteroid belt is set for an imminent further launch attempt, after giving up its previous slot to a Mars probe. The US spacecraft, which will visit the small worlds of Ceres and Vesta, will launch atop a Delta rocket from the Cape Canaveral Airforce Station.
Its journey of eight years and nearly five billion kilometres will help decipher how the Solar System formed.
The asteroids are thought to be the leftovers after the planets were made.
Dawn will reach Vesta in 2011 before going on to visit Ceres in 2015.
....
Dawn is expected to send back high-resolution images of these worlds, showing not just craters but also mountains, canyons, and clear evidence of volcanism.
Dawn will make a fly-past of Mars on its long journey. The spacecraft will orbit Vesta for about nine months, and Ceres for at least five months. The mission is scheduled to end in July 2015.
Visiting two objects and orbiting both is a capability that is only now becoming possible through the development of solar-electric engines.
Unlike the chemical rockets of yesteryear which gave probes a short, sharp impulse, the new generation of ion thrusters provide gentle but sustained propulsion. They work by ejecting charged atoms (ions) of xenon.
They take time to build up speed - hence Dawn's long mission profile - but they are very efficient and flexible, allowing spacecraft to hop from target to target.
(Excerpt) Read more at news.bbc.co.uk ...
Rocket engines work by pushing propellant away from the spacecraft. The action of the propellant leaving the engine causes a reaction that pushes the spacecraft in the opposite direction. This is what causes a balloon to rush around as the air is allowed to escape; the air pushes on the balloon as it leaves.
An ion engine uses this same principle, but the great innovation is in how efficiently this happens. The gas xenon (which is like helium or neon, but heavier) flows into the ion engine, where it is given an electrical charge. Charged atoms are called ions. As soon as the xenon atoms become xenon ions, they can be pushed around by an electrical voltage. A pair of grids in the ion engine, electrified to almost 1300 volts, accelerates the ions to very high speed and shoots them out of the engine. As the ions race away from the engine, they push back on the spacecraft, propelling it in the opposite direction. [The electricity for this remarkable system can be provided by solar arrays, as on Deep Space 1 and Dawn, or by a nuclear power system, as on Project Prometheus. The principle of operation of the ion engine is the same.]
The xenon ions travel at about 35 kilometers/second (77,000 miles/hour). This is about 10 times faster than the exhaust from conventional rocket engines, so the xenon gives about 10 times as much of a push to the spacecraft as chemical propellants do. That means that it takes only one tenth as much propellant for an ion engine to work as it does for a chemical propulsion system. To accomplish some of the more ambitious and exciting missions in the solar system, we simply cannot build and launch spacecraft large enough to carry the chemical propellants that would be needed for the mission. Ion propulsion is one of the ways to get around this problem.
Now the ion engines use only a very small amount of xenon at a time. That means that the thrust is very very low. If you rest a piece of paper on your hand, the paper pushes on your hand about as hard as the ion engine pushes on the spacecraft! It may take 4 days or more just to use up 1 kilogram (about 2 pounds) of xenon. Unlike chemical engines, which can be operated for minutes, or in extreme cases, for an hour or so, ion engines can be operated for years. The effect of the gentle thrust slowly builds up, eventually attaining speeds far beyond the reach of conventional propellants.
Star Wars!!
The Empire used Twin Ion Engines on their fighters. (Hence the TIE fighter.)
I never knew that.
Interesting how different concepts in science fiction end up becoming reality years later.
Except these ion engines are incredibly slow.
.....The asteroids are thought to be the leftovers after the planets were made....
Many think the asteroids are currently being pulled in and will be part of planets some day. Hence they are not left over. They are currently bein gusd but the time scale is very long.
It was a beautiful launch - pulled over on the way to work to get a look at it. Sunrise lit up the plume and the skies were clear.
Dawn has three ion engines that will provide a gentle yet increasingly accelerating thrust. Electrons will bombard Dawn’s modest supply of xenon gas, and the resulting ions will shoot out into space, nudging the spacecraft along.
Even “Star Wars” had only twin ion engines with its T.I.E. Fighters, [chief engineer Marc] Rayman noted with a smile earlier in the week.
“PREPARE FOR LUDICROUS SPEED!”
Isn’t it those launches just before dawn or after sundown that show the spectacular effect of huge, glowing exhaust clouds in the upper atmosphere lit up by the setting/rising sun while the earth below is in darkness?
Yes, I watched one just at sunset a few years back where the plume started in the shadows of the dark, but extended up into the bright sunlight as it ascened. I just happened to be on a causeway and had a great view.
ascened=ascended
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