Wednesday, July 13, 2011

Scientist At A Hamburger Stand

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass ("go") between electrodes in a solution, when an electric field is applied. It is from Greek ιον, meaning "going."

Dawn is a robotic spacecraft sent by NASA on a space exploration mission to the two most massive members of the asteroid belt: Vesta and the dwarf planet Ceres. Launched on September 27, 2007, Dawn is scheduled to reach Vesta on 16 Jul 2011, which it will then explore until 2012. It is scheduled to reach Ceres in 2015. It will be the first spacecraft to visit either body.

Dawn is innovative in that it will be the first spacecraft to enter into orbit around a celestial body, study it, and then re-embark under powered flight to proceed to a second target. All previous multi-target study missions—such as the Voyager program—have involved rapid planetary flybys.

A total of three ion propulsion engines are required to provide enough thruster lifetime to complete the mission and still have adequate reserve. However, only one thruster will be operating at any given time. Dawn will use ion propulsion for years at a time, with interruptions of only a few hours each week to turn to point its antenna to Earth. Total thrust time through the mission will be about 2,100 days, considerably in excess of Deep Space 1's 678 days of ion propulsion operation.

The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 10 times that of chemical engines. The electrical level and xenon fuel feed can be adjusted to throttle each engine up or down. The engines are thrifty with fuel, using only about 3.25 milligrams of xenon per second (about 10 ounces over 24 hours) at maximum thrust. The Dawn spacecraft carries 425 kilograms (937 pounds) of xenon propellant.

At maximum thrust, each engine produces a total of 91 millinewtons -- about the amount of force involved in holding a single piece of notebook paper in your hand. You would not want to use ion propulsion to get on a freeway -- at maximum throttle, it would take Dawn's system four days to accelerate from 0 to 60 miles per hour.

As slight as that might seem, over the course of the mission the total change in velocity from ion propulsion will be comparable to the push provided by the Delta II rocket that carried it into space -- all nine solid-fuel boosters, plus the Delta's first, second and third stages. This is because the ion propulsion system will operate for thousands of days, instead of the minutes during which the Delta performs.

Well, she's got her daddy's car
And she cruises to the hamburger stand, now
Seems she forgot all about the library
Like she told her old man, now
And with the radio blasting
Goes cruising just as fast as she can, now

And she'll have fun, fun, fun
Til her daddy takes the T-bird away

The Dawn spacecraft goes from-zero-to-sixty
in four days and cruised to the asteroid belt
to do science, playing the cards it was dealt,
looking at rocks to see what there is to see.

If the Dawn spacecraft could nix tranquility,
toss away its cards for something more heartfelt
than watching, maybe, ice on a big rock melt,
should it spark off to play in complexity?

A witch could summon storms, though they might not come,
an alchemist transmute lead to gold, or not.
Both kept watch, too, on the planets, Moon and Sun.

There’s an engine. Some hear it rumble and hum.
There’s a library, and there’s another spot—
hamburgers, donuts. What if Dawn turned to fun?

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Dawn Spacecraft at JPL


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