Dawn Spacecraft Successfully Launched

[krispos42]

Source: National Aeronautics and Space Administration

CAPE CANAVERAL, Fla. - NASA's Dawn spacecraft began its 3 billion kilometer (1.7 billion mile) journey through the inner solar system to study a pair of asteroids Thursday at 7:34 a.m. Eastern Time (4:34 a.m. Pacific Time).

The Delta 2 rocket, fitted with nine strap-on solid-fuel boosters, safely climbed away from the Florida coastline and launch complex 17B at the Cape Canaveral Air Force Station. "We have our time machine up and flying," said Dawn Principal Investigator Christopher Russell of the University of California, Los Angeles.

Dawn is scheduled to begin its exploration of Vesta in 2011 and Ceres in 2015. The two icons of the asteroid belt are located in orbit between Mars and Jupiter and have been witness to so much of our solar system's history.

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"The Dawn spacecraft launched successfully from the launch complex at Cape Canaveral Air Force Station, Fla. Image credit: NASA TV"


Read more: http://www.nasa.gov/mission_pages/dawn/news/dawn-20070927.html

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The mission home page is here:

http://www.nasa.gov/mission_pages/dawn/main/index.html


NASA is calling it the "Prius of space" because of its ultra-efficient ion engine for long-term thrust.

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At first glance, Dawn's full throttle, pedal-to-the-metal, performance is a not-so-inspiring 0-to-60 mph in 4 days. But consider this - because of its incredible efficiency, it expends only 40 ounces of xenon propellant during that time. And then take into consideration that after those four days of full-throttle thrusting, it will do another four days - and then another four. By the end of 12 days the spacecraft will have increased its velocity by over 180 miles per hour, with more days and weeks and months of continuous thrusting to come. After a year Dawn's ion propulsion system will have increased the spacecraft's speed by 5,500 mph while consuming the equivalent of only 15 gallons of fuel. By the end of its mission Dawn will have accumulated more than 5 years of total thrust time, giving it an effective change in speed of about 23,000 mph.

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http://www.nasa.gov/mission_pages/dawn/news/dawn-20070913f.html

[BobRossi]

1. Ah yes, my hard earned at work...

What a waste, there are a pair of astroids in the White House that would cost much less to study.

[trusty elf]

2. Disasteroid

[sakabatou]

3. LMAO

[Thor_MN]

9. Many Asters are blue, purple or white. Blood (heme) is red, thus a pair of Hemorrhoids in the WH

is more correct....

[Lionel Mandrake]

4. The spacecraft has three ion engines, but uses only one at a time?

It says that somewhere on the NASA web site.

Why don't they use all three engines together to triple the thrust and get to the asteroids sooner?

[krispos42]

5. I missed that part...

but I would assume it's a function of how much power they can pull through the solar panels to accelerate the xenon. Plus redundancy, of course.

[Lionel Mandrake]

8. I agree.

Another NASA web page states:

"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."

Here's the link: http://www.nasa.gov/mission_pages/dawn/spacecraft/index.html

I suspect you are right about electrical power being the limiting factor.

[krispos42]

10. And it will only get worst the further out it gets

It's an inverse-squared thing, so twice as far is 1/4th the watts per square meter. Even if they could run all three initially, Ceres is about 3 AU away, which cuts solar intensity to 1/9th that of Earth.

At least it's close enough to use solar cells instead of plutonium batteries.

[Lionel Mandrake]

11. Yes, solar cells are inappropriate for trips to the outer planets.

For a while, NASA was contemplating a mission to Pluto using solar cells. That spacecraft design was truly ridiculous.

[krispos42]

12. I didn't know we could make solar panes the size of Rhode Island

:rofl:

[Solon]

6. Ion engines don't work that way...

They are a low but constant acceleration type of propulsion, with a high fuel efficiency. Whether its one engine or 3 going off at the same time, the increase in acceleration wouldn't be worth the extra energy expenditure. With standard chemical rockets, acceleration is everything, especially to counteract Earth's gravitational field. Once in a micro-gravity environment, and attaining orbital speed, such rockets are used in short bursts, because they are, to put it one way, gas guzzlers, and then the spacecraft "coasts" to whatever destination its intended to go, using, possibly, gravitational boosts from nearby celestial bodies to help it along.

Ion engines are different, they don't work in atmosphere, so are useless for taking off from the surface of Earth, however, once in a micro gravity environment, in the near perfect vacuum of space, the modest propellant(usually a neutral gas, such as Xenon), only thrusts at about the same force it take for you to pick up a piece of paper. However, that force is constant, the thruster shoots out the propellant for weeks, months, even years, this leads to constant acceleration. While the acceleration is modest, it adds up over time, and can lead to having spacecraft that get to their destination much faster than those solely propelled by chemical rocket engines.

[Lionel Mandrake]

7. Yes and no.

Yes, I know how ion engines work and what their advantages and disadvantages are compared to chemical rockets.

Your statement that "whether its one engine or 3 going off at the same time, the increase in acceleration wouldn't be worth the extra energy expenditure" is incorrect. There is no extra energy expenditure for a given delta V. There is a requirement for extra power, but the power would only be required for one third as much time. Hence the energy is the same. The difference is that you get to your destination sooner.

I agree with your statement that "while the acceleration is modest, it adds up over time, and can lead to having spacecraft that get to their destination much faster than those solely propelled by chemical rocket engines". That would be the case if the acceleration were somewhat less modest than it is here. Ion engines use a lot of electrical power, and I suspect that this spacecraft is underpowered, despite the huge solar array. I believe RTGs would have been a better choice for electrical power. A nuclear reactor would be better yet, but NASA doesn't have one on the shelf as far as I know.