Subscribe free to our newsletters via your
. 24/7 Space News .




IRON AND ICE
Dawn On A Smooth And Steady Course
by Dr. Marc D. Rayman
Pasadena CA (JPL) Dec 06, 2010


File image

Dawn is maintaining its smooth and steady course through the solar system as it gradually closes in on Vesta. With the utmost patience and persistence, it continues thrusting with its ion propulsion system, heading toward its July rendezvous with the second most massive member of the main asteroid belt.

Even as the spacecraft climbs farther from the sun, Earth's orbit is beginning to bring the planet closer to the probe.

Having thrust for two-thirds of its time in space, Dawn has now achieved the velocity equivalent of about 5.5 kilometers per second (more than 12,000 miles per hour). We have seen before that this does not represent the actual change in speed, but it is still a very useful measure of the effect of the thrusting.

Although it has long since surpassed the record for propulsive change in velocity, Dawn is only now at the halfway point in the planned profile of thrusting for its ambitious eight-year expedition. By the time it completes its mission at dwarf planet Ceres (the asteroid belt's most massive resident) in 2015, it will have accomplished twice the effective velocity change it has achieved so far.

At the beginning of this month, mission controllers installed new parameters in the software used to control the spacecraft's orientation (which engineers refer to as "attitude") in the zero-gravity conditions of spaceflight. The attitude control system has four methods of keeping Dawn stable or turning to point in a new direction.

When the spacecraft is not thrusting with the ion propulsion system, it has two techniques: it can rely on its reaction wheels or on its reaction control system.

The wheels are gyroscope-like devices which, when electrically spun faster or slower, rotate (or stop the rotation of) the spacecraft. As we saw early last year, the reaction wheels alone are not sufficient, so when they are in operation, the reaction control jets also are used, although only occasionally. (The jets also are known as "thrusters," nomenclature used in previous logs, but to avoid confusion with the ion thrusters in this discussion, we will refer to them only as "jets.")

The conventional rocket propellant hydrazine is fired through the jets to impart a small force to the ship, causing it to turn or to stop turning.

Not content simply to coast through the solar system, as most interplanetary probes do, Dawn devotes the majority of the time to using one of its ion thrusters to change its course, constantly applying a light pressure to its orbit to bring it closer and closer to that of its destination.

In addition to accelerating the ship, an ion thruster can be used to rotate it by slightly changing the angle of the thrust. This provides the attitude control system two other means of control when the spacecraft is in powered flight: it can use either the thruster plus the wheels (again with the occasional help of the reaction control system) or the thruster plus the more frequent use of the jets.

Over the summer, engineers powered the reaction wheels off, preserving them for use in orbit around Vesta and Ceres. The reaction control system took over quite smoothly and has been keeping the craft stable ever since, most of the time in concert with the ion propulsion system.

Even before deciding to deactivate the wheels for the rest of the interplanetary phases of the mission, engineers began working on a technique to use the hydrazine more efficiently, ensuring that the supply would last to the end of the long journey. When it left Earth more than three years ago, Dawn carried 45.6 kilograms (101 pounds) of hydrazine.

It still has more than 38 kilograms (nearly 84 pounds) onboard, and frugal operators want to continue to use the precious resource sparingly. They devised a means to reduce the rate of propellant consumption when the reaction control system is the primary control method. (The expenditure of hydrazine during ion thrusting was already so low that there was no need for an improvement in that control mode.)

The new control parameters were finalized after extensive analysis and simulation at Orbital Sciences Corporation and JPL. Without needing to change the software, operators radioed the values to Dawn in October and timed them to go into effect on Nov. 1 during the normal weekly hiatus in thrusting (and thus when attitude control is reliant exclusively on the jets) to point the main antenna to Earth.

Thanks to this successful upgrade, the system now uses only about one-eighth as much hydrazine during those periods that it is holding steady and not applying ion thrust.

To rotate from one attitude to another requires firing some jets to start the huge ship (the largest NASA has ever sent on an interplanetary voyage) turning and then others to stop it. To achieve a further savings in the hydrazine, controllers reduced the rate at which Dawn executes its turns.

The standard speed had been a whiplash-inducing 0.1 degrees per second; that's the same pace at which the minute hand of a clock moves (except for some of the clocks sold in Dawn gift shops, and we're still processing your refund requests on those). On Nov. 1, the speed was lowered to half that, meaning less propellant is needed to initiate a rotation and less is needed to terminate it.

The only turns in a typical week are those required to shift between the attitude required for ion thrusting and the attitude required for pointing the main antenna to Earth, and a little extra time spent turning is easily affordable.

On Nov. 8, all four reaction wheels were powered on for a short time. For wheels 1, 2, and 3, this served as routine maintenance, keeping them in top condition so they will be ready to return to duty as the Vesta phase of the mission begins next summer. Running wheel 4 provided additional data on its condition so that engineers could assess its long-term prospects.

Dawn is 0.083 AU (12 million kilometers or 7.7 million miles) from Vesta, its next destination.

It is also 3.05 AU (456 million kilometers or 283 million miles) from Earth, or 1235 times as far as the moon and 3.09 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

.


Related Links
Dawn
Asteroid and Comet Mission News, Science and Technology






Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News





IRON AND ICE
Hayabusa's Harvest
Moffett Field CA (SPX) Nov 22, 2010
The Japan Aerospace Exploration Agency (JAXA) has confirmed that the tiny particles inside the Hayabusa spacecraft's sample return container are in fact from the asteroid Itokawa. Scientists examined the particles to determine if the probe successfully captured and brought back anything from the asteroid, and in a press release said "about 1,500 grains were identified as rocky particles, a ... read more


IRON AND ICE
Robotic Excavations Could Help Get Helium 3 From Moon To Earth

A Softer Landing on the Moon

Neptec Wins Canadian Space Agency Contract To Develop A New Generation Of Lunar Rovers

Mission to far side of moon proposed

IRON AND ICE
Drilling For The Future Of Science

Opportunity Imaging Small Craters On Way To Endeavour

Opportunity Making Progress To Endeavour Crater

Spain Supplies Weather Station For Next Mars Rover

IRON AND ICE
SwRI Researchers Continue Starfighters Suborbital Space Flight Training

X-37B Orbital Test Vehicle Completes First Flight

Website Hosts Space Transcripts

Roscosmos And NASA To Seal Deal On Joint Projects

IRON AND ICE
China Builds Theme Park In Spaceport

Tiangong Space Station Plans Progessing

China-Made Satellite Keeps Remote Areas In Venezuela Connected

Optis Software To Optimize Chinese Satellite Design

IRON AND ICE
NASA Seeks Nonprofit To Manage ISS National Lab Research

Expedition 25 Returns Home

Crews approved for space station mission

Soyuz crew land safely on earth from ISS

IRON AND ICE
ISRO Hands Two Contracts To Arianespace

US company readies first space capsule launch

Kazakh Space Agency Seeks Extra Funding For New Baikonur Launch Pad

Aerojet Propulsion Raises Japan's First Quasi-Zenith Satellite MICHIBIKI

IRON AND ICE
Super-Earth Has An Atmosphere, But Is It Steamy Or Gassy

First Super-Earth Atmosphere Analyzed

Super Earth Could Be Steaming Hot Or Full Of Gas

500th 'extrasolar' planet discovered

IRON AND ICE
Video games get kids to eat more veg, fruit: study

Cell phone exposure linked to bad behavior in kids: study

Next-Gen Earth Imaging Satellite Advances To Critical Design Review Phase

Google unveils new smartphone, the Nexus S




The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement