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Where is Dawn?

Frequently asked questions:

Question: I see that a gravitational trajectory assist is scheduled for February 2009 (angular momentum transfer) with Mars. Normally, outward-bound probes pass by the planet while closer to the sun than the planet. The Dawn probe seems to be further away from the sun than Mars, so it would be traveling faster than Mars before the transfer. Wouldn't that slow the probe down, instead of speeding it up with respect to the sun? A more succinct question would be: How much delta V are you expecting from the Mars encounter (heliocentric velocity).
Answer: There is a wide range of geometries that can make planetary gravity assists effective, and while approaching from outside the orbit of the planet may appear unusual, Dawn is not unique in doing so. The specifics of the gravity assist include not only the relative speed between the probe and the planet but also the direction each one is moving at the time of the encounter.

In our case, the principal benefit of the gravity assist is to change the plane of Dawn's orbit around the Sun. Based on your choice of words, you seem to have some understanding already of the key principles, so you probably already know that most planets orbit the Sun close to the plane of Earth's orbit, also known as the ecliptic. You may also know that changing the plane of an orbit can be energetically very expensive. Vesta and Ceres orbit farther from the ecliptic than most planets do.

If we had launched in 2006, the ion propulsion system could have achieved the plane change by itself. The mission is a little more difficult with a 2007 launch, because there is less time to complete the required ion thrusting before the relative alignment of Vesta and Ceres makes the trip between them inconveniently long. Therefore, we take advantage of the gravity of Mars to change Dawn's inclination. The delta-v is about 1.1 km/s, providing a plane change of less than 5°, but that significantly reduces the time Dawn needs to thrust, allowing it to reach Vesta at about the same time, even after launching a year later.

Question: How did the Dawn Mission get it's name?
Answer: The mission was not named after a person, but rather it was so named to present a simple image of the purpose of the mission: to provide information on the dawn of the solar system.

Questions by Category
More frequently asked questions about the Dawn mission are grouped below by category. Please select a question to find out the answer. (Tip: To temporarily allow a site to display pop-ups, click the Information Bar when it notifies you that a pop-up has been blocked. Then, click Temporarily Allow Pop-ups.)

MissionCMISSION
Question M1: What is the reason for going to Vesta before Ceres?
Question M2: Will there be opportunities to visit other asteroids, either en route to Ceres or as part of an extended mission?
Question M3: Why does it take so long to get to Vesta and then from Vesta to Ceres? The travel time scheduled appears to be significantly longer than would be required by a Hohmann transfer orbit.

IIon PropulsionCON PROPULSION
Question IP1: In the broadest sense, how does ion propulsion work?
Question IP2: Why did the engineers choose Xenon to propel Dawn's ion engines?
Question IP 3: What makes ion propulsion advantageous for the Dawn mission?
+ See additional NASA FAQs about Ion Propulsion

Answers provided by Chris Russell - Dawn Principal Investigator, Lucy McFadden - E/PO Director, Joe Wise - E/PO Manager, and Marc Rayman - Dawn Project System Engineer.

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