In comparison to the twin Voyagers, Dawn's flight path circled the sun in an anti-clockwise manner. The map of its path shows its targets, Ceres and Vesta were in a trajectory that was close to a straight path, if Dawn had taken a clockwise direction, with a close flyby of Mars. I can't find a reason to show it was studying the space around Lagrange point 3 (in relation to Earth), as according to this flight path map. So what was the reason for a longer direction?

Dawn's flight path

  • $\begingroup$ Where do you see it studying the L3 point? After leaving earth Dawn never crossed the earth obit again. Specifically not the point opposite to Earth. $\endgroup$ – SpaceBread Oct 15 '19 at 13:54
  • $\begingroup$ You mention the comparison to the Voyagers. There are two big differences between them and Dawn: (1) their rockets did (almost) all the acceleration at the start of their flights. Dawn's thrusters work very slowly for a very long time, slowly changing its direction (2) they were in the outer solar system where the Sun's gravity is much weaker, so things move in something more like a straight line. $\endgroup$ – Steve Linton Oct 16 '19 at 10:42
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    $\begingroup$ I'm voting to close this question as off-topic because It really belongs on space travel, not astronomy. If someone has the knowledge/privileges to migrate it, that would be helpful. $\endgroup$ – Steve Linton Oct 16 '19 at 10:48

Dawn was not exploring L3. But instead it explored both Vesta and Ceres. In order to do so in a cost-saving manner, Dawn was also a mission demonstrator for an interplanetary mission equipped with ion propulsion.

Because of the low specific impulse of ion propulsion, Dawn cannot fly in a straight line, as this would imply a high initial velocity and the spacecraft couldn't deaccelerate in time. Therefore, Dawn started with low $v_0$ at Earth, and raised its orbit slowly and continuously, creating the spiral you see.


The Earth orbits the Sun in a counter-clockwise direction. Covering $2 \pi \times 150\times 10^6$ million kilometers in $365.25 \times 24 \times 3600$ seconds, or 29.9 km/sec.

If you wanted to leave the Earth in a clockwise heliocentric orbit you'd have to accelerate Dawn by an extra 60 km/sec to do that, which is unfeasible with current rockets.

Likewise approaching Ceres you'd also be going the wrong direction to try to enter into orbit, you'd face the problem of Ceres moving counterclockwise at 17.9 km/sec while Dawn would be moving roughly the same speed clockwise, so another 35.8 kilometers per second relative motion that you would have to "dispose of" somehow.

As this answer points out, Dawn used electric propulsion which can only slowly change velocity. If you are in a heliocentric orbit and tried to spend many years decelerating to a dead stop and slowly re-accelerating the other direction, in the mean time you'd fall towards (but probably not into) the Sun, if it were possible at all (it would probably run out of propellant).

Dawn was not built to withstand a close pass with the Sun, and so it would not have survived.

Launches often try to use every last bit of "free" velocity from Earth's motion, not only by the deep-space launches orbiting the Sun in a prograde direction (the same direction the Earth and all other planets rotate) but by first launching into orbits around the Earth in a prograde direction, taking advantage of the Earths rotational velocity around its axis.

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    $\begingroup$ also, the image represents the asteroids and planets at the time DAWN gets to Ceres (It appears to be a screenshot of this video vimeo.com/117835245 ). A spacecraft taking the OP's proposed clockwise route while the worlds are in or near the shown position would would probably need to be moving far faster than New Horizons did, and would have maybe a few hours to observe Vesta instead of 14 months. $\endgroup$ – notovny Oct 16 '19 at 2:48
  • $\begingroup$ @notovny you can probably expand that into an additional answer! If not, please feel free to edit this one and include that. $\endgroup$ – uhoh Oct 16 '19 at 6:41

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