Dawn Cruise Options

[dmuller]

On the question of the orbit always being circular, I didn't mean to imply that I thought that was the case with Dawn -- it's that it seemed to me that that would be the optimal way to use a low-thrust engine, and I'm curious why that's not the case in practice. Is it solely to make the Mars flyby work?

Greg, if you have the Orbiter simulator (http://www.orbitersim.com) you can simulate this in Earth orbit. Get a scenario where your ship is docked to the ISS, set your Orbit display to PRJ ship and DST altitude (will look more or less like a circle), undock, hit "prograde", get your engines on the lowest thrust setting (i.e. press and hold "Ctrl" and then literally hit your "+" button on the Numpad as short as possible), speed up the simulation to 100x (unless you have a lot of time - but dont go over 100x), and watch the PeA (perigee altitude) and ApA (apogee altitude) increase ... everytime you nearly get a circle (circle: PeA = ApA), youl'll find the ApA running away, half an orbit later the PeA starts to catch up.

[siravan]

Regarding ion engines and circular orbits, one may make a qualitative non-mathematical argument why that is the case. We all know that for an elliptical orbit, firing engines at periapsis modifies the apoapsis (rising or dropping the apoapsis depending on the direction of the thrust), whereas firing the engines at apoapsis changes the periapsis. Now, for the case of a continuous low-thrust ion engine, it affects both apoapsis and periapsis. Let assume the case where both increase (as in Dawn). The spacecraft spends more time in the vicinity of the apoapsis that periapsis, as it is slower farther in its orbit from the center. Therefore the effect on periapsis is larger than the periapsis. The net effect is that with continuous thrusting, periapsis starts to catch up with apoapsis, i.e. the orbit becomes more circular.

[dmuller]

BTW, whilst getting the Dawn data from the Horizons system, I noticed the following description of the propulsion system. I particularly love the bold part, leaves every high-performance car lover drooling!

PROPULSION
Dawn uses 3 ion thrusters to reach Vesta once separated from the Delta II. It
will use the thrusters to spiral to lower altitudes on Vesta, leave Vesta,
cruise to Ceres, and spiral to a low altitude orbit at Ceres.

Weight : 8.9 kg each
Dimensions : 33 cm long, 41 cm diameter
Specific impulse : 3100 s
Thrust : 19-91 mN
Acceleration : 0-60 mph in 4 days
Operational time : 2000 days of thrust (entire mission)

[dmuller]

Since there is much talk about the Dawn cruise, I have finally uploaded the Dawn Realtime simulation onto my website at http://www.dmuller.net/dawn

That's been 4 posts to this thread in 24 hours ... I'll give it a break now for a while

[Greg Hullender]

Erm...I don't think so. I would call it "outbound" since Dawn approaches the flyby from within the orbit of Mars (assuming the link I gave shows you the same as it shows me!) I only comment since I spent quite some time trying to figure out the flyby trajectory for the earlier June launch window (as an "interested layman" who finds these things fascinating )

I see it now. The graphic is a bit confusing because sometimes the dotted line is Dawn's trajectory and other times it's a planetary orbit. I was seeing the solid line as Mars' orbit -- but it's actually Dawn.

--Greg

[Toma B]

I still haven't found what I was looking for....can anybody help me?
Will there be any "non-targeted" flyby of any known asteroid or comet on the way to Vesta or Ceres?

[Phil Stooke]

Earlier discussions of this mission, before launch, suggested there could be several other flybys, but now they are not emphasized. Saving fuel is probably the big reason.

Phil

[kwp]

Earlier discussions of this mission, before launch, suggested there could be several other flybys, but now they are not emphasized.

I asked a mission specialist a few weeks ago and was told that, sadly (my word, not hers), no en route flybys are planned.

-Kevin

[Marz]

I asked a mission specialist a few weeks ago and was told that, sadly (my word, not hers), no en route flybys are planned.

-Kevin

The Mars gravity assist is coming up in a few months (Feb 09). Are any observations or kodak-moments planned?

Only 2.8 more years until Vesta is reached!

[djellison]

Yes - an extensive campaign is planned for Dawn at the mars flyby

http://cosis.net/abstracts/EPSC2008/00442/...8-A-00442-1.pdf

One extra detail I discovered at Europlanet is a colour movie of a full mars day on the departure leg.

Doug

[ugordan]

One extra detail I discovered at Europlanet is a colour movie of a full mars day on the departure leg.

That will make for a great companion to MESSENGER's Earth departure movie!

[tedstryk]

That will make for a great companion to MESSENGER's Earth departure movie!

It will also be fun to compare with the Rosetta images (given the recent flurry of activity, I am guessing that the psa release will be soon).

[Greg Hullender]

Someone apparently asked the Dawn team our earlier question about Dawn's "inbound" gravitational assist, and the Dawn people actually posted a very nice response in their FAQ section.

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.

This certainly answered all of my questions! I don't usually look at the FAQ sections for space missions because most of them are too elementary to interest me, but this was a welcome exception.

Their whole FAQ list is here: http://dawn.jpl.nasa.gov/mission/faqs.asp

--Greg

[peter59]

27 November 2008 - 1001 days to Dawn's arrival at Vesta.

[Juramike]

"We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty."

-Dr. Marc D. Rayman, Dawn Journal entry for November 30, 2008 posted in the Planetary Society website : http://www.planetary.org/blog/article/00001755/