Dawn approaches Ceres, From opnav images to first orbit Options

[Phil Stooke]

On Tuesday (two days from now, for visitors from the future), the first optical navigation image will be taken... hopefully we'll have it in our hands soon after that. So it's time for a new topic. Over the next few months we'll have progressively closer images and full orbit characterization sequences, no doubt including multispectral image sets.

A new world...

This is a bit of reprocessing I have been doing with the Hubble images from a few years ago.




Phil

[nprev]

Paul, you may know this right off the bat: Where does Ceres rate on the PPP scale? Is it on par with Mars, Europa & Titan, rated somewhat lower, or not even on the radar at this time?

Not directly applicable to Dawn, of course, save that it might be a factor in EOM planning.

[Tom Tamlyn]

* * * Where does Ceres rate on the PPP scale? Is it on par with Mars, Europa & Titan, rated somewhat lower, or not even on the radar at this time?

Not directly applicable to Dawn, of course, save that it might be a factor in EOM planning.

Once google helped me to remember what PPP stood for , a couple of interesting items turned up with a quick additional search. A summary of planetary protection categories is here.

Io and asteroids of the "[u]ndifferentiated, metamorphosed kind" are in Category I, the category of least concern. Other asteroids are in Category II, along with the Earth's Moon, Jupiter, and a host of other bodies.

Titan is in Category II* (an intermediate category for icy satellites) for all types of missions. Mars and Europa are in Category III for flybys and orbiters, Category IV for landers and probes.

A NASA planetary protection powerpoint from November 2014 lists Dawn at Ceres as Category II (a body of “significant interest * * * where there is only a remote chance that contamination carried by a spacecraft could compromise future investigations”), based on a determination that "Dawn will not impact Ceres due to orbital mechanics constraints." The 2009 flyby of Mars, by contrast is listed as Category III.

A powerpoint specific to the Dawn Mission from January 2013 is a little confusing to the lay reader; a specific category isn't assigned with respect to Ceres. On a page that characterizes the Mars flyby as a Category III mission, it notes that the mission was required to demonstrate “Orbital lifetime around Ceres of greater than 20 years post-orbital insertion."

This restriction is subject to change, based on assessment of... let's just call it conditions on Ceres as discovered by the mission. Based on what the mission finds, it could apply for a release from the 20 year orbit requirement. On the other hand, the mission might (the presentation specifically mentions observations of recent melting as relevant here) instead be required to demonstrate (i) a stable orbital lifetime for an additional 20 year period, (ii) a "long-term stable orbit around Ceres" ["long term" undefined] or (iii) a plan to "leave Ceres orbit."

So the answer to your question seems to be "Category II," subject to change.

In the course of researching this post, I found the November 2013 minutes of the planetary protection subcommittee of the NASA Advisory Council. I'm confused by the highlighted words in the following sentence at p.7: "If the Dawn mission successfully enters orbit around Ceres, it will be going so fast that impact avoidance is assured." I can't figure out what "so fast" has to do with Dawn's method of gradually slipping into a stable orbit.

Can anyone interpret that, or is it just a (perfectly understandable) instance of an over-worked minute taker writing something down that's a little confused?

Edited for a typo, to fix a quote, and to avoid potential issue with forum guidelines.

This post has been edited by Tom Tamlyn: Jan 22 2015, 07:36 AM