Comments on: MESSENGER update

By: Sloan

Sloan — Tue, 15 Jan 2008 17:43:11 +0000

Lugosi on 14 Jan 2008 at 6:12 pm:
“I heard a rumor that Messenger was actually supposed to probe Venus…”

*snicker*

By: Gordan

Gordan — Tue, 15 Jan 2008 17:25:19 +0000

I’m only asking this because even calibrated imagery (say one that gives a white Enceladus in RGB) doesn’t produce Saturn looking like all those releases (or VIMS “proper” CIE XYZ output), but rather the blue channel ends up too dim. Same for Titan. In the end some fudging/guessing/channel mixing is probably inevitable, be it with perfectly calibrated imagery or not.

But, I digress… As I write this, MESSENGER ought to be about an hour into its data playback. Tentatively, some images might appear as early as tonight (EST), though more likely tomorrow.

By: John W

John W — Tue, 15 Jan 2008 17:10:48 +0000

Yep, that’s me. I’m not directly involved in producing public images, but I work with the guys who do it. The ground-based calibrations are used, but unfortunately they’re only so reliable since things change over the course of the launch and cruise. So star calibrations are used somewhat, but especially calibrations based off of moons. (We can compare them to HST photometry at very low phase.) It’s a pain in the tail, though, because there’s ambiguity on how to weight the various data sets. The result is that each filter has an error associate with its calibration. If you take extreme errors in the various filters, the color changes are noticeable to the human eye. As a result, it isn’t entirely automated.

By: Gordan

Gordan — Tue, 15 Jan 2008 16:39:09 +0000

“We sincerely try our best on Cassini”

OT, John, but are you working for CICLOPS? (John Weiss, perhaps?). If so, I was always interested in the way you generate color images – was it using groundbased visual spectra of Saturn or did you “borrow” detailed spectra VIMS has to offer? I notice VIMS color results look pretty close to ISS color shots in most cases (apart from “blue cranium” that is).

By: John W

John W — Tue, 15 Jan 2008 16:31:33 +0000

Note, however, that relativistic effects on Mercury amount to 43 arcseconds per century. It’s pretty insignificant over a few years, especially once you’re in orbit around Mercury. I’d bet that the errors in the trajectory due to measurement errors on position/velocity and due to anomalous forces on the spacecraft overwhelm this. That said, I’m think that JPL nav. teams do actually take relativity into account (I know some scientists have to in order to get sufficiently accurate location information on spacecraft). I could ask, but I wouldn’t have an answer very quickly.

As for color of Mercury, I don’t think that there have been a lot of multi-filter images taken, so the gray you usually see is probably just because it was a black-and-white image. On the other hand, Mercury is a lot like the Moon, as far as I know, so gray is quite possibly the dominant color. Sadly, getting accurate color from spacecraft is crazy difficult. We sincerely try our best on Cassini, but it’s amazing how much uncertainty there really is.

By: Gordan

Gordan — Tue, 15 Jan 2008 16:28:02 +0000

Mark Martin: “It’s cheaper to just do the GR math.”

I don’t believe they use GR in trajectory mechanics, Newtonian mechanics still serves perfectly for this purpose. Relativistic effects at Mercury’s distance are far, far smaller than other undesired perturbations such as solar light pressure or Mercury mascons (once in orbit, of course).

GR *may* be taken in account when measuring Doppler shift for precise ranging, but IIRC all trajectory design is still done via Newtonian equations. There really is no point in GR, it would simply be overkill with all the other unmodelable perturbations.

By: aiabx

aiabx — Tue, 15 Jan 2008 15:03:16 +0000

Don’t worry Lugosi, I got your joke, and responded with the laughter it deserved.