I'm back from the Europa Focus Group meeting... Options

[Guest_BruceMoomaw_*]

...which I decided to attend literally at the last possible minute, which is why I didn't alert you guys in advance. Very interesting -- both the discussions about the likely design of the mission (and how to retrieve it from cancellation), and many of the actual science presentations (which aren't on the Web yet, although they probably soon will be). I'll give you some more information tomorrow -- although I can't resist telling Alex that Tom Spilker's subgroup took my ideas about a Europa penetrator, and the printed information I gave them on the subject, seriously enough to recommend making further inquiries to NASA HQ on it. (And without my browbeating them, either. Nyaah.) The case for it, however, is still extremely far from certain.

As I say, more tomorrow.

[Guest_BruceMoomaw_*]

The tether idea, alas, would work about as well as one of Wile E.'s schemes (but would be a lot more costly -- after all, he always survives somehow). The moment the lower end of the tether brushes the surface, the penetrator gets flung downwards -- in an entirely uncontrolled way -- on the top end.

As for the Bowling Ball's instrumentation: it would unfold nothing. What it would have on its surface is a number of little camera ports and patch antennas, and about half a dozen little sharp-edged "sample cups" that could scoop up surface samples as a result of the impact itself -- after which electrodes in the cups would analyze the ice's pH, salinity and redox content, and a fiber-optic lead from the cups to an onboard Raman spectrometer would provide further composition analysis. Add an onboard short-lived seismometer or geophone, and that's the total instrument payload of the current model (although an active pulse generator might allow the geophone to double as a simple sonar system to help measure the rock content of the ice, as Spilker thinks is wise).

I'm starting to think that the biggest problem with a penetrator might be not its impact angle, but its remaining horizontal drift on landing -- which a broad surface-contact foreplate couldn't correct on impact, especially if the edge of the plate in the direction of the horizontal motion was also tilted slightly downwards. That is, the penetrator might "trip" and thus "flip". It may be that the only way to solve that problem is to absolutely minimize remaining horizontal motion during the deorbit burn -- which, in turn, might require a forward-facing Doppler radar sensor. (The Bowling Ball, by contrast, has tremendously more tolerance for horizontal landing velocity.)

It starts to look as though the only question is whether the possibility of getting some astrobiological data on the very first landing -- using a sample from a still quite shallow depth, and involving the analysis of a very small amount of ice collected by the penetrator -- would be worth the considerably increased complexity of the Penetrator as opposed to the Bowling Ball. In short, I'm starting to acquire new serious doubts about my Brilliant Idea.

[JRehling]

As for the Bowling Ball's instrumentation: it would unfold nothing. What it would have on its surface is a number of little camera ports and patch antennas, and about half a dozen little sharp-edged "sample cups" that could scoop up surface samples as a result of the impact itself -- after which electrodes in the cups would analyze the ice's pH, salinity and redox content, and a fiber-optic lead from the cups to an onboard Raman spectrometer would provide further composition analysis. Add an onboard short-lived seismometer or geophone, and that's the total instrument payload of the current model (although an active pulse generator might allow the geophone to double as a simple sonar system to help measure the rock content of the ice, as Spilker thinks is wise).

I wonder, qualitatively speaking, how many different ways the Bowling Ball idea could be tweaked to something intermediate between a hard-smash lander, a penetrator, and an airbag lander.

Could a spherical instrument package be placed inside a larger sphere with free capacity to rotate and a bottom-loaded center of gravity so that when the outer sphere makes first contact, the instrument sphere would rotate into a desired orientation while the outer sphere crumples? It seems like if it were desirable to end up with a constrained (but not tightly constrained) final orientation of the instrument package, something like this could be designed to happen automatically during the crumple/deceleration phase of landing.

It seems to me a penetrator is designed to be destroyed simultaneous with its initial compression and to have destruction exceed compression (leaving a lot of Gs for the instrument-bearing portion to absorb). An airbag is designed to compress fully with no destruction (Gs are still experienced by the instruments in each bounce, especially the first, but not as part of the lithobraking). Wouldn't the ideal design balance these exactly, like an airbag that fails precisely at the instant when it would begin to decompress on the bottom surface (and thus bounce)? There's nothing about the bounce up that you particularly desire (if you have another means to orient the instrument package), and the ability to do so means redundant springiness/durability. With the airbag representing the redundant-spring design and the penetrator representing the partial-spring design, is it feasible to try to design a system that gets the spring/destruction phase Just Right?

With a self-orienting instrument package (think of the Ask The Eight Ball), it seems like that's the ideal design if the margins can be predicted precisely. There's no atmosphere of varying density to screw up that aspect of the landing...

[Bob Shaw]

Could a spherical instrument package be placed inside a larger sphere with free capacity to rotate and a bottom-loaded center of gravity so that when the outer sphere makes first contact, the instrument sphere would rotate into a desired orientation while the outer sphere crumples? It seems like if it were desirable to end up with a constrained (but not tightly constrained) final orientation of the instrument package, something like this could be designed to happen automatically during the crumple/deceleration phase of landing.

Er... ...Ranger A?

Bob Shaw