The Rosetta comet probe – alternative designs for the Philae lander

The Philae lander has now left the Rosetta comet-chasing space probe and is heading for a rendezvous with the comet in six hours or so.
Watching television reports about how the probe is designed to land on the comet doesn’t exactly fill me with confidence. The three-legged design looks as though it wouldn’t take much to tip it over, and the surface of the comet certainly looks as though it’s littered with numerous trip hazards.
I can’t quite work out why the designers didn’t go for a more terrain-forgiving landing mechanism that had more latitude for error. Obviously there are issues of cost restraints and factors of technical feasibility that I know nothing about, but here are a few suggestions of my own.
Ideas that come to mind are such things as using a harpoon to anchor the craft to the comet before the craft actually touches down, then the craft could reel itself in no matter what the lie of the land. This method would obviously need the material of which the comet is composed to be harpoonable, which may be why the method was rejected. (The craft actually does have anchoring harpoons attached, but these are designed to be deployed after it’s landed, so they are more of a back-up system than a primary device).
Other landing systems that may have been appropriate could involve flexible or cushioning landing mechanisms such as inflated pillows (that deflate on impact), widely splayed, long, articulated legs (much more flexible and articulated than the rather rigid-looking ones that the spacecraft actually has) that buckle on impact to take on the topography of the landing site, or even the astronautical equivalent of long chains with grappling hooks that could fan out around the craft as it comes in to land to snag on any suitable features of the terrain.
In all of these designs the scientific instruments in the probe could be housed in a self-righting capsule to ensure optimum orientation after touchdown. The design of the probe that’s actually going to land in a few hours’ time doesn’t seem to have any system for realigning itself if it lands awkwardly.

Anyway, Philae is due to land soon, so I hope my misgivings prove to be unfounded. I’m sure the landing site that’s been chosen for the probe is the best possible site that’s available, so – fingers crossed!.


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