Planetary probes present a very diverse range of structural problems and solutions. This is in contrast to free-flying spacecraft (i.e. satellites and deep-space probes) which generally have a simple box or drum structure because there is only a single dominant loading (launch). On the other hand, landers and probes can range from resembling spiders to cannonballs, with the range generally being driven by thermal as well as structural requirements. Landers may be spidery open frames with equipment boxes bolted on, like the Surveyor landers; in contrast, entry probes for hot, deep atmospheres are constructed as pressure vessels and have thus been spherical in shape.
On most satellites the largest accelerations and thus structural loads are encountered during launch (typically 5–10 g): however, entry probes to Venus or Jupiter may encounter decelerations of 100–500 g. In such situations, load paths must be kept as short as possible to minimize the structural mass. The Pioneer Venus and Galileo probes (which had thermal constraints) used thick-walled pressure vessels supporting solid deck plates to which equipment was bolted. Spherical geometries are also appropriate where landing attitude is not initially controlled (e.g. Luna 9, 13; though note that the interiors of these spacecraft were pressurized, which also tended to favour a spherical design).
The Huygens probe did not need to exclude the atmosphere and therefore had an unsealed, thin-walled shell to preserve an aerodynamic shape and support light foam insulation.
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