In Hal Clement's novel with this title, a space probe is landed on a planet with a surface gravity 400 times that of Earth. It's purpose is to make measurements of the gravitational field in a place where it's very intense (but won't destroy the probe) in hopes that scientists will come closer to understanding it.
Gravity is the weakest of the four forces known to modern day physics, and that makes it very hard to study. In order to get a lot of this force, you need a heck of a lot of matter, or you need to amplify your measurements by a huge factor. The pioneer space probes have been subject to the effects of gravity for a long time, and over a large range of field strengths.
They are also exquisitely sensitive measurement devices. Space probes are lauched with precisely known masses and velocities, and they are aimed very precisely. They routinely hit their targets dead-on, carrying enough fuel to change their speeds by a total of a few hundred feet per second. There just isn't that much of an ability to make course corrections. The fact that space probes wind up anywhere near where they're supposed to be is a testimony to how well known the effects of gravity must be.
The pioneer probes are not where they're supposed to be.
The difference between where they are and where they're supposed to be is the same as the distance between the Earth and the Moon.
New probes are being proposed to study this further. Whatever the cause turns out to be, it's bound to be Interesting.
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