Question

The Pioneer 10 space probe was launched in 1972 to explore the asteroid belt and Jupiter. The probe continued traveling away from the Solar System and into deep space. In 2003, when it was over seven billion miles from home, its power supply failed and all contact was lost. Just before the power failure, data from the spacecraft indicated a reduction in its velocity that was too rapid to be explained by the Sun's gravitational pull alone. Which of the following is the best scientific conclusion based on the unexpected data?


The spacecraft was slowed by a force other than gravity.


The instruments measuring Pioneer 10's velocity malfunctioned.


The law of universal gravitation is incorrect and should be revised.


The data collected regarding the Sun's gravitational force is incorrect.

Answer 1

The spacecraft was slowed by a force other than gravity.

Reason: The data show a deceleration larger than can be explained by the Sun’s gravity, so the best scientific conclusion is that an additional force acted on Pioneer 10 (e.g., thermal recoil, outgassing, or drag). Instrument failure or revising gravity are possible but require further evidence; they are not the simplest, most immediate conclusion from the data.