Step 1 :The significant figures in a number are digits that carry meaningful information about its precision. In the given measurement $9.0 \times 10^3 \mathrm{~km}$, the significant figures are the digits in the number before the exponential part.
Step 2 :The number 9.0 has two significant figures: the 9 and the 0 after the decimal point.
Step 3 :The 3 in the exponent is not considered a significant figure because it is part of the exponential notation, which indicates the order of magnitude, not the precision of the number.
Step 4 :Final Answer: The number of significant figures in the measurement $9.0 \times 10^3 \mathrm{~km}$ is \(\boxed{2}\).