Sonar Sensor Detecting In Coming Object Going Right or Left

Hello. I am currently working on a project that entails a stepper motor, a motor driver, and a sonar sensor. Currently I am using the Arduino Sonar Sensor. I am unsure on how i can have the sensor give information if an object that is coming towards the sensor is come more right than left.

For example:

The sensor is attached to the motor. There is an in coming object moving towards the sensor. I want to be able to have the sensor give data that allows the motor to move in a linear motion (left or right) away from the in coming object.

Is there a way I can only use one sonar sensor? Or would I need two to be able to calculate the distance (in a triangular field) that will help pinpoint the exact location of the in coming object.

Please help, I've been stuck on this for weeks!!

A single sensor gives only distance information. You need two or more distance measurements to localize an object.