Momentum is calculated using the formula:
\[ p = m \cdot v \]
where \( p \) is momentum, \( m \) is mass, and \( v \) is velocity.
If the object's mass \( m \) stays the same, but the speed \( v \) is cut in half, the new momentum \( p' \) can be expressed as:
\[ p' = m \cdot \left( \frac{v}{2} \right) = \frac{1}{2} (m \cdot v) = \frac{1}{2} p \]
This shows that the new momentum is half the original momentum.
Thus, the momentum halves.