How to calculate acceleration distance(frictionless, massless)

Question

Object A is moving via. 2D rigidbody by velocity V and in script is defined breaking power P. Every frame of breaking (P * Time.deltaTime) is subtracted from velocity V (in other words, ignoring mass). How i can calculate distance D required to slow down to target velocity tV ?

I'm not actually using force/power but i setting velocity directly with this line of code :

Vector3 dir = transform.TransformDirection(Vector3.up);
ship.rb.velocity = Vector2.MoveTowards(ship.rb.velocity, (dir * maximalSpeed) * ship.massModifier, speedGain / ship.massModifier);

(Don't worry about ship.massModifier, Basically heavier ship hulls are slower)

I'm self-learning Indie developer and i need this for AI to my game. Enemy in space ship have nothing like space-breaks so only way how to slow down is to turn ship by 180 degrees and thrust forward in counter-direction.I need to know this to calculate distance from target required to slow down and prevent possible collision with target.

Answer 1

Use the formula

$$ t = \frac{v_0}{a}$$

$$ s = v_0 \cdot t + \frac{at^2}{2}$$

where \$v_0\$ is the starting velocity, \$a\$ is the acceleration/deceleration and \$s\$ is the distance