Why ion propulsion?

Is there any resistance to movement in space for DS1?

Most of what resists movement on Earth is absent in space. Take, for example, what slows or stops things on Earth:

1, FRICTION: When you hit the brakes in a car, the wheels slow and friction between the road and the wheels slows the entire car. Friction, caused by things in physical contact with each other, is largely absent in space. Space is almost totally empty, so there is no "road" for the space ship to rub against.

2. AIR RESISTANCE: Air resistance is like friction. It is caused by the molecules in the air pushing against a craft in flight. It is what slows parachutists down and keeps them from crashing into the Earth. There is no air resistance in space because there's no air in space.

3. GRAVITY: Gravity, which will slow down a ball thrown up in the air, is present in space. But since gravity decreases with distance from a planet or star, the farther out into space DS1 is, the less gravity will slow it down. Also, if DS1 is close to an object with significant gravity like Mars and heading towards it, the gravity will actually speed it up. Gravity only acts as resistance to movement in space when DS1 is close to a planet and moving away from it.

Could ion propulsion move something on Earth?
Could NASA use ion propulsion to put a ship into space?
What is a propellant?
How does gravity work in space?
How long do space missions take?

How fast does DS1 go?
How fast do conventional rockets go?
What is resistance?
What would happen if DS1 ran out of fuel?

How does mass and distance affect gravity?