Mass and Inertia?

inertia means the tendency of object to remain in its state without change , but mass means that the resistance of motion when an object is exerted by external force

I´m not a physicist but I think mass and inertia are not the same thing. They are obviously related but they are different entities. Mass is the measure of the amount of matter an object has and inertia is the resistance an object "makes" to changes in its motion state (if an object is at rest or moving with constant speed, it'll stay that way unless a force is applied to it). As far as relativistic speeds are concerned, the mass of an object increases with velocity, it doesn´t decrease. Of course, it´s inertia also increases and so it would be increasingly difficult to accelerate the object. This is why (or at least one of the reasons why) a material object cannot travel at the speed of light. The energy provided to accelerate the object would be converted into mass, eventually leading that mass to be infinite at the speed of light. All of this, obviously, according to Einstein´s Theory of Relativity and according to the world's most famous equation: E=mc2!

Inertia: in physics, the resistance of a body to any alteration in its state of MOTION, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or of direction of motion

Mass: n physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. There are two ways of referring to mass, depending on the laws of physics defining it. The gravitational mass of a body may be determined by comparing the body on a beam balance with a set of standard masses; in this way the gravitational factor is eliminated (see GRAVITATION; WEIGHT). The inertial mass of a body is a measure of the body's resistance to acceleration by some external force. All evidence seems to indicate that the gravitational and inertial masses are equal. According to the special theory of RELATIVITY, mass increases with speed according to the formula m = m0/Ö(1-v2/c2), where m0 is the rest mass (mass at zero velocity) of the body, v its speed, and c the speed of light in vacuum. The theory also leads to the Einstein mass-energy relation E = mc2, where E is the energy and m the relativistic mass.

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Now to calculate the mass in space you can used Being Balance. or the simple balance is two pans...

You are confusing "rest mass" with "relativistic mass."

At relativistic speeds, an object with small rest mass has large relativistic mass.

Mass is a measured quantity that characterizes a physical property, inertia.

An object's inertia is properly quantified by its relativistic mass, not its rest mass.