Scientists measure the mass of celestial bodies like planets or stars by observing the gravitational effects they have on other objects. One common method involves studying the orbits of objects around these celestial bodies. For example, the mass of a planet can be determined by observing the orbit of its moon. By applying Kepler's laws of planetary motion and Newton's law of universal gravitation, scientists can calculate the mass of the planet based on the orbital characteristics of its moon, like the orbital period and distance from the planet. Similarly, for stars, astronomers observe the motion of planets or other stars orbiting around them. These observations, combined with gravitational theories, allow the calculation of the star's mass. These methods are based on the principle that the gravitational force exerted by a body is directly proportional to its mass, thereby enabling mass measurements from gravitational interactions, even across vast cosmic distances.

An object's weight changes on different planets due to the variation in gravitational field strength of each planet, while its mass remains the same because mass is an intrinsic property of the object. The gravitational field strength varies from planet to planet, depending on factors like the planet's mass and radius. For example, larger planets like Jupiter have stronger gravitational fields, making objects weigh more there compared to Earth. On smaller planets or moons, like Mars or Earth's moon, the gravitational field is weaker, so objects weigh less. However, the object's mass, which is the quantity of matter in it, does not change with location. It remains constant irrespective of external influences like gravity. This distinction is crucial for understanding celestial mechanics and is a fundamental concept in astrophysics, where the effects of different planetary gravities on objects are studied.

If gravity on Earth were to suddenly increase, our weight would increase proportionally, but our mass would remain unchanged. Weight is the force exerted by gravity on an object's mass, calculated as Weight = Mass x Gravitational Field Strength (g). An increase in Earth's gravitational field strength (g) would result in a proportional increase in the weight of objects and people on Earth, as the force acting upon their mass would be greater. However, mass is a measure of the quantity of matter in an object and is independent of gravitational forces. Therefore, even with increased gravity, the amount of matter that constitutes our bodies (our mass) would stay the same. This hypothetical scenario highlights the fundamental difference between mass and weight: mass is an intrinsic property of matter, while weight is a force that results from the interaction of mass with a gravitational field.

Weightlessness in space is a phenomenon that occurs due to the minimal gravitational force experienced there, significantly affecting an object's weight but not its mass. In orbit, astronauts and objects experience a state of continuous free fall towards Earth, creating the sensation of weightlessness. However, this does not imply that their mass is zero. Their mass remains constant as it is a measure of the amount of matter in their bodies, which does not change. What changes is the weight - the gravitational force acting on that mass. Since the gravitational force in space is much weaker compared to Earth's surface, the weight of objects and astronauts becomes negligibly small, leading to the sensation of weightlessness. This concept is pivotal in understanding how gravitational fields work and how they affect objects. It also has significant implications for space travel and living in microgravity environments, where conventional concepts of weight and up/down orientation do not apply.

Yes, an object retains its mass regardless of its presence in a vacuum. Mass is a measure of the amount of matter in an object, and it is an intrinsic property that does not depend on external factors such as air presence or gravitational field. In a vacuum, where there is an absence of matter, the object's mass remains unchanged. This is because mass is related to the number and types of atoms and molecules composing the object, and these do not change whether the object is in a vacuum or surrounded by air. However, in a vacuum, the absence of air resistance can affect the object's motion, but not its mass. This concept is crucial in space exploration and physics experiments conducted in vacuum environments, where the behaviour of objects in the absence of air resistance is studied, but their mass remains a constant factor in calculations and observations.