Suppose that you used some geometry and kinematics to estimate that the Earth goes around the Sun with an orbital speed of approximately 30,000 m/s (60,000 mph), and that the Sun is approximately 150 million kilometers away from the Earth. Use this information to estimate the mass of the Sun.
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Satellite Motion: Speed & Period
Problem 70
Textbook Question
The Lunar Reconnaissance Orbiter (LRO) is a NASA Moon-orbiting spacecraft in an orbit with an altitude of 50 km above the Moon’s surface. What is the period of the LRO as it orbits the Moon?

1
Step 1: Identify the relevant formula for orbital period. The orbital period (T) of a satellite can be determined using Kepler's Third Law, which is expressed as:
Step 2: Gather the necessary data. You need the mass of the Moon (M), the gravitational constant (G), and the orbital radius (r). The orbital radius is the sum of the Moon's radius and the altitude of the orbit: , where is the Moon's radius and is the altitude of the orbit.
Step 3: Substitute the values into the formula. Use the known values for (gravitational constant), (mass of the Moon), and (orbital radius) to calculate the orbital period.
Step 4: Simplify the expression. Perform the necessary algebraic manipulations to isolate (orbital period) and simplify the terms under the square root.
Step 5: Compute the result. Use a calculator or computational tool to evaluate the simplified expression and determine the orbital period in seconds. If needed, convert the result into minutes or hours for better interpretation.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Orbital Mechanics
Orbital mechanics is the study of the motion of objects in space under the influence of gravitational forces. It involves understanding how celestial bodies interact and the laws governing their orbits, such as Kepler's laws. For a spacecraft like the LRO, the altitude and mass of the Moon are crucial for calculating its orbital period.
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Gravitational Force
Gravitational force is the attractive force between two masses, described by Newton's law of universal gravitation. The force depends on the masses involved and the distance between their centers. For the LRO, the Moon's mass and the distance from its center to the spacecraft determine the gravitational pull acting on it, which influences its orbital speed and period.
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Orbital Period
The orbital period is the time it takes for a spacecraft to complete one full orbit around a celestial body. It can be calculated using the formula T = 2π√(r³/GM), where T is the period, r is the distance from the center of the Moon to the spacecraft, G is the gravitational constant, and M is the mass of the Moon. Understanding this concept is essential for determining how long the LRO takes to orbit the Moon.
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