Question

In 1957, Russia launched Sputnik I. Its elliptical orbit around the earth reached maximum and minimum distances from the earth of 583 miles and 132 miles, respectively. Assuming that the center of the earth is one focus and that the earth is a sphere of radius 4000 miles, find the eccentricity of the orbit.

Short answer

The eccentricity of Sputnik I's orbit is approximately 0.00036.

Step-by-step solution

Understanding the Problem

We need to find the eccentricity of the elliptical orbit of Sputnik I. We know the maximum and minimum distances from the Earth, which are 583 miles and 132 miles, respectively. This defines the semi-major axis and allows us to find the necessary parameters for eccentricity calculation.

Identify the Semi-Major Axis (a) and Semi-Minor Axis (b)

In an ellipse, the semi-major axis (\(a\)) is half the sum of the maximum and minimum distances from the center. So first we compute \(a\): \[a = \frac{583 + 132}{2} + 4000\] since Earth’s radius is not included in the orbit measurement at those distances.

Calculate the Foci Distance (c)

The distance of each focus from the center of the ellipse is \(c\). Since one focus is the center of the earth at a distance of 4000 miles from the center of the ellipse, we compute \(c = a - \text{distance of closest approach to the earth}\): \[c = a - (132 + 4000)\]

Compute the Eccentricity (e)

The eccentricity (\(e\)) of an ellipse is given by \(e = \frac{c}{a}\). Using the value of \(c\) calculated in the previous step and \(a\), compute \(e\).

Substitute and Solve

We substitute the known values into the expressions formulated to find \(a\), \(c\), and \(e\). After substituting the values, calculate each expression to derive the eccentricity. Specifically,\[e = \frac{\left(a - 4132\right)}{a}\].

Key concepts

Elliptical Orbits

Elliptical orbits are fascinating paths that celestial bodies, like planets and satellites, follow around a larger mass, thanks to the force of gravity. Their shape is not a perfect circle, but rather an ellipse, which can be thought of as a flattened circle.
In the case of Sputnik I, launched in 1957, its path around Earth took on this elliptical form. This orbit allows for varying distances from the Earth at different points in the satellite's journey, known as the apoapsis (farthest) and periapsis (nearest).

The concept of elliptical orbits was first introduced by Johannes Kepler in the 17th century. He discovered that:

• Planets move in ellipses with the Sun at one focus.
• The speed of the orbiting body changes as it travels, moving faster when closer to the larger mass.

Understanding elliptical orbits allows us to calculate critical parameters, like the eccentricity, which measures how much the orbit deviates from being circular.

Semi-Major Axis

The semi-major axis of an ellipse is a crucial parameter that represents half of the longest diameter across the ellipse. It's essentially the average of the farthest and closest points in the orbit from a central body.

For the orbit of Sputnik I, this involved calculating the average of the maximum and minimum distances from the Earth, adding the Earth's radius (because these distances are measured from the Earth's surface), and then dividing by two. Mathematically, it is explained as:

• The formula for the semi-major axis, \(a\), in an orbital context: \[a = \frac{d_{max} + d_{min}}{2} + R_{earth}\]where \(d_{max}\) is 583 miles, \(d_{min}\) is 132 miles, and \(R_{earth}\) is 4000 miles.

This calculation helps us understand the scale of the orbit and plays a vital role in determining the shape of the ellipse through its eccentricity.

Focus of an Ellipse

The focus of an ellipse is one of two fixed points, around which the shape of the ellipse is perfectly arranged, ensuring that the total distance from a point on the ellipse to each focus remains constant.

In astronomy, especially when considering orbits, one of these foci is often the center of the Earth or the Sun. For Sputnik I, Earth serves as one focus of its elliptical orbit. This is crucial because:

• The gravitational pull from this focus (Earth, in this case) helps maintain the shape and course of the orbit.
  
• Knowing the position of the focus allows us to determine the distance from this point to any position on the orbit, particularly the closest (periapsis) and farthest (apoapsis) distances.

Understanding the focus helps us calculate other parameters like the semi-major axis and eccentricity, emphasizing its importance in orbital analysis.

Orbital Mechanics

Orbital mechanics is a branch of mechanics that deals with the motions of objects in space, guided by the laws of physics. It encompasses many fundamental principles that dictate the behaviors of architectural orbits, such as those for planets and satellites.

In the elliptical orbits of satellites like Sputnik I, these principles become especially important:

• Newton's Law of Universal Gravitation explains how two bodies attract one another, pulling the satellite toward the Earth and keeping it in a consistent orbit.
  
• Kepler's Laws, particularly the first law, confirm that orbits are ellipses with the attracting body at a focus.

Through orbital mechanics, we can predict satellite paths, calculate transfer orbits, and understand the dynamics of space missions. Learning about these mechanics provides a practical foundation for understanding how natural and artificial satellites navigate the cosmos.