This set of Spaceflight Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Seleno-Centric Trajectories”.

1. What is a seleno-centric orbit?

a) Orbit around Earth

b) Orbit around Moon

c) Orbit around Mars

d) Orbit around Sun

View Answer

Explanation: The orbit around moon which is influenced by moon’s gravity is known as a seleno-centric orbit. When manned and unmanned missions are launched which orbit the moon, they are said to be in a seleno-centric orbit.

2. Which of these terms is not used to refer to the apoapsis of the lunar orbit?

a) Apolune

b) Aposelene

c) Apocynthion

d) Apomoon

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Explanation: Apogee is the point in the orbit which is the farthest from center of attraction. There are special terms used to refer to the apogee of the moon which are-apolune, aposelene and apocynthion.

3. Which of these terms is not used to refer to the periapsis of the lunar orbit?

a) Periselene

b) Perilunar

c) Perilune

d) Pericynthion

View Answer

Explanation: Perigee is the point in the orbit which is the closest from center of attraction. There are special terms used to refer to the apogee of the moon which are-periselene, perilune and pericynthion.

4. At what altitude are Low Lunar Orbits (LLO) present?

a) Below 100 km

b) 100-150 km

c) 150-500 km

d) 500-1000 km

View Answer

Explanation: Low lunar orbits are the ones which are at an altitude of less than 100 km. Their orbital period is about 2 hours. There are often several perturbations that make that satellites orbiting very unstable.

5. Which of these trajectories makes use of moon’s gravity to return it back to earth’s trajectory?

a) Trans-lunar trajectory

b) Slingshot trajectory

c) Circumlunar trajectory

d) Free-trajectory

View Answer

Explanation: Circumlunar trajectory is a type of free-return trajectory which sends the spacecraft from Earth around far-side of the moon to return it back to the Earth’s orbit using gravity only once the initial trajectory is set. For a circumlunar flight, we need to compute both the perigee condition and condition upon exit from lunar sphere of influence.

6. What is the formula used to compute the eccentricity of the selenocentric orbit?

a) e = \(\sqrt{1 + 2\varepsilon h^2/\mu_m^2}\)

b) e = 1 + 2εh^{2}/\(\mu_m^2\)

c) e = \(\sqrt{1 – 2\varepsilon h^2/\mu_m^2}\)

d) e = 1 – 2εh^{2}/\(\mu_m^2\)

View Answer

Explanation: The eccentricity of the selenocentric orbit is calculated using the formula:

e = \(\sqrt{1 + 2\varepsilon h^2/\mu_m^2}\)

Where, μ

_{m}is the gravitational parameter of the moon

ε is the energy

h is the angular momentum

7. Transition from a geocentric motion to selenocentric motion makes use of patched conic approximation to determine an approximate trajectory.

a) True

b) False

View Answer

Explanation: Transition from a geocentric motion to selenocentric motion makes use of patched conic approximation. The gradual process takes plane over a finite arc of the trajectory where Earth and Moon affect path equally. Patched conics give a nice approximation for preliminary analysis.

8. Patched-conic approximation can be used for calculating the return trajectory to Earth from Moon.

a) True

b) False

View Answer

Explanation: There is a high perturbation which is encountered when the spacecraft is in the moon’s sphere of influence. Due to this reason patched–conic approximation cannot be used for return trajectories back to the earth.

9. Which of these assumptions are not employed for using patched–conic approximation for a selenocentric orbit?

a) The Earth is fixed in space

b) The eccentricity of the Moon orbit around the Earth is neglected

c) The flight of the space vehicle takes place in the Moon orbital plane

d) The Moon is fixed in space

View Answer

Explanation: There are various assumptions employed for using patched–conic approximation for a selenocentric orbit. Out of these, the assumption that is invalid is that the moon is not fixed in space.

10. For a trajectory involving the spacecraft to move from the earth to the moon, how many phases are involved in the trajectory?

a) 1

b) 2

c) 3

d) 6

View Answer

Explanation: The trajectory has two distinct phases when it moves from the earth to the moon or vice versa-geocentric and selenocentric trajectories. The geocentric phase corresponds to the portion of the trajectory which begins at the point of application of the first impulse and extends to the point of entering the Moon’s sphere of influence. The selenocentric phase corresponds to the portion of trajectory in the Moon’s sphere of influence and ends at the point of application of the second impulse.

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