# Lunar Trajectories Questions and Answers

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

1. In order to get the spacecraft to land on moon, how do we time the launch from the Earth?
a) Measure time for the spacecraft to cross the moon at intercept point
b) Measure time for the spacecraft to cross the moon when at t-1 seconds
c) Measure time for the spacecraft to cross the moon when at t-5 seconds
d) Measure time for the spacecraft to cross the moon when at t-10 seconds

Explanation: When the spacecraft is launch from the Earth, Moon is also moving in its own orbit. To make sure that the spcecraft is able to land on the moon, we have to time it in such a way that the spacecraft and moon intersect at some time t.

2. Which of these assumptions is made while approximating the landing error due to the injection conditions?
a) Neglecting Sun’s gravity
b) Neglecting lunar gravity
c) Velocity of the spacecraft is constant
d) Orbital time of lunar orbit is approximated

Explanation: In order to predict the error in guidace due to injection errors, a simplified model of Earth-Moon system is used. One assumption made while calculating this is that the moon’s gravitational force is considered to be negligible.

3. The injection errors cause a deviation in the time- of- flight of the spacecraft only.
a) True
b) False

Explanation: Due to injection errors, there’s a deviation in both the sweep angle and the time-of- flight of the spaceccraft. This will result in the spacraft intersecting the moon in its orit at different time and location than initially predicted.

4. Eastward launches do not get affected by injection errors.
a) True
b) False

Explanation: Earth rotates anticlockwise resulting in surface velocity of 1600 km per hour. When there is a direct eastward launch of a spacecraft from the the Earth, the effects of injection errors are countered and the orbit does not face any perturbation.

5. What happens to the sweep angle when the injection velocity is too high?
a) Is larger than predicted
b) Is smaller than predicted
c) Remains unaffected
d) Does not intersect the moon’s orbit

Explanation: When the injection velocity of the spacecraft from the Earth is too high, the geocentric sweep angle is smaller than predicted. This results in spacecraft crossing moon’s orbit westward than initially predicted. The difference is of the amount Δψ.

6. What happens to the time-of-flight when the injection velocity is too high?
a) Is larger than predicted
b) Is shorter than predicted
c) Remains unaffected
d) Becomes infinity as it never intersects the moon’s orbit

Explanation: When the injection velocity of the spacrcarft form the Earth is too high, the geocentric sweep angle is smaller than predicted which results in a decrease in the time-of-flight as well. This difference is of the amount Δt.

7. Which error is the miss distance at moon a function of?
a) Time-of-flight
b) Inclination
c) Flight-path angle
d) True anomaly

Explanation: The miss distance at moon is a function of flight-path angle error only. For understanding, an error od about 1 deg in flight-path angle can cause the spacrcraft to miss the distance at moon by 1,300 km.

More MCQs on Lunar Trajectories:

Sanfoundry Global Education & Learning Series – Spaceflight Mechanics.