Correct Answer : 1/80
Explanation : The moon’s mass is 1/80 the mass of the Earth. It is 7.35 × 1022 kg compared to the Earth’s mass which is 5.972 × 1024 kg. The gravitational constant on moon is 1/6th that of the Earth.
Correct Answer : 382,400 km
Explanation : Earth and moon revolve around a common center of mass. The mean distance between the centers of these two is 382,400 km. This value can often vary over the course of the orbit so the mean value of 382,400 km is considered. 398,600 km3 s is the gravitational parameter of the Earth.
Correct Answer : 27.3 days
Explanation : Earth and moon both revolve around a common center of mass once in 27.3 days. 27 days is the revolution period of moon and 360 days is the revolution period of the earth.
Correct Answer : Tidal bulge
Explanation : Tidal bulge on earth’s ocean surface caused due to moon is carried eastward due to earth’s rotation. This leads to a shift in the center of gravity of earth to the east of the lining which joins earth and moon. This leads to acceleration of moon in the direction of the orbital motion which leads to moon spiraling outward.
Correct Answer : Barycenter
Explanation : Barycenter is a term used to refer to the center of mass of two or more bodies. In this case it is the earth- moon system. This point lies inside earth about 4,700 km from its center.
Correct Answer : When moon comes between sun and earth
Explanation : Solar eclipse occurs when moon comes between earth and sun. Moon’s shadow is casted on earth. They usually happen once in every 18 months. These occur at new moon.
Correct Answer : When the earth comes between sun and moon
Explanation : Lunar eclipse occurs when earth comes between moon and sun. Earth’s shadow is casted on the moon’s surface. Earth, moon and sun are perfectly aligned. There are two types of lunar eclipses- total lunar eclipse and partial lunar eclipse.
Correct Answer : 6
Explanation : Just like Earth, even moon has orbital elements used to describe the orbit and the position. The moon’s orbit is defined by six orbital elements as follows: semi-major axis (a), eccentricity (e), inclination (i), longitude of the ascending node (Ω), right ascension at epoch (α) and the argument of perigee (ω).
Correct Answer : Semi-major axis
Explanation : The six orbital elements are used to describe the shape, size and orientation. The semi-major axis (a) is used to determine the size of the orbit. Eccentricity is is determine the orbit shape, and the argument of periapsis determines the orbit’s orientation.
Correct Answer : Eccentricity
Explanation : The six orbital elements are used to describe the shape, size and orientation. The shape of the orbital element is determined using the eccentricity. an style="border: 0px; m
Correct Answer : Longitude of ascending node
Explanation : Longitude of ascending node is known as the angle in the fundamental plane between the vernal equinox and the point where satellite crosses through the fundamental plane in northerly direction (ascending node. That is why it is also known as right ascension of ascending node. It is measured counter clockwise when it is viewed from the north side of the fundamental plane.
Correct Answer : Evection
Explanation : The moon’s orbit is perturbed due to the effect of Sun’s gravitational pull. This results in small period changes in the orbital eccentricity at intervals of 31.8 days. This effect is known as evection which was discovered by Hipparchus.
Correct Answer : When moon is at the nodal crossing
Explanation : When the moon is at the either nodal crossing point eclipse occurs. The node where the moon moves from south to north is called ascending node and the node where the moon moves from north to south is known as descending node. Eclipse occurs only a these two points because only in this case the sun, moon and earth are aligned.
Correct Answer : 8.9 years
Explanation : The line joining the perigee and apogee of the orbit is known as line of apsides. This rotates in the direction of moon’s orbital motion causing argument of perigee to vary by 30 deg in 8.9 years.
Correct Answer : Time taken for moon to go around orbit from one node to another
Explanation : The time taken for moon to travel from one node in the orbit to another is known as draconitic period or nodal period. Draconitic period is 27.21222 days which is lesser than the sidereal month.
Correct Answer : Orbit around Moon
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.
Correct Answer : Apomoon
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.
Correct Answer : Perilunar
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.
Correct Answer : Below 100 km
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.
Correct Answer : Circumlunar trajectory
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.
Correct Answer : The Moon is fixed in space
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.
Correct Answer : Moon’s rotational speed
Explanation : There are several design restrictions which are there for carrying out luncar missions. This includes, phases of the moon which determines the lighting conditions, declination of moon and direction of launch from the particular site.
Correct Answer : Geocentric sweep angle
Explanation : One of the most important parameter which is used to deciding the acceptable launch time is geocentric sweep angle. This is the angle swept by the spacecraft from launch to the lunar intercept.
Correct Answer : Angular velocity
Explanation : The parameter required in order to complute the time of flight to the moon’s sphere of influence is-parameter p, eccentricity, semi-major axis and eccentric anomaly.
Correct Answer : Measure time for the spacecraft to cross the moon at intercept point
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.
Correct Answer : Neglecting lunar gravity
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.
Correct Answer : Is smaller than predicted
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.
Correct Answer : Is shorter than predicted
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.
Correct Answer : Flight-path angle
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.
Correct Answer : Increase fuel efficiency
Explanation : Gravity assist maneuver also known as fly-by or swing-by makes use of the gravity of a planet, sun or moon to propel the spacecraft. This leads to saving the propellant mass and increase the efficiency. The satellite/spacecraft enters planet’s sphere of influence along hyperbolic arrival asymptote and leaves the planet along hyperbolic departure asymptote.
Correct Answer : Alter the orbital time
Explanation : Using gravity assist maneuver, the spacecraft’s speed can be altered i.e., either accelerate or deaccelerate. This is also used to alter the path using the gravitational pull. But the aim of carrying out gravity assist maneuver is not to alter the orbital time as it is dependent on various other factors.
Correct Answer : Increases
Explanation : Every celestial body has its own gravitational sphere where its gravitation is dominated as compared to other body’s gravitational field. When the spacecraft enters this sphere, there is a strong pull due to gravity thus increasing its velocity.
Correct Answer : Decreases
Explanation : When the spacecraft leaves the planet’s sphere of influence along the hyperbolic escape trajectory, it experiences a strong pull due to the planet’s gravity. This decreases the velocity of the spacecraft due to reversibility of orbits.
Correct Answer : Mars
Explanation : Voyager 2 was one of the first interspace missions which made use of gravity assist by Jupiter, Saturn, Uranus and Neptune. They were all aligned perfectly which is a rare occurrence.
Correct Answer : Limitation of correct alignment of planets
Explanation : On of the major setbacks for using the gravitational slingshot is that there are very rare occurrences for the planets to align in way expected for the optimum slingshot to occur. It can take several years, decades to achieve a perfect alignment of the celestial bodies.On the other hand, slingshot mostly accelerates the spacecraft thus making it less time consuming. And since it makes use of lesser fuel, it is ore economical.
Correct Answer : Time taken for phase angle to return to original value
Explanation : When any planet/moon revolves, the time it takes to return to the same angular position from the sun as seen from Earth is called as synodic period. The phase angle returns to its original value.
Correct Answer : 586 days
Correct Answer : Planet’s ephemeris
Explanation : Synodic period has been of high importance among the astronomers in predicting the planet’s ephemeris in advance. It is the time taken by the celestial object to return to the same initial angular position from Sun as seen from the Earth. The term ‘synodic period’ is often used for moon, known as lunar month = 29.531 days.
Correct Answer : 583.92 days
Explanation : Synodic period of Venus is the time taken for it to be seen from Earth in the same position with respect to Sun. It is known to be 583.92 days which is about 19 months. 365.25 days is the synodic period of the Earth.
Correct Answer : Time between successive full moon
Explanation : Synodic period or lunar period is the time taken between successive recurrences of the same phase. This could be time between two consecutive full moon or new moon. For moon the synodic period is 29.531 days which is an average value.
Correct Answer : 29.5 days
Explanation : The time taken for the moon between two successive phases is 29.5 days. On the other hand 27.3 days is the lunar sidereal which is the time taken for moon to complete a 360 degree orbit around the earth relative to the sun.
Explanation : Mars is the only planet in our solar system whose synodic period is more than its sidereal period. Its sideral period is 687 days whereas its synodic period is 780days. The synodic period is the time taken by the planet or any celestial object to come back to the initial angular position whereas the sidereal period is the time taken by the planet to orbit 360 degrees around the Earth as seen from Sun or any other fixed star.
Correct Answer : Solar conjunction
Explanation : For outer planets there is a point when the Earth and sun are in a straight line and the planet I at exact opposite part of the sky to the sun. Within each synodic cycle, thus the planet disappears into sun’s glare once at solar conjunction. Half a cycle later, it is visible for much of the night when it reaches opposition.
Correct Answer : Difference between true anomaly between injection and lunar intercept
Explanation : We know that the geocentric angular travel ψc is arbitrarily chosen whereas free-flight ψff is selected based on the initial injection conditions. This free-flight is just difference between true anomaly between injection and lunar intercept.
Correct Answer : Local sidereal time
Correct Answer : Rotation speed of the earth
Explanation : There are several constraints which carrying out lunar missions. These include-launch site location, accuracy tolerance, limited range of injection conditions, phase of the moon determining the lighting condition, direction of launch from a site.
Correct Answer : Non-coplanar trajectory
Explanation : Coplanar trajectory only occurs when the launch is at a suitable condition where launch latitude matches the moon’s orbital inclination. This is a rare condition which occurs only at Cape Kennedy whose latitude for launch is 28.5 deg. When launches are done at any other location or when the suitable conditions is not met, it results in non-coplanar trajectory.
