Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT_ZIP/jplnews/0161.ZIP/0161.PR

Text File | 1993-04-21 | 7KB | 123 lines

OFFICE OF PUBLIC EDUCATION AND INFORMATION
CALIFORNIA INSTITUTE OF TECHNOLOGY JET PROPULSION LABORATORY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE MURRAY 1-3661, EXTENSION 3111

FOR RELEASE:  A.M.'s of July 19, 1962

                         VENUS TRAJECTORY
         The boost portion of the Mariner mission consists of
three phases:  ascent into a circular parking orbit of approxi-
mately 115 miles, coast in the parking orbit to a pre-determined
point in space, and burning out of the parking orbit to greater
than escape speed.
         The Atlas D/Agena B space booster will rise vertically
and pitch over in the required direction determined by the exact
time of launch.  The vehicle will gain speed and altitude until a
signal from the ground guidance system commands shutdown of the
Atlas engines and separation of the Agena/Mariner from the Atlas.
The Agena engine ignites after a short coast period and acceler-
ates itself and the spacecraft into the parking orbit at a speed
of 17,450 mph.
         The Agena/Mariner will be traveling in a southeasterly
direction over the Atlantic Ocean towards the coast of South
Africa.  Just before reaching Africa, at a point in space deter-
mined by the launch date, time of launch, and desired flight time
to Venus, the Agena engine will re-ignite and accelerate the
spacecraft to a speed of about 25,820 mph.
         Shortly after the Agena engine shuts down, the Mariner
spacecraft is separated from the Agena.  This is "injection."
The speed of the spacecraft exceeds the escape velocity at this
altitude by 1215 mph and the spacecraft moves off in the hyper-
bolic orbit relative to earth.  Because of the rapid change of

VENUS TRAJECTORY               -2-

altitude, the rate at which it moves around the earth decreases
until it is traveling essentially in a straight line outward from
earth.  During the time from injection to escape, the radius
vector from the earth's center to the spacecraft moves through an
angle of about 145?o\.
         At the same time it is moving out, the spacecraft is
slowing down relative to earth because of earth's gravity.  When
it reaches a distance of about 600,000 miles, after about three
days, and has essentially "escaped earth", the velocity will have
decreased from the original 25,820 mph to 6550 mph.  The time of
the second Agena burn will have been chosen so that this velocity
relative to earth is in a direction opposite to that of the earth
in its orbit about the sun.  Thus, the spacecraft will be moving
about the sun 6550 mph slower than the earth's approximate 66,000
mph; that is, about 59,100 mph.
         Because of the lower orbital velocity about the sun, the
spacecraft will be moving too slowly to maintain a circular orbit
against the sun's gravity.  It will, therefore, start falling
inward toward the orbital Venus.  The combination of the inward
motion and the circular motion around the sun produces an
ecliptic orbit that will intersect the orbit of Venus some
100 days later.
         About eight days after launch the accumulated tracking
data will be used to compare the trajectories of the spacecraft
with the trajectory necessary to provide the planned Venus
encounter.  The midcourse maneuver will depend on the different
between these two trajectories.

VENUS TRAJECTORY               -3-

         Now Mariner will begin to curve in towards the sun and
gradually increase its speed.  Eventually, due to the inward
curving path, Mariner's speed will exceed that of the earth and
it will catch up and pass earth.  Later, it will catch up with
rapidly moving Venus, approaching the planet on its dark side at
a speed of over 83,000 mph relative to the sun.
         Entering the sphere of gravitational influence of the
planet, Mariner's path will begin to be deflected due to its
pull.  Its speed will be increased even greater, reaching over
90,000 mph relative to the sun, as it passes Venus on its sunny
side at a distance of about 10,000 miles from the surface.  In
addition, Mariner's path will be bent about 36 degrees in
traveling past the planet.
         At about 65 minutes before closest approach, or at a
distance of 18,600 miles from the planet's surface, the planetary
experiments will begin to scan Venus.  They will operate for
30 minutes, after which the mission is officially over.
         The path of the spacecraft in the vicinity of Venus has
been designed so that Venus will not block the spacecrafts' view
of either the sun or earth.  This is necessary to insure contin-
uous communication with earth and proper functioning of the sun
and earth sensors.  The latter provide reference directions for
attitude control of the spacecraft.  The communication distance
at the time of arrival is about 36 million miles.
         After leaving the sphere of influence of Venus, the
spacecraft will have even greater speed than when it entered.
In essence, it will experience an increase in energy and speed

VENUS TRAJECTORY               -4-

due to the bending of its course by Venus.  This phenomena is
similar to that sometimes experienced by comets which travel too
close to the planet Jupiter.  The energy increase is sometimes
sufficient to cause the comet to escape the solar system.  Such
will not be the case for Mariner, however,
         Designing an interplanetary trajectory is a complex task
that taxes the capabilities of high-speed computers.  The trajec-
tory engineer faces a task complicated by the interactions of the
motion of the earth about the sun, the motion of Venus, the spin
of the earth, and the effect of gravitational fields of the
earth, sun, moon, Venus, Jupiter and even the pressure of the
sun's radiation, on the path of the spacecraft.
         The trajectory designer, therefore, must calculate a
trajectory from minute to minute for that portion of each day
during the launch opportunity that launch could occur.  He must
keep his trajectory with range safety limits (the early portions
of the launch must be over water, not land masses) and he must
keep the trajectory within range of the tracking stations.
         Meshing all these factors into a successful trajectory,
spanning millions of miles and nearly four months in time, is a
formidable task.





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