Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0521.PR

Text File | 1993-04-21 | 13KB | 252 lines

OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE  [phone]

                                                 April 1, 1969

          Commands were transmitted from Earth to the Mars-bound
Mariner 7 today to complete the star acquisition sequence begun
shortly after the spacecraft was launched on March 27.
          For the first five days of flight, Mariner 7's star
tracker has been "locked onto" the star Vega.  One of the com-
mands, generated at the Jet Propulsion Laboratory, Pasadena,
Calif., caused the spacecraft to drop lock on Vega and continue
its roll search until it acquired and locked onto the star
Canopus.
          The commands were transmitted from the NASA-JPL Deep
Space Network station at Woomera, Australia, across a distance of
about 1,120,000 miles.
          Canopus, one of the brightest stars in the galaxy,
provides a second celestial reference--along with the sun--upon
which to base the midcourse maneuver and the pointing of the
science instruments at Mars.
          Mariner 7 was allowed to remain locked onto Vega for
several days while engineers analyzed some unexpected behavior
observed in the Mariner telemetry during the launch phase of the
mission.
          Most of the anomalies are now understood and the
spacecraft is operating correctly.

                               -2-

          During the powered phase of flight, the telemetry data
indicated a decrease in the radio transmitter power lasting for
about eight minutes.  However, the decrease has not been verified
by ground receiver measurements and the performance at present is
correct.  Although the telemetry indication has not recurred,
this inconsistency remains under investigation.
          An unexpected change in the power profile was observed
when sunlight fell on one of the spacecraft sensors.  This
occurrence was normal, but was not recognized as such at the time.
          The Central Computer and Sequencer memory also has been
checked out and found to be correctly loaded.  It is believed
that a spurious electrical signal at the time Mariner separated
from the Centaur caused some unintended switch settings in the
CC&S.  A command from the ground today reset the switches and all
are now in the normal position.
          Preliminary analysis of the Mariner 7 trajectory
indicates that the launch accuracy was as good or better than the
near-perfect launch of Mariner 6 a month earlier.  Without a
midcourse maneuver, Mariner 7 would fly over the south polar cap
of Mars at an altitude of about 16,000 miles on the night of
August 4.  A maneuver, however, is planned for next week to
change the closest approach altitude to about 2000 miles.
          At 12 noon PST today, Mariner 7 was 1,120,000 miles
from Earth and traveling at a velocity of 9311 miles per hour
relative to Earth.  Distance to Mars was 76 million miles.

                               -3-

Mariner 7 will clock 195 million miles in its 130-day voyage.  Of
this distance, the spacecraft has completed 8.5 million miles.
          Mariner 6, now in its 36th day of flight, is 6.2 million
miles from Earth and 71 million miles from Mars.  Its flight path
is 226 million miles long and duration of flight from Earth to
Mars is 156 days.  Mariner 6 has already flown 64 million miles.

                               ###





OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE  [phone]

                                                 April 8, 1969

          Mariner 7, one of two automatic scientific spacecraft
destined to rendezvous with Mars this summer, today executed a
midcourse maneuver adjusting its flight path for a 2000-mile-
altitude swing by the planet.
          Now in its twelfth day of flight and more than 2 1/2
million miles from Earth, Mariner 7 acted upon commands from the
ground to turn the proper position then fire its rocket engine
for 7.7 seconds for a velocity change of about 10 miles per hour.
          Mariner Project officials at the Jet Propulsion Labora-
tory in Pasadena, California, said the maneuver shortens the
Earth-to-Mars flight time by 15 minutes and brings the spacecraft
closer to the planet.  On its pre-maneuver trajectory, Mariner 7
would have passed Mars at a distance of 16,500 miles at 10:20 p.m.
PDT on August 4, 1969.  The new arrival time is 10:05 p.m. at a
distance of about 2000 miles.
          Maneuver commands, prepared at the Space Flight
Operations Facility at JPL, were transmitted from the Deep Space
Network station at Woomera, Australia.  The DSN station at
Johannesburg, South Africa, also had the spacecraft in view and
both stations monitored the performance of the maneuver.
          Mariner 7 was launched March 27 from Cape Kennedy.  It
carries an array of science instruments to obtain high resolution

                               -2-

television pictures of Mars and measure surface temperatures and
atmospheric composition and density.  Mariner 7 will conduct its
experiments in the southern hemisphere of the planet, ioncluding
a portion of the south polar cap.
          Mariner 6, an identical twin, is now in its 43rd day of
flight and is more than seven million miles from Earth.  Mariner 6
will arrive at Mars at 10:18 p.m. PDT on July 30, with instruments
conducting similar experiments near the Martian equator.

                               ###





OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE  [phone]

FOR IMMEDIATE RELEASE                            April 25, 1969

          A series of radio commands has been transmitted to
Mariner 6--one of two U.S. spacecraft on the way to Mars--to alter
the method of spacecraft stabilization as Mariner flys past the
planet this summer.
          Transmitted as a precautionary measure because of a
possible problem with the spacecraft receiver and a known problem
with the Canopus sensor, the commands programmed the on-board
computer to turn on the gyroscopes automatically for the science-
taking phase of the mission.
          Using the gyros to provide a stable platform for
pointing the science instruments at Mars and the high-gain
antenna toward Earth is an alternate attitude stabilization
procedure.  Normally three-axis stabilization is provided by
optical sensors that lock onto the sun and the star Canopus.
          According to the Mariner Project manager, H. M. (Bud)
Schurmeier, of the Jet Propulsion Laboratory, the Canopus sensor
was "distracted" by tiny bright dust particles apparently shaken
off the spacecraft both at the time of the mid-course maneuver
and when Mariner 6's scan platform was unlatched.  Both events
correlate with the firing of explosive devices.
          When the platform was unlatched, the spacecraft was
locked onto Canopus.  The particles caused Mariner to go into a

                               -2-

roll search for the star and Canopus was reacquired properly.  At
the time of the maneuver, the spacecraft was on autopilot-gyro
control and the particles were seen but ignored.
          Another squib-firing event is associated with the Mars
encounter sequence and could cause Mariner 6 to begin a star
search at the last moment if particles are again in view of the
sensor while it is locked onto a star.
          Mariner 7, which will reach Mars five days after
Mariner 6, also experienced the dust particle problem when it
executed a mid-course maneuver.  Mariner 7, however, is still
programmed for the normal sun-Canopus attitude control scheme,
but will be reprogrammed for gyroscopes at a later date.
          The commands to Mariner 6 were sent early in the
mission because of a recent change in the telemetry reading from
the radio receiver indicating the possibility that at a later
date the spacecraft might not accept commands from the ground.
The problem, however, has not recurred.
          The Canopus sensor on Mariner 6 also has failed to
accept a "look angle" update command, one of a series issued
periodically by the spacecraft to keep the star within the field
of view of the sensor.  Backup commands from the ground indicated
the sensor can be switched to only two of its five view positions.
          An analysis of other stars that can be used as a roll
reference when Canopus is out of view is underway at JPL.  Because
it is now planned to complete the Mars passage on inertial con-
trol, with the gyros, a star reference is necessary only to
establish the initial roll position for the gyros.

                               -3-

          Mariner 6 was launched from Cape Kennedy on February 24
and will arrive at Mars at about 10:18 p.m. PDT on July 30.
Mariner 7 was launched on March 27 and makes its closest approach
to the planet at about 10 p.m. PDT on August 4.  Bh Mariners
will pass Mars at a lowest altitude of about 2000 miles.
   ow in its 59th day of flight, Mariner 6 is more than
10 million miles from Earth and about 50 million miles from Mars.
Mariner 7 is 6.7 million miles from Earth and 53.8 million miles
from Mars.

                               ###





OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE  [phone]

FOR RELEASE:  SUNDAY, JUNE 1, 1969

          Two Mariner spacecraft enroute to Mars logged a combined
total of 272,363,358 miles of space travel at noon today with 167
million miles--and 8 weeks--to go to Mars encounter.
          Mariner VI is 21,731,091 miles from Earth and will fly
past Mars--at its point of closest approach--at 10:18 p.m., PDT,
July 30, after traveling 241,838,160 miles since launch last
February 24.
          Mariner VII is 19,526,893 miles from Earth and will fly
past Mars at 10:00 p.m., PDT, August 4.  Its total travel
distance will be 197,137,830 miles.  At noon today it had covered
110,808,438 miles since launch on March 27.
          The distance from Mars to Earth at encounter will be
approximately 60 million miles.
          Each of the 850 pound spacecraft are identically equip-
ped with two television cameras to photograph Mars at medium and
high resolution and with instruments to measure the temperature
of the surface and to determine the composition of the upper and
lower atmosphere.  Atmospheric density will also be measured by
transmitting the spacecraft radio signal through the atmosphere
of Mars as the spacecraft curves behind the planet.
          The first data on Mars will be transmitted to Earth on
the night of July 29 in the form of 33 pictures of the full disc
of the planet taken by Mariner VI during its approach to Mars.

                               -2-

          The pictures will be stored on a tape recorder in the
spacecraft and played back when Mariner VI is over the 210-foot
diameter communications antenna at the Goldstone station of the
Deep Space Net in the California Mojave desert.  (Additional
pictures are then recorded on the tape.)  Use of the sensitive
210-foot antenna will allow transmission of one picture each five
minutes at a rate of 16,200 bits per second.  By comparison it
required 8 1/2 hours to transmit each picture from Mars in 1965.
          The two spacecraft are programed to return a total of
141 pictures of the disc of Mars in five transmission sessions as
they approach Mars.  When the spacecraft fly past Mars the tape
recorders will then store 24 surface pictures for each spacecraft
and science data from the other instruments.  These will be
transmitted to Earth when the spacecraft have gone beyond Mars.
          The 1965 pictures contained 240,000 bits each compared
with 3,900,000 bits for the pictures expected from the current
missions.


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