FLIGHT INTO SPACE
With the Mariner R Project officially activated in the fall of 1961 and
the launch vehicles selected, engineers proceeded at full speed to meet
the difficult launch schedule.
A preliminary design was adopted in late September, when the scientific
experiments to be carried on board were also selected. By October 2, a
schedule had been established that would deliver two spacecraft to the
assembly building in Pasadena by January 15 and 29, 1962, respectively,
with the spares to follow in two weeks.
During the week of November 6, tests were underway to determine problems
involved in mating a mock-up of the spacecraft with the Agena shroud and
adapter assembly. A thermal control model of the spacecraft had already
gone into the small space simulator at JPL for preliminary temperature
tests.
MR-1, the first Mariner scheduled for flight, was in assembly
immediately after January 8, 1962, and the process was complete by the
end of the month, when electrical and magnetic field tests had been
started. At the same time, assembly of MR-2 was underway. Work on MR-1
was a week ahead of schedule by the end of the month.
A full-scale temperature control model of the spacecraft went into the
large space simulator on February 26. In mid-March, system tests began
on both spacecraft and it was decided that the flight hardware would be
tested only in the small simulator, with the temperature control model
continuing in the large chamber.
[Illustration: _Technician wears hood and protective goggles while
working on Mariner spacecraft in Space simulator chamber at Jet
Propulsion Laboratory, Pasadena._]
On March 26, MR-1 was subjected to full-scale mating tests with the
shroud (cover) and the adapter for mounting the spacecraft on top of the
Agena. MR-2 was undergoing vibration tests during the week of April 16.
By April 30, MR-1 had completed vibration tests and had been mapped for
magnetic fields so that, once compensated for, they would not interfere
with the magnetometer experiment in space.
A dummy run of MR-1 was conducted on May 7 and the spacecraft, space
flight center, and computing equipment were put through a simulated
operations test run during the same week.
By May 14, clean-up and final inspection by microscope had begun on
MR-1, MR-2, and MR-3 (the latter spacecraft had been assembled from the
spares). Soon after, the first two van loads of equipment were shipped
to Cape Canaveral. The final system test of MR-1 was completed on May 21
and the test of MR-2 followed during the same week.
During the week of May 28, all three spacecraft and their associated
ground support equipment were packed, loaded, and shipped to the
Atlantic Missile Range (AMR). At the same time, the Atlas designated to
launch MR-1 went aboard a C-133 freight aircraft at San Diego. On the
same day, an Air Force order grounded all C-133’s for inspection and the
plane did not depart until June 9.
By June 11, 1962, the firing dates had been established and both
spacecraft were ready for launching. The Atlas booster had already been
erected on the launch pad. The dummy run and a joint flight acceptance
test were completed on MR-1 during the week of July 2. Final flight
preparations and system test of MR-1 and the system test of MR-2 were
concluded a week later.
Thus, in 324 days, a new spacecraft project had been activated; the
design, assembly, and testing had been completed; and the infinite
number of decisions pertaining to launch, AMR Range Operations,
deep-space tracking, and data processing activities had been made and
implemented.
Venus was approaching the Earth at the end of its 19-month excursion
around the Sun. The launch vehicles and Mariners I and II stood ready to
go from Canaveral’s Launch Complex 12. The events leading to the first
close-up look at Venus and intervening space were about to reach their
first crisis: a fiery explosion over the Atlantic Ocean.
MARINER I: AN ABORTIVE LAUNCH
After 570 hours of testing, Mariner I was poised on top of the
Atlas-Agena launch vehicle during the night of July 20, 1962. The time
was right, the Range and the tracking net were standing by, the launch
vehicles were ready to cast off the spacecraft for Venus.
[Illustration: _Atlas for launching Mariner II arrives at Cape
Canaveral in C-133 aircraft._]
The countdown was begun at 11:33 p.m., EST, July 20, after several
delays because of trouble in the Range Safety Command system. At the
time, the launch count stood at T minus 176 minutes—if all went well,
176 minutes until the booster engines were ignited.
Another hold again delayed the count until 12:37 a.m., July 21, when
counting was resumed at T minus 165 minutes. The count then proceeded
without incident to T minus 79 minutes at 2:20 a.m., when uncertainty
over the cause of a blown fuse in the Range Safety circuits caused the
operations to be “scrubbed” or cancelled for the night. The next launch
attempt was scheduled for July 21-22.
The second launch countdown for Mariner I began shortly before midnight,
July 21. Spacecraft power had been turned on at 11:08 p.m., with the
launch count at T minus 200 minutes. At T minus 135 minutes, the weather
looked good. A 41-minute hold was required at minus 130 minutes (12:17
a.m., July 22) in order to change a noisy component in the ground
tracking system.
When counting was resumed at T minus 130 minutes, the clock read 12:48
a.m. A previously scheduled hold was called at T minus 60 minutes,
lasting from 1:58 to 2:38 a.m. The good weather still held.
At T minus 80 seconds, power fluctuations in the radio guidance system
forced a 34-minute hold. Time was resumed at 4:16 a.m., when the
countdown was set back to T minus 5 minutes.
At exactly 4:21.23 a.m., EST, the Atlas thundered to life and lifted off
the pad, bearing its Venus-bound load. The boost phase looked good until
the Range Safety officer began to notice an unscheduled yaw-left
(northeast) maneuver. By 4:25 a.m., it was evident that, if allowed to
continue, the vehicle might crash in the North Atlantic shipping lanes
or in some inhabited area. Steering commands were being supplied but
faulty application of the guidance equations was taking the vehicle far
off course.
Finally, at 4:26.16 a.m., after 293 seconds of flight and with just 6
seconds left before separation of the Atlas and Agena—after which the
launch vehicle could not be destroyed—a Range Safety officer hit the
“destruct” button.
A flash of light illuminated the sky and the choppy Atlantic waters were
awash with the glowing death of a space probe. Even as it fluttered down
to the sea, however, the radio transponder of the shattered Mariner I
continued to transmit for 1 minute and 4 seconds after the destroy
command had been sent.
Mariner I did not succumb easily.
MARINER II: A ROLL BEFORE PARKING
Ever since Mariner II had arrived at the Cape on June 4, test teams of
all organizations had labored day and night to prepare the spacecraft
for launch. The end of their efforts culminated after some 690 hours of
test time, both in California and in Florida.
Thirty-five days after Mariner I met its explosive end, the first
countdown on Mariner II was underway. At 6:43 p.m., EST, August 25,
1962, time was picked up. The countdown did not proceed far, however.
The Atlas crew asked for a hold at T minus 205 minutes (8:39 p.m.)
because of stray voltages in the command destruct system caused by a
defective Agena battery. After considerable delay, the launch effort was
scrubbed at 10:06 p.m.
[Illustration: _Two assembly operations and system checkouts are
performed separated by a trip to the pad to verify compatibility
with the launch vehicle_]
[Illustration: _A complete electronic checkout station in the hangar
supports the spacecraft to ensure operability_]
[Illustration: _Mariner takes form as the solar panels are attached
and the final hangar checkout operations are performed before the
launch._]
[Illustration: _Wrapped in a dust cover, the spacecraft is
transferred from Hangar AE at AMR to the explosive safe area for
further tests._]
[Illustration: _Inside the bunker-like explosive safe area, the
powerful midcourse maneuver rocket engine is installed in the center
of the spacecraft._]
[Illustration: _Final assembly and inspection complete, Mariner is
“canned” in the nose shroud that will protect it through the Earth’s
atmosphere and into space._]
[Illustration: _At the pad, the shrouded spacecraft is lifted past
the Atlas ..._]
[Illustration: _... and the Agena._]
[Illustration: _Twelfth floor: Mariner reaches its mating level._]
[Illustration: _The spacecraft is eased over to the top of the Agena
..._]
[Illustration: _... and carefully mated to it._]
The second launch attempt started at 6:37 p.m., August 26, with the
Atlas-Agena B and Mariner II ready on the pad. At 9:52 p.m., T minus 100
minutes, a 40-minute hold was called to replace the Atlas main battery.
By 10:37, with 95 minutes to launch, all spacecraft systems were ready
to go.
A routine hold at T minus 60 minutes was extended beyond 30 minutes in
order to verify the spacecraft battery life expectation. At 11:48 p.m.,
with the count standing at T minus 55 minutes, the spacecraft, the
vehicles, the Range, and the DSIF were all given the green light.
When good launching weather was reported at 12:18 a.m., August 27, just
25 minutes from liftoff, a cautious optimism began to mount in the
blockhouse and among the tired crews.
But the tension began to build again. The second prescheduled hold at T
minus 5 minutes was extended beyond a half-hour when the radio guidance
system had difficulty with ground station power. Counting was “picked
up” and the clock continued to move down to 60 seconds before liftoff.
Suddenly, the radio guidance system was in trouble again. Fluctuations
showed in its rate beacon signals, and another hold was called. Still
another hold for the same reason followed at T minus 50 seconds. This
time, at 1:30 a.m., the count was set back to T minus 5 minutes.
One further crisis developed during this hold—only 3 minutes of
pre-launch life remained in Atlas’ main battery. A quick decision was
made to hold the switchover to missile power until T minus 60 seconds to
help conserve the life of the battery.
At 1:48 a.m., the count was resumed again at T minus 5 minutes. The long
seconds began to drag. Finally, the Convair test director pressed the
fire button.
Out on the launch pad, the Atlas engines ignited with a white puff and
began to strain against the retaining bolts as 360,000 pounds of thrust
began to build up. In a holocaust of noise and flame, the Atlas was
released and lifted off the launch pad on a bearing of 106.8 degrees at
exactly 1 hour, 53 minutes, 13.927 seconds in the morning of August 27,
1962.
Mariner II was on its way to listen to the music of the spheres.
As the launch vehicle roared up into the night sky, the JPL Launch
Checkout Station (DSIF O) tracked the spacecraft until Mariner
disappeared over the horizon. A quick, preliminary evaluation of
spacecraft data showed normal readings and Atlas seemed to be flying a
true course. The AMR in-flight data transmission and computational
operations were being performed as expected. With liftoff out of the
way, the launch began to look good.
After the radio signal from the ground guidance system cut off the
engines and the booster section was jettisoned, the remaining Atlas
forward section, plus the Agena and the spacecraft began to roll.
However, it stabilized itself in a normal attitude. Although the Atlas
had not gone out of the Range Safety restrictions, it was within just 3
degrees of exceeding the Agena horizon sensor limits, which would have
forced another aborted mission.
After the booster separation, the Atlas sustainer and vernier engines
continued to burn until they were shut off by radio guidance command.
Shortly thereafter, spring-loaded bolts ejected the nose-cone shroud
which had protected the spacecraft against frictional heating in the
atmosphere. Simultaneously, the gyroscopes in the Agena were started
and, at about 1:58 a.m., the Agena and the spacecraft separated from the
now-spent Atlas, which was retarded by small retro-rockets and drifted
back into the atmosphere, where it was destroyed by friction on reentry.
THE PARKING ORBIT
As the Agena separated from the Atlas booster vehicle, it was programmed
to pitch down almost 15 degrees, putting it roughly parallel with the
local horizon. Then, following a brief coasting period, the Agena engine
ignited at 1:58.53 a.m. and fired until 2:01.12 a.m. Cut-off occurred at
a predetermined value of velocity. Both the Agena and the spacecraft had
now reached a speed of approximately 18,000 miles per hour and had gone
into an Earth orbit at an altitude of 116.19 statute miles.
The second stage and the spacecraft were now in a “parking orbit,” which
would allow the vehicle to coast out to a point more favorable than Cape
Canaveral for blasting off Mariner for Venus.
During the launch, Cape radar had tracked the radar beacon on the Agena,
losing it on the horizon at 2:00.53 a.m. Radar stations at Grand Bahama
Island, San Salvador, Ascension, the Twin Falls Victory ship, and
Pretoria (in South Africa) continued to track down range. Meanwhile,
Antigua had “locked on” and tracked the spacecraft’s radio transponder
and telemetry from 1:58 to 2:08 a.m. when it went over the Antigua
horizon.
[Illustration: _Mariner II is accelerated to Earth-escape velocity
and out of orbit near St. Helena. Rotation of earth causes flight
path to appear to double back to west over Africa._]
[Illustration: _The sequence of events in the launch phase of the
Mariner flight to Venus._]
EVENT
1. LIFTOFF
2. ATLAS BOOSTER ENGINE CUTOFF
3. ATLAS SUSTAINER ENGINE CUTOFF
4. ATLAS VERNIER ENGINE CUTOFF
5. SPACECRAFT SHROUD EJECTION
6. ATLAS-AGENA B SEPARATION
7. AGENA B FIRST IGNITION
8. AGENA B FIRST CUTOFF
9. AGENA B SECOND IGNITION
10. AGENA B SECOND CUTOFF
11. SPACECRAFT SEPARATION
12. INITIATE AGENA YAW MANEUVER
13. COMPLETE AGENA YAW MANEUVER
14. EXPEL UNUSED AGENA PROPELLANT
The second coasting period lasted 16.3 minutes, a time determined by the
ground guidance computer and transmitted to the Agena during the vernier
burning period of Atlas. Then, Agena restarted its engine and fired for
a second time. At the end of this firing period, both the Agena and
Mariner, still attached, had been injected into a transfer trajectory to
Venus at a velocity exceeding that required to escape from the Earth’s
gravity.
The actual injection into space occurred at 26 minutes 3.08 seconds
after liftoff from the Cape (2:19.19 a.m., EST) at a point above 14.873
degrees south latitude and 2.007 degrees west longitude. Thus, Mariner
made the break for Venus about 360 miles northeast of St. Helena, 2,500
miles east of the Brazilian coast, and about 900 miles west of Angola on
the west African shore.
During injection, the vehicle was being tracked by Ascension, telemetry
ship Twin Falls Victory, and Pretoria. Telemetry ship Whiskey secured
the spacecraft signal just after injection and tracked until 2:26 a.m.
Pretoria began its telemetry track at 2:21 and continued to track for
almost two hours, until 4:19 a.m.
Injection velocity was 7.07 miles per second or 25,420 miles per hour,
just beyond Earth-escape speed. The distance at the time of injection
from Canaveral’s Launch Complex 12 was 4,081.3 miles.
The Agena and Mariner flew the escape path together for another two
minutes after injection before they were separated at 2:21 a.m. Agena
then performed a 140-degree yaw or retro-turn maneuver by expelling
unused propellants. The purpose was to prevent the unsterilized Agena
from possibly hitting the planet, and from following Mariner too closely
and perhaps disturbing its instruments.
Now, Mariner II was flying alone and clear. Ahead lay a journey of 109
days and more than 180 million miles.
ORIENTATION AND MIDCOURSE MANEUVER
As Mariner II headed into space, the Deep Space Instrumentation Facility
(DSIF) network began to track the spacecraft. At 2:23.59 a.m., DSIF 5 at
Johannesburg, aided by the Mobile Tracking Station, installed in vans in
the vicinity, was “looking” at the spacecraft, just four minutes after
injection.
Johannesburg was able to track Mariner until 4:04 p.m. because, as the
trajectory took Mariner almost radially away from the Earth, our planet
began in effect to turn away from under the spacecraft. On an Earth map,
because of its course and the rotation of the Earth, Mariner II appeared
to describe a great arc over the Indian Ocean far to the west of
Australia, then to turn north and west and to proceed straight west over
south-central Africa, across the Atlantic, and over the Amazon Basin of
northern South America. Johannesburg finally lost track at a point over
the middle of South America.
While swinging over the Indian Ocean on its first pass, the spacecraft
was acquired by Woomera’s DSIF 4 at 2:42.30 a.m., and tracked until 8:08
a.m., when Mariner was passing just to the north of Madagascar on a
westerly course. Goldstone did not acquire the spacecraft until it was
approaching the east coast of South America at 3:12 p.m., August 27.
With Mariner slowly tumbling in free space, it was now necessary to
initiate a series of events to place the spacecraft in the proper flight
position. At 2:27 a.m., 44 minutes after launch, the Mariner Central
Computer and Sequencer (CC&S) on board the spacecraft issued a command
for explosively activated pin pullers to release the solar panels and
the radiometer dish from their launch-secured positions. At 2:53, 60
minutes after liftoff, the attitude control system was turned on and the
Sun orientation sequence began with the extension of the directional
antenna to a preset angle of 72 degrees.
[Illustration: _Mariner II was launched while Venus was far behind
the Earth. During the 109-day flight, Venus overtook and passed the
Earth. It rendezvoused with the spacecraft at a point about
36,000,000 miles from the Earth._]
[Illustration: _During the midcourse maneuver, the trajectory of
Mariner II was corrected so that the spacecraft would approach
within 21,598 miles of Venus._]
ROLL MANEUVER ANTENNA UP
PITCH MANEUVER
MOTOR BURN
SUN REAQUISITION ANTENNA REPOSITION
EARTH REAQUISITION
The Sun sensors then activated the gas jets and moved the spacecraft
about until the roll or long axis was pointed at the Sun. This maneuver
required only 2½ minutes after the CC&S issued the command. The solar
panel power output of 195 watts was somewhat higher than anticipated, as
were the spacecraft temperatures, which decreased and stabilized six
hours after the spacecraft oriented itself on the Sun.
On August 29, a command from Johannesburg turned on the cruise
scientific experiments, including all the instruments except the two
radiometers. The rate of data transmission was then observed to decrease
as planned and the data conditioning system was functioning normally.
For seven days, no attempt was made to orient the spacecraft with
respect to the Earth because the Earth sensors were too sensitive to
operate properly at such a close range. On September 3, the CC&S
initiated the Earth acquisition sequence. The gyroscopes were turned on,
the cruise scientific instruments were temporarily switched off, and a
search for the Earth began about the roll axis of the spacecraft.
During this maneuver, the long axis of the spacecraft was held steady in
a position pointing at the Sun and the gas jets rolled the spacecraft
around this axis until the sensors, mounted in the directional antenna,
could “see” the Earth. Apparently, the Earth sensor was already viewing
the Earth because the transmitter output immediately switched from the
omni- to the directional antenna, indicating that no search was
necessary.
However, the initial brightness reading from the Earth sensor was 38, an
intensity that might be expected if the spacecraft were locked onto the
Moon instead of the Earth. As a result, the midcourse maneuver was
delayed until verification of Earth lock was obtained.
Mariner’s injection into the Venus trajectory yielded a predicted miss
of 233,000 miles in front of the planet, well within the normal miss
pattern expected as a result of the launch. Because the spacecraft was
designed to cross the orbit of Venus behind the planet and pass between
it and the Sun, it was necessary to correct the trajectory to an
approximate 8,000- to 40,000-mile “fly-by” so the scientific instruments
could operate within their design ranges.
After comparison of the actual flight path with that required for a
proper near-miss, the necessary roll, pitch, and motor-burn commands
were generated by the JPL computers. When, on September 4, it had been
established that the spacecraft was indeed oriented on the Earth and not
the Moon, a set of three commands was transmitted to the spacecraft from
Goldstone, to be stored in the electronic “memory unit” until the start
command was sent.
At 1:30 p.m., PST, the first commands were transmitted: a 9.33-degree
roll turn, a 139.83-degree pitch turn, and a motor-burn command to
produce a 69.5-mile-per-hour velocity change.
At 2:39 p.m., a fourth command was sent to switch from the directional
antenna to the omni-antenna. Finally, a command went out instructing the
spacecraft to proceed with the now “memorized” maneuver program.
Mariner then turned off the Earth and Sun sensors, moved the directional
antenna out of the path of the rocket exhaust stream, and executed a
9.33-degree roll turn in 51 seconds.
Next, the pitch turn was completed in 13¼ minutes, turning the
spacecraft almost completely around so the motor nozzle would point in
the correct direction when fired.
The spacecraft was stabilized and the roll and pitch turns controlled by
gyroscopes, which signalled the attitude control system the rate of
correction for comparison with the already computed values.
With the solar panels no longer directly oriented on the Sun, the
battery began to share the power demand and finally carried the entire
load until the spacecraft had again been oriented on the Sun.
At the proper time, the motor—controlled by the CC&S—ignited and burned
for 27.8 seconds, while the spacecraft’s acceleration was compared with
the predetermined values by the accelerometer. During this period, when
the gas jets could not operate properly, the spacecraft was stabilized
by movable vanes or rudders in the exhaust of the midcourse motor.
The velocity added by the midcourse motor resulted in a decrease of the
relative speed of the spacecraft with respect to the Earth by 59 miles
per hour (from 6,748 to 6,689 miles per hour), while the speed relative
to the Sun increased by 45 miles per hour (from 60,117 to 60,162 miles
per hour).
This apparently paradoxical condition occurred because, in order to
intercept Venus, Mariner had been launched in a direction opposite to
the Earth’s course around the Sun. The midcourse maneuver turned the
spacecraft around and slowed its travel away from the Earth while
allowing it to increase its speed around the Sun in the direction of the
Earth’s orbit. Gradually, then, the spacecraft would begin to fall in
toward the Sun while moving in the same direction as the Earth, catching
and passing the Earth on the 65th day and intersecting Venus’ orbit on
the 109th day.
At the time of the midcourse maneuver, the spacecraft was travelling
slightly inside the Earth’s orbit by 70,000 miles, and was behind the
Earth by 1,492,500 miles.
THE LONG CRUISE
After its completion of the midcourse maneuver, Mariner reoriented
itself on the Sun in 7 minutes and on the Earth in about 30 minutes.
During the midcourse maneuver, the omnidirectional antenna was used;
now, with the maneuver completed, the directional antenna was switched
back in for the duration of the mission.
Ever since the spacecraft had left the parking orbit near the Earth and
been injected into the Venus trajectory, the Space Flight Operations
Center back in Pasadena had been the nerve center of the mission.
Telemetered data had been coming in from the DSIF stations on a 24-hour
schedule. During the cruise phase, from September 5 to December 7, a
total of 16 orbit computations were made to perfect the planet encounter
prediction. On December 7, the first noticeable Venus-caused effects on
Mariner’s trajectory were observed, causing a definite deviation of the
spacecraft’s flight path.
On September 8, at 12:50 p.m., EST, the spacecraft lost its attitude
control, which caused the power serving the scientific instruments to
switch off and the gyroscopes to switch on automatically for
approximately three minutes, after which normal operation was resumed.
The cause was not apparent but the chances of a strike by some small
space object seemed good.
As a result of this event, a significant difference in the apparent
brightness reading of the Earth sensor was noted. This sensor had been
causing concern for some time because its readings had decreased to
almost zero. Further decrease, if actually caused by the instrument and
not by the telemetry sensing elements, could result in loss of Earth
lock and the failure of radio contact.
After the incident of September 8, the Earth sensor brightness reading
increased from 6 to 63, a normal indication for that day. Thereafter,
this measurement decreased in an expected manner as the spacecraft
increased its distance from the Earth.
Mariner II was now embarked on the long cruise. On September 12, the
distance from the Earth was 2,678,960 miles and the spacecraft speed
relative to the Earth was 6,497 miles per hour. Mariner was accelerating
its speed as the Sun’s gravity began to exert a stronger pull than the
Earth’s. On October 3, Mariner was nearly 6 million miles out and moving
at 6,823 miles per hour relative to the Earth. A total of 55,600,000
miles had been covered to that point.
Considerable anxiety had developed at JPL when Mariner’s Earth sensor
reading had fallen off so markedly. This situation was relieved by the
unexplained return to normal on September 8, although the day-to-day
change in the brightness number was watched closely. The apparent
ability of the spacecraft to recover its former performance after the
loss of attitude control on September 8 and again on September 29 was an
encouraging sign.
Another disturbing event occurred on October 31, when the output from
one solar panel deteriorated abruptly. The entire power load was thrown
on the other panel, which was then dangerously near its maximum rated
output. To alleviate this situation, the cruise scientific instruments
were turned off. A week later, the malfunctioning panel returned to
normal operation and the science instruments were again turned on.
Although the trouble had cleared temporarily, it developed again on
November 15 and never again corrected itself. The diagnosis was a
partial short circuit between one string of solar cells and the panel
frame, but by now the spacecraft was close enough to the Sun so that one
panel supplied enough power.
By October 24, the spacecraft was 10,030,000 miles from the Earth and
was moving at 10,547 miles per hour relative to the Earth. The distance
from Venus was now 21,266,000 miles.
October 30 was the 65th day of the mission and at 5 a.m., PST, Mariner
overtook and passed the Earth at a distance of 11,500,000 miles. Since
the spacecraft’s direction of travel had, in effect, been reversed by
the midcourse maneuver, it had been gaining on the Earth in the
direction of its orbit, although constantly falling away from the Earth
in the direction of the Sun.
The point of equal distance between the Earth and Venus was passed on
November 6, when Mariner was 13,900,000 miles from both planets and
travelling at 13,843 miles per hour relative to the Earth. As November
wore on, hope for a successful mission began to mount. Using tracking
data rather than assumptions of standard midcourse performance, the
Venus miss distance had now been revised to about 21,000 miles and
encounter was predicted for December 14. But the DSIF tracking crews,
the space flight and computer operators, and the management staff could
not yet relax. The elation following the successful trajectory
correction maneuver on September 4 had given way alternately to
discouragement and guarded optimism.
Four telemetry measurements were lost on December 9 and never returned
to normal. They measured the angle of the antenna hinge, the fuel tank
pressure, and the nitrogen pressure in the midcourse and attitude
control systems. A blown fuse, designed to protect the data encoder from
short circuits in the sensors, was suspected. However, these channels
could not affect spacecraft operation and Mariner continued to perform
normally.
The rising temperatures recorded on the spacecraft were more serious.
Only the solar panels were displaying expected temperature readings;
some of the others were as much as 75 degrees above the values predicted
for Venus encounter. The heat increase became more rapid after November
20. By December 12, six of the temperature sensors had reached their
upper limits. It was feared that the failure point of the equipment
might be exceeded.
The CC&S performed without incident until just before encounter, when,
for the first time, it failed to yield certain pulses. JPL engineers
were worried about the starting of the encounter sequence, due the next
day, although they knew that Earth-based radio could send these
commands, if necessary.
On December 12, with the climax of the mission near, the spacecraft was
34,218,000 miles from the Earth, with a speed away from the Earth of
35,790 miles per hour, a Sun-relative speed of 83,900 miles per hour.
Only 635,525 miles from Venus at this point, Mariner II was closing fast
on the cloud-shrouded planet. But it was a hot spacecraft that was
carrying its load of inquisitive instruments to the historic encounter.
ENCOUNTER AND BEYOND
On its 109th day of travel, Mariner had approached Venus in a precarious
condition. Seven of the over-heated temperature sensors had reached
their upper telemetry limits. The Earth-sensor brightness reading stood
at 3 (0 was the nominal threshold) and was dropping. Some 149 watts of
power were being consumed out of the 165 watts still available from the
crippled solar panels.
At JPL’s Space Flight Operations Center, there was reason to believe
that the ailing CC&S might not command the spacecraft into its encounter
sequence at the proper time. Twelve hours before encounter, these fears
were verified.
Quickly, the emergency Earth-originated command was prepared for
transmission. At 5:35 a.m., PST, a radio signal went out from
Goldstone’s Echo Station. Thirty-six million miles away, Mariner II
responded to the tiny pulse of energy from the Earth and began its
encounter sequence.
After Mariner had “acknowledged” receipt of the command from the Earth,
the spacecraft switched into the encounter sequence as engineering data
were turned off and the radiometers began their scanning motion, taking
up-and-down readings across the face of the planet. As throughout the
long cruise, the four experiments monitoring the magnetic fields, cosmic
dust, charged particles, and solar plasma experiments continued to
operate.
[Illustration: _Mariner II approached Venus from the dark side,
crossed between the planet and the Sun while making three radiometer
scans of the disk._]
As Mariner approached Venus on its night side, it was travelling about
88,400 miles per hour with respect to the Sun. At the point of closest
approach, at 11:59.28 a.m., PST, the distance from the planet was 21,598
miles.
During encounter with Venus, three scans were made: one on the dark
side, one across the terminator dividing dark and sunlit sides, and one
on the sunlit side. Although the scan went slightly beyond the edge of
the planet, the operation proceeded smoothly and good data were received
on the Earth.
With encounter completed, the cruise condition was reestablished by
radio command from the Earth and the spacecraft returned to transmitting
engineering data, together with the continuing readings of the four
cruise scientific experiments.
After approaching closer to a planet and making more meaningful
scientific measurements than any man-made space probe, Mariner II
continued on into an orbit around the Sun.
December 27, 13 days after Venus encounter, marked the perihelion, or
point of Mariner’s closest approach to the Sun: 65,505,935 miles. The
Sun-related speed was 89,442 miles per hour. As Mariner began to pull
away from the Sun in the following months, its Sun-referenced speed
would decrease.
Data were still being received during these final days and the Earth and
Sun lock were still being maintained. Although the antenna hinge angle
was no longer being automatically readjusted by the spacecraft, commands
were sent from the Earth in an attempt to keep the antenna pointed at
the Earth, even if the Earth sensor were no longer operating properly.
At 2 a.m., EST, January 3, 1963, 20 days after passing Venus, Mariner
finished transmitting 30 minutes of telemetry data to Johannesburg and
the station shut down its operation. When Woomera’s DSIF 4 later made a
normal search for the spacecraft signal, it could not be found.
Goldstone also searched in vain for the spacecraft transmissions, but
apparently Mariner’s voice had at last died, although the spacecraft
would go into an eternal orbit around the Sun.
It was estimated that Mariner’s aphelion (farthest point out) in its
orbit around the Sun would occur on June 18, 1963, at a distance of
113,813,087 miles. Maximum distance from the Earth would be 98,063,599
miles on March 30, 1963; closest approach to the Earth: 25,765,717 miles
on September 27, 1963.
THE RECORD OF MARINER
The performance record of Mariner II exceeded that of any spacecraft
previously launched from Earth:
—It performed the first and most distant trajectory-correcting maneuver
in deep space, firing a rocket motor at the greatest distance from the
Earth: 1,492,000 miles (September 4, 1962).
—The spacecraft transmitted continuously for four months, sending back
to the Earth some 90 million bits of information while using only 3
watts of transmitted power.
—Useful telemetry measurements were made at another record distance from
the Earth: 53.9 million miles (January 3, 1963).
—Mariner II was the first spacecraft to operate in the immediate
vicinity of another planet and return useful scientific information to
the Earth: approximately 21,598 miles from Venus (December 14, 1962).
—Measurements were made closest to the Sun: 65.3 million miles away
(December 27, 1962).
—Mariner’s communication system operated for the longest continuous
period in interplanetary space: 129 days (August 27, 1962, to January
3, 1963).
—Mariner achieved the longest continuous operation of a spacecraft
attitude-stabilization system in space, and at a greater distance from
the Earth than any previous spacecraft: 129 days (August 27, 1962, to
January 3, 1963), at 53.9 million miles from the Earth.