By Marty McDowell/NASA
On November 3, 1973, the United States launched Mariner 10. Mariner 10,
though launched after Pioneer 11, was the
first craft to successfully complete multiple planetary flybys. It was
also the first to encounter Mercury.
Mariner 10 was the seventh successful launch in the Mariner series, the
first spacecraft to use the gravitational pull of one planet (Venus) to
reach another (Mercury), and the first spacecraft mission to visit two
planets. Mariner 10 was the first (and as of 2003 the only) spacecraft to
visit Mercury. The spacecraft flew by Mercury three times in a retrograde
heliocentric orbit and returned images and data on the planet. Mariner 10
returned the first-ever close-up images of Venus and Mercury. The primary
scientific objectives of the mission were to measure Mercury's
environment, atmosphere, surface, and body characteristics and to make
similar investigations of Venus. Secondary objectives were to perform
experiments in the interplanetary medium and to obtain experience with a
dual-planet gravity-assist mission.
Spacecraft and Subsystems
The spacecraft structure was an eight-sided forger magnesium framework
with eight electronics compartments. It measured 1.39 m diagonally and
0.457 m in depth. Two solar panels, each 2.69 m long and 0.97 m wide, were
attached at the top, supporting 5.1 sq m of solar cell area. Fully
deployed the spacecraft measured 8.0 m across the solar panels and 3.7 m
from the top of the low-gain antenna to the bottom of the heat-shield. A
scan platform with two degrees of freedom was mounted on the anti-sunward
face. A 5.8 m long hinged magnetometer boom extended from one of the
octagonal sides of the body. Total launch mass was 502.9 kg, of this 29 kg
were propellant and attitude control gas. The total mass of instruments
onboard was 79.4 kg.
The rocket engine was a 222-N liquid monopropellant hydrazine motor
situated below a spherical propellant tank which was mounted in the center
of the framework. The nozzle protruded through a sunshade. Two sets of
three pairs of orthogonal reaction nitrogen gas jets, mounted on the tips
of the solar panels, were used to stabilize the spacecraft on three axes.
Command and control were the responsibility of an on-board computer with a
512 word memory augmented by ground commands
Mariner 10 carried a motor driven high-gain dish antenna, a 1.37 m
diameter aluminum honeycomb-disk parabolic reflector, which was mounted on
a boom on the side of the spacecraft. A low-gain omnidirectional antenna
was mounted at the end of a 2.85 m boom extending from the anti-solar face
of the spacecraft. Feeds enabled the spacecraft to transmit at S- and
X-band frequencies, data could be transmitted at a maximum rate of 117.6
kilobits per second. The spacecraft carried a Canopus star tracker,
located on the upper ring structure of the octagonal satellite, and
acquisition sun sensors on the tips of the solar panels. The interior of
the spacecraft was insulated with multilayer thermal blankets at top and
bottom. The sunshade was deployed after launch to protect the spacecraft
on the solar-oriented side. Louvered sides on five of the eight
electronics compartments also helped control the interior temperatures.
Instruments on-board the spacecraft measured the atmospheric, surface,
and physical characteristics of Mercury and Venus. Experiments included
television photography, magnetic field, plasma, infrared radiometry,
ultraviolet spectroscopy, and radio science detectors. An experimental
X-band, high-frequency transmitter was flown for the first time on this
Mariner 10 (also known as Mariner Venus Mercury 1973) was placed in a
parking orbit after launch for approximately 25 minutes, then placed in
orbit around the Sun en route to Venus. The protective cover of the
sunward-facing electrostatic analyzers did not open fully after launch,
and these intruments, part of the Scanning Electrostatic Analyzer and
Electron Spectrometer experiment, could not be used. It was also
discovered that the heaters for the television cameras had failed, so the
cameras were left on to prevent low temperatures from damaging the optics.
Mariner 10, shown in this
artist's rendering, was the last in a series of
Mariner missions designed to survey other planets
in the solar system.
Image courtesy of NASA.
A trajectory correction maneuver was made 10 days after launch.
Immediately following this manuever the star-tracker locked onto a bright
flake of paint which had come off the spacecraft and lost lock on the
guide star Canopus. An automated safety protocol recovered Canopus, but
the problem of flaking paint recurred throughout the mission. The on-board
computer also experienced unscheduled resets occasionally, which would
neccesitate reconfiguring the clock sequence and subsystems. Periodic
problems with the high-gain antenna also occurred during the cruise. In
January 1974 Mariner 10 made ultraviolet observations of Comet Kohoutek
and another mid-course correction was made on 21 January. The spacecraft
passed Venus on February 5, 1974, at a closest range of 5768 km at 17:01
UT and returned the first close-up images of Venus. This also marked the
first time a spacecraft used a gravity assist from one planet to help it
Enroute to Mercury an attitude control anomaly occurred for the second
time, using up attitude control gas. Some new procedures were used from
that point on to orient the spacecraft, including Sun-line maneuvers and
the use of solar wind on the solar panels to orient the spacecraft.
Mariner 10 crossed the orbit of Mercury on March 29, 1974, at 2046 UT, at
a distance of about 704 km from the surface. A second encounter with
Mercury, when more photographs were taken, occurred on September 21, 1974,
at an altitude of 48,069 km. Unfortunately the lighted hemisphere was
almost the same as the first encounter, so a large portion of the planet
remained unimaged. A third and last Mercury encounter at an altitude of
327 km, with additional photography of about 300 frames and magnetic field
measurements occurred on March 16, 1975. Engineering tests were continued
until March 24, 1975, when the supply of attitude-control gas was depleted
and the mission was terminated.
Mariner 10 results showed a Hadley-type circulation existed in Venus'
atmosphere and showed that Venus had at best a weak magnetic field, and
the ionosphere interacted with the solar wind to form a bow shock. At
Mercury, it was confirmed that Mercury had no atmosphere and a cratered,
dormant Moon-like surface was shown in the images. Mercury was shown to
have a small magnetic field and a relatively large iron-rich core. Total
research, development, launch, and support costs for the Mariner series of
spacecraft (Mariners 1 through 10) was approximately $554 million. The
total cost of the Mariner 10 mission was roughly $100 million.