[Illustration: 17. _Mariner 10_ returned data and photographs from
the vicinities of Venus and Mercury.]
[Illustration: 18. This computer-enhanced image of Mercury’s surface
was returned by _Mariner 10_ from 200,000 kilometers (124,000 miles)
and 6 hours away from closest approach to Mercury on March 29,
1974.]
_Mariner 10_ returned closeup pictures of the cloud cover around Venus
and of Mercury’s sunbaked surface. _Mariner 10_ was the first spacecraft
to photograph Mercury, the innermost planet. The spacecraft’s
instruments also measured particles, fields, and radiation from these
planets.
_Mariner 10_ flew by Venus on February 5, 1974, after a three-month,
240-million-kilometer (150-million-mile) journey that took the
Spacecraft halfway around the Sun. _Mariner 10_ swung around the planet,
taking a variety of measurements and photographs of the clouds that
obscure the planet’s face. Using the planet’s gravity to “bend” its
flight path, _Mariner 10_ flew on toward encounter with Mercury.
On March 29, 1974. _Mariner_ sped across the night side of the little
planet closest to the Sun. Only 703 kilometers (436 miles) above the
rugged surface, _Mariner_’s cameras captured the first closeup views of
the planet’s daylight hemisphere. The pictures show craters,
scarps—cliffs nearly 3 kilometers (2 miles) high and stretching as far
as 500 kilometers (300 miles) across the surface—basins, and hilly
furrowed terrain.
After providing our first glimpse of Mercury’s surface, _Mariner_ raced
on around the Sun and back out across Venus’ orbit. With some trajectory
adjustments using on-board thrusters. _Mariner_ returned to within
48,000 kilometers (30,000 miles) of Mercury on September 21, 176 days
after the first encounter, again returning pictures and data.
_Mariner_’s orbit brought it back to the planet for a third pass in
another 176 days. On-board propellant exhausted, the spacecraft
continues its orbit of the Sun and innermost planet.
_Mariner 10_ is the first complex spacecraft designed to travel to the
inner reaches of the solar system. At closest approach to the Sun, the
spacecraft received five times as much light and heat as it did on
leaving Earth. Thus the solar panels, which collect and convert solar
radiation into electrical energy for the spacecraft’s instruments and
controls, were designed to tilt more and more away from the sunlight as
_Mariner_ approached the Sun.
_Mariner_ could transmit much more information to Earth than earlier
flyby spacecraft. This higher data rate enabled the craft to send back
more live pictures of the planets as it flew by them. Some information
was stored on magnetic tape for later transmission. This capability
permitted _Mariner_ to collect data when it was hidden from Earth behind
a planet, and send the information when it emerged.
Prime contractor for _Mariner 10_ was Hughes Aircraft Company.
_Mariner 10_ is from the National Aeronautics and Space Administration.