AS MARINER'S CAMERAS snapped the last pictures of Venus, the thoughts of the scientists and engineers turned toward the mission's priority target-Mercury. Forty-three days of cruise and a third trajectory correction maneuver remained before mission completion. Analysis of the failures and anomalies experienced to date continued at an urgent pace, while at the same time the complicated Mercury science sequences were subjected to detailed scrutiny in search of adjustments required to accommodate the corresponding changes in spacecraft performance.
A Troubled Journey
About one week after Venus encounter, a decision had to be made as to how the spacecraft should be redirected toward Mercury with the least expenditure of maneuvering gas.
The oscillation problem and the attendant risk of losing all attitude-control gas if there should be a loss of celestial reference resulted in a number of changes in mission operations. One was the cancellation of further roll calibration maneuvers; another was the introduction of a period of "solar sailing" during which the spacecraft roll, pitch, and yaw axis rates and limit cycle magnitude were reduced by differential tilting of the solar panels to use the pressure produced on the panels by solar radiation pressure in a controlled manner, like wind on a sail. This technique significantly reduced the amount of gas which would have been used in the standard celestially controlled cruise mode.
On February 14, 1974, Mariner's gyros were tested preparatory to making the third trajectory correction maneuver. The gyros did not oscillate during the first two tests, but did so during the third test and during a commanded turn of the spacecraft. As a result, the planned trajectory correction maneuver was cancelled; it would have caused the loss of too much gas in gyro oscillations. Instead, a Sun-line maneuver was decided upon, to be executed in mid-March. At that time the position and orientation of the spacecraft would be such that the normal position of the rocket engine, relative to the Sun and Canopus, would be suitable to apply the right amount and direction of thrust to change trajectory without requiring the spacecraft to roll or pitch to do so. By making the trajectory change in this way, project personnel would be able to send Mariner to rendezvous with Mercury at the correct point on the dark side on March 29, but approximately 17 min later than the time desired. All the science data originally planned for Mariner to gather at Mercury could still be obtained.
 Shortly after midnight, in the early morning of February 18, duty operators were startled to observe from the telemetered data that Mariner 10 had lost celestial reference on the star Canopus. During the next three hours, project staff members rushed to JPL and watched helplessly as Canopus "drifted" through the star tracker's field of view twice and the spacecraft gyrated in short gyro-off/gyro-on cycles. A normal roll search for Canopus could not be started until communications were reestablished between the spacecraft and a big ground antenna; Mariner had been in communication with a 26-m (85-ft) antenna at the time of the trouble. After a big (64-m) antenna had been obtained for the troubled spacecraft, a command was sent to initiate a roll search, and Canopus was acquired 1.3 min later. The gyros had been on for I hr and 48 min, but fortunately no oscillations were....
...observed until the final acquisition of Canopus. Approximately 70 millipounds of nitrogen gas were lost because of this incident, which was thought to have been caused by a bright particle passing the spacecraft. The occurrence of bright particle distraction had increased from a rate of 1 or 2 a week immediately after launch to about 10 a week by the end of February. On March 6, a group of bright particles again disturbed the star tracker and caused the spacecraft to roll and waste attitude control gas for 40 min.
On March 13, the project staff held a final conference to approve the Sun-line course change. On March 16, at 04:54 a.m. PDT, the propulsion system was ignited and burned for 51 sec to change the velocity of Mariner by 17.8 m/sec (59 ft/sec) directly away from the Sun. This would change the Mercury flyby from the sunlit to the dark side of the planet (Fig. 7-1). The aim point had been carefully chosen to get the best possible science data and also to allow a return to Mercury six months later.
Conditions were now very critical. Because the angle between the velocity change due to the trajectory correction maneuver and the line between spacecraft and Earth was 103 deg, the doppler shift measured at Earth would show only a small component. So no precise estimate of how successful the maneuver had been could be obtained until tracking data had been analyzed for about 10 days after the maneuver. This might be too late to make corrections and still reserve sufficient gas for a second encounter with Mercury. Another possibility was to use the high data rate engineering telemetry to measure the pressure within the rocket thrust chamber and use this to determine the actual magnitude of the velocity change produced by the rocket thrust. If the engine burned too " hot, " there would be an overshoot that could not subsequently be corrected by a Sun-line maneuver. If, however, the engine burned "cold," the undershoot could be corrected by a further firing of the rocket engine within 24 to 48 hr.
The rocket engine fired by command as scheduled. Preliminary analysis using data gathered from the engineering telemetry supplemented by the doppler shift measurement indicated that the maneuver had been about one percent short of that required. Thus the flyby was expected to be 200 km (124 mi) closer to Mercury than planned. Since this still satisfied all  the requirements of the science experiments at Mercury, no additional maneuvers were planned.
On Sunday, March 17, the day after the maneuver, the nonimaging science experiments were turned on in preparation for the encounter. All instruments were checked and confirmed to be in excellent operating condition. A little less than one week later the first TV image of Mercury was displayed on screens at JPL. By now the high-gain antenna had mysteriously recovered (never to fail again, as it turned out), and high-resolution full coverage of Mercury was expected.
First pictures of Mercury were about the same as pictures obtained from Earth, but gradually, as more pictures came back from the spacecraft, observers could distinguish bright spots which had apparent diameters up to 400 km (250 mi) (Fig. 7-2). Some of the bright spots lined up with light streaks to merge into great circle arcs like the bright rays on the Moon.
By March 25, the pictures showed a surface of mottled character, suggestive of a fuzzy picture of a cratered surface such as Earth's Moon ( Fig. 7-3 ). Mercury appeared as a wide crescent as Mariner 10 approached. By this time, Mariner was 3.5 million km (2.17 million mi) from...
 ...Mercury, and the images of the planet (Fig. 7-4) exceeded the highest resolution previously obtained by Earth-based telescopes. These and subsequent images soon revealed Mercury to be a Moon-like body, heavily cratered, with large flat circular basins similar to those on the Moon and Mars.
The bright spot which was the first feature seen on Mercury in the earliest photographs was soon recognized to be a small, 25-km (15-mi) bright-rayed crater. ( It was later named after the astronomer Gerard Kuiper, who had done so....
....much to encourage lunar and planetary exploration at the beginning of the space age and was a member of the Mariner 10 TV team. Dr. Kuiper had died several months earlier.)
During the next few days the pictures of Mercury progressed from revealing to fantastic (Fig. 7-5). The densely cratered surface showed a profusion of detail. Moonlike, yet at the same time somehow different from the Moon, the face of Mercury was built up in picture after picture. Hurriedly, scientists at the Video Analysis Facility assembled the many photographs into large photomosaics that provided detailed views of almost the whole lighted hemisphere of this small world. As Mariner sent back pictures on leaving Mercury, scientists were excited to find a huge circular feature about 1300 km (800 mi) across located on the terminator. This great basin was surrounded by mountains and with radial structures very similar to the Mare Orientale of the Moon.
Mariner 10 began taking pictures of Mercury on March 23, from a distance of 5.3 million km (3.3 million mi). Photography was intermittent for the next four days but became an almost continuous operation on March 28, one picture being taken every 42 sec. However, Mariner was unable to photograph Mercury during the half hour around closest approach at 1.46 p.m. PDT on March 29, because the flight path had been targeted to pass behind the planet on the night side.
While Mariner 10 was still occulted from Earth by the planet, the cameras started taking pictures of Mercury's far side from the closest possible altitude of about 5790 km (3600 mi). Since the planet blocked radio communications to Earth at that time, the TV frames had to be recorded on tape within the spacecraft for transmission later. Periodic photographic operations continued for another five days until April 3, when the spacecraft was 3.5 million km (2.17 million mi) past Mercury. In all, more than 2000 pictures of Mercury were transmitted from Mariner 10. The....
Fig 7-5. During the next few hours the details increased. Taken shortly before 12:00 noon on March 28 at 952,000 km (590,240 mi) (a) shows the bright spot between limb and terminator close to the center as a bright-rayed crater. In (b) taken March 29 at 500,000 km (310,000 mi), a lunarlike surface on which features as small as 11 km (6.8 mi) can be seen. The picture no longer shows the whole of the planet. In (c), taken four hours before closest approach, at 198,000 km (122,000 mi), a profusion of craters in the southwestern quadrant of Mercury can be seen.
 ....photogeometry of the flyby and the angles at which the images of Mercury were obtained are shown in Fig. 7-6.
Mercury had appeared as a fat crescent as Mariner 10 approached the planet (Fig. 7-7). After the spacecraft passed by on the dark side of Mercury, it left in a direction that showed slightly more than half the planet illuminated (Fig. 7-8). Shortly afterwards it was discovered that the Mariner 10 cameras had shown the relative brightness of Mercury and the way the light reflected from the planet is polarized are identical to the Moon.
 The brightest crater on Mercury (Kuiper, Fig. 7-9) reflects almost 25% of the sunlight falling on it, just a little more than the brightest feature on the Moon (Aristarchus, Fig. 7-10). Because albedo boundaries between plains and highlands are less clearly defined on Mercury than on the Moon, the planet overall appears of low contrast compared with the Moon.
Photographs of increasing detail revealed that, although generally like the Moon, Mercury has some distinctly nonlunar features (Fig. 7-11) including, for example, large scarps or cliffs nearly 3 km (2 mi) high and stretching as far as 500 km (300 mi) across the surface, which, because of their lobate form, appear to be compressional (thrust fault) features, perhaps resulting from...
Fig. 7-9. The bright object was the first feature to be recognized on Mercury and turned out to be a young rayed crater. It was named Kuiper in memory of Dr. Gerard Kuiper, a leading advocate of interplanetary spacecraft and a member of the imaging team for Mariner 10. In (a), the crater is related to the incoming mosaic; (b) shows the crater in close-up as seen at a distance of 88,450 km (55,000 mi) some 2-1/2 hours before closest approach. Kuiper is about 41 km (25 mi) in diameter and is located on the rim of a larger (80-km) and older crater.
 ...forces on the surface materials as a hot central core of the planet cooled.
The major features of Mercury revealed by Mariner 1O's camera were basins, craters, scarps, ridges, lunar-like highlands, and plains. The highlands are cratered about as heavily as their lunar counterparts. The largest basin-named Caloris (the Greek word for "hot") because it is one of the two areas on Mercury that face the Sun at perihelion-is 1300 km ( 800 mi ) across. It resembles the Mare Imbrium basin on the Moon except for an unusual pattern of cracks on its floor (Fig. 7-12). Mercury displays extensive ray systems (Fig. 7-13), similar to those on the Moon, and there are innumerable secondary impact craters, crater chains and great circle alignments of bright features closely resembling lunar ray systems.
There is also a jumbled terrain (Fig. 7-14), informally termed " weird terrain " by the TV team, which is somewhat analogous to similar areas on parts of the Moon. It is characterized by hills and lineations on which rims of craters are broken and dissected. On the Moon the jumbled terrain is antipodal to the basins of Mare Imbrium and Mare Orientale, believed to be the results of major impacts. On Mercury, it is antipodal to the Caloris Basin, which also is believed to be a major impact basin.
Donald Gault of Ames Research Center has postulated that the "weird terrain" could have been caused by seismic forces transmitted through the body of the planet and along the surface crust, which focussed at the antipodes of the major impact basin.
Mare-like surfaces of large extent have now been observed on the Moon, Mars, and Mercury. All show a surprising similarity in the numbers of small craters that pepper them (Fig. 7-15). This implies that all these planets received similar intensities of meteorite bombardment. Prior to the Mariner mission to Mercury, scientists thought that the amount of bombardment might differ at various distances from the Sun. Now it appears that the meteorites were spread evenly throughout the inner Solar System, at least during the final stages of planetary formation.
All the terrestrial planets, including Earth and Venus, may thus have experienced a period of widespread impact cratering and basin formation. The evidence of this process, recorded on the Moon, Mercury, and Mars, has been largely wiped out on Earth and can be demonstrated only by sophisticated geological mapping on ancient surfaces such as the Canadian shield.
By direct computer link through the NASA Communication Center, it was possible to monitor the nonimaging science data being returned from the spacecraft in real-time. The data were processed at JPL and then transmitted to the various principal investigators' facilities at the University of Chicago, Los Alamos Scientific Laboratory, and Goddard Space Flight Center.
 This operation permitted continuous data coverage in real-time during critical calibrations and at encounters. It allowed rapid assessment of observations at encounter, which was especially important for the magnetometer and plasma science experiment during this first encounter with Mercury.
Magnetic measurements made in the vicinity of Mercury produced an unexpected and surprising result. Mercury's effect on the solar wind revealed the presence of a planetary magnetic field about one-sixtieth of Earth's field. This field produces a bow shock and fills the plasma cavity expected behind an airless small body like Mercury. The source of the magnetic field was a mystery, the first order question being whether it was internally generated or a result of electric currents induced in the surface or in the tenuous atmosphere of Mercury by the solar wind. Another visit to Mercury would be required to resolve the question.
The high-energy charged particle experiment recorded four unusual events during the first encounter. The first of these events was a low....
 ...counting rate, which was probably a 600-mi-wide population of residually trapped low-energy electrons. The next two were impulsive events of large fluxes of approximately 300-keV electrons and 550-keV protons. The events are notable because of their fast onset times and the periodic nature of the counting rates. These facts impose severe constraints on the acceleration mechanism. All three events were observed in the magnetosphere. The fourth event was observed in the boundary between the magnetosphere and the bow shock. This event was composed of 300-keV electrons whose counting rate varied with a marked 5-sec periodicity.
Radio tracking of Mariner showed that Mercury is also much closer to being a perfect sphere than is the Earth. The mass of Mercury was measured to 100 times greater accuracy than previously, i.e., to within one ten-thousandth of the mass of the Sun ( Fig. 7- 16). Mariner determined that Mercury does not possess an ionosphere greater than one hundred-thousandth that of the Earth. Although Mercury is virtually without an atmosphere, the planet does have more helium than the Moon, possibly originating from radioactive decay of uranium and thorium or capture from the solar wind.
A night temperature low of 90 K ( - 297°F) was measured by Mariner's infrared radiometer just before dawn on Mercury. The maximum daytime temperature in late afternoon was 460 K (369°F).
This temperature difference between night and day is enormous. But at times, when Mercury makes its closest approach to the Sun, the range can reach 650 K (1170°F): greater than on any other planet in the Solar System.
The temperature gradient measured between Mercury's light and dark sides offers further proof that its surface is very similar to the Moon: an insulating blanket of dust pulverized by meteoritic impacts. A few outcroppings of rocks and freshly formed craters cause slight temperature variations. But generally the soil is most probably very light and porous, with an appearance and bearing strength similar to lunar soil. An astronaut's footprint on Mercury would be almost indistinguishable from one on the Moon.
Man's first glimpse of Mercury at close hand was quite brief, yet Mariner 10 returned several thousand photographs and tens of thousands of non-imaging measurements of the planet's surface and environment. The planet had been revealed to be an intriguing combination of Earthlike and Moonlike characteristics, a body whose early history, the record of which is preserved on its ancient surface, is an important piece in the puzzle that is the origin of our Solar System. As had been the case with earlier planetary missions, a few hours of spacecraft observations had added more to man's store of knowledge about a littleknown planet than centuries of Earth-based observations.