Mar 23 1974
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(New page: 23 March-5 April: NASA'S Mariner 10, launched 3 Nov. 1973, became the first spacecraft to explore Mercury, during a 12-day flyby of the planet-returning some 1700 TV photos and much ne...)
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23 March-5 April: NASA'S Mariner 10, launched 3 Nov. 1973, became the first spacecraft to explore Mercury, during a 12-day flyby of the planet-returning some 1700 TV photos and much new information on its atmosphere, surface, mass, and shape.
The incoming encounter sequence began 23 March at 5 600 000 km from Mercury, transmitting the clearest photos yet taken of the tiny planet. Although most of the photos were recorded on tape before transmission, real-time photos with reduced resolution were sent to analyze the reflective properties of Mercury at different lighting angles. By 28 March photos were showing the planet to be heavily cratered like the moon.
During the incoming near-encounter, from 17 to 3% hrs before encounter, 18 frames were transmitted. Mosaics were made for locating smaller areas photographed later at higher resolutions. Spacecraft instruments identified an extremely thin atmosphere surrounding the planet, containing helium at concentrations above lunar levels and small amounts of neon, argon, and possibly zenon.
Mariner 10 passed within 704 km of Mercury at 4:47 pm EDT 29 March, within 56 km of the aim point. Data were transmitted without interruption over the 150 000 000-km distance until earth occultation began at 4:50 pm EDT. Radio signals were recovered following the 12-min occultation. Photos of the planet-described by scientists at Jet Propulsion Laboratory as "superb"-showed details less than 1.6 km in size and covered almost half the planet's surface, which was pocked with craters as deep as 32 km and between 8 and 160 km wide. Giant cracks in the surface suggested high heat beneath. Some ancient volcanic activity was seen in a few rille-like ravines that crossed the craters and dome-like hills. Absence of halo craters indicated young craters probably had been covered by the dust storms that constantly swept the planet. At a JPL press briefing following the encounter, Dr. James C. Fletcher said onboard instruments showed that Mercury had an unexpected magnetic field, estimated to be 1% that of the earth. The magnetic field was measured at 90 to 100 gammas at the spacecraft's closest approach, with an extrapolated value at the surface of 100 to 200 gammas.
As Mariner 10 raced by Mercury on 30 March, photos of the morning side of the planet showed a much smoother region than the rugged area seen under the afternoon sun. The north pole region appeared much less pitted and flatter than the south pole. Instruments found that Mercury was two-thirds iron, making its density greater than previously believed and probably responsible for the unexpected magnetic field. Concentrations of uranium and thorium were comparable to those on the earth.
Preliminary results, presented at a 31 March press conference at JPL, included a Mercury mass measurement 100 times better than previous ones, a detached well-defined bow shock wave, a temperature varying from 90 K on the night side to 700 K on the day side ( - 300°F to 800°F) with no thermal anomalies detected, opaque surface soil with the top few centimeters 1.0 to 1.5 times as dense as water and 50% as porous, and high-energy electrons in the mev range observed from 700 to 5000 km above the surface. An ultraviolet scanner had detected what appeared to be an egg-shaped moon moving in orbit around Mercury, but the Mariner navigational team later identified the object as a dim but very hot star, known as 31 Crater in the constellation Corvus.
Despite a power anomaly, Mariner 10 continued to transmit TV pictures in real-time at 117.6 kilobits per sec and tape-recorded frames at 22.05 kbps. TV imagery was discontinued 5 April when the resolution became equal to that of earth-based telescopes.
All mission events were accomplished as planned and the mission was adjudged successful 22 April. The spacecraft was expected to swing back by Mercury in September. (NASA MOR, 24 April 74; JPL Press Kit, 19 March 74; NYT, 24 March 74, 30 March-1 April .74; W Post, 29 March-1 April 74; W Star-News, 28 March 74; LA Times, 20 March-1 April 74)
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