Mariner 5

At 17:34:56 GMT on 19 October 1967, spacecraft time, Mariner 5 made its closest pass of Venus, 10,151 km from the planet's centre. About three minutes earlier its radio signal, still aimed at Earth, had begun to graze the night-side atmosphere. As the gas bent the beam its frequency drifted, by about 16,000 Hz at what JPL called the moment of occultation; Mariner 4's signal at Mars in 1965 had shifted by about 5 Hz. Goldstone caught the signal again 20.6 minutes after closest approach, climbing out of the sunlit side. Siddiqi's chronicle names this radio-occultation experiment as the mission's primary goal; it let a spacecraft with no camera change what people thought Venus was like.

The spacecraft was left over from Mars. Mariner 4 had flown past Mars on 15 July 1965, and its flight spares were in storage. NASA authorized the Mariner Venus 67 project late in December 1965, about 18 months before the June 1967 launch window, and on 30 December NASA's H. E. Newell told the Mariner 4 investigators by teletype that NASA planned to send the spare, MC-4, to Venus. The aim was to extend what Mariner 2 had found in 1962; the 1969 project report notes that Mariner 5 was the only spacecraft NASA scheduled for the 1967 opportunity.

Choosing the payload took about five weeks. On 8 January 1966 about 30 people met at Caltech and agreed that the most useful measurement would be the pressure or density at the surface, best made by dropping a probe, which did not appear feasible this time. The near-unanimous view was that an S-band occultation experiment mattered most and that television pictures of the clouds were highly desirable. Budget studies then forced a choice between a camera and Stanford University's dual-frequency radio experiment. JPL told NASA on 2 February that it preferred the camera; NASA's steering committee chose the radio experiment, and on 4 February Newell approved seven investigations without a camera, asking JPL to add television if data storage and power allowed. JPL's director replied on 15 February that, with 14½ months to launch, it could not be done without seriously jeopardizing the mission.

Converting a Mars craft for Venus meant turning it over. To point its antennas at Earth, the spacecraft had to fly the other way up, so the side that had stayed in shade on the way to Mars now faced the Sun. Closer to the Sun less panel was needed, and the solar-cell area fell from 70.4 to 43.6 square feet (about 6.5 to 4.1 m²). A sunshade unfolded with the panels, and JPL reckoned it kept the bus about 30 °F (17 °C) cooler at Venus.

Two of the seven experiments needed no instrument of their own: the S-band occultation used the ordinary telemetry transmitter, and celestial mechanics used range and Doppler tracking. The dual-frequency experiment ran the other way. Stanford's 150-foot dish transmitted at 49.8 and 423.3 MHz, and a new receiver on the spacecraft measured how the two signals changed. The magnetometer, plasma probe and trapped radiation detector were Mariner 4 instruments, and the ultraviolet photometer, from hardware built for Mariner 4 but never flown on it, looked for sunlight scattered by hydrogen and atomic oxygen.

Mariner 5 left Cape Kennedy at 06:01:00 UT on 14 June 1967 on an Atlas-Agena D, two days after Venera 4 had left Baikonur. JPL's target was a pass 8,165 km from the centre of the planet, but the launch was aimed 75,000 km wide so that neither the unsterilized spacecraft nor the Agena could strike Venus, with a correction planned from the outset. The motor fired on 19 June at 23:08:03 GMT in one section of JPL's 1969 report and 23:08:06 in another (SP-190's 23:08:28 is the report's time for the burn's end). It imparted 15.41 m/s against 16.13 m/s planned, and the flyby would come no closer than 10,000 km from the centre; the report suggests the shortfall may explain the wider pass, although its trajectory chapter describes the target as about 10,200 km from the centre. The primary objectives were still met, and the optional second burn was never used.

The encounter sequence began at 02:49:00 GMT on 19 October, commanded through the Deep Space Station at Woomera in Australia. Mariner 5 approached the night side from north of the planet's orbit and stayed more than 2,400 km clear of Venus's shadow, which mattered because losing the Sun would have cost it attitude control. The radio ray met the atmosphere at about 37° N on the night side and emerged at about 32° S in daylight.

Turning a frequency shift into a weather report needs an assumption about what the air is made of. The signal first felt the neutral atmosphere about 6,140 km from the centre, roughly 85 km up, where the pressure was about a thousandth of Earth's. The rate at which density fell off there implied 69 to 87 percent carbon dioxide if the rest was nitrogen. Assuming 90 percent, the 1969 report finds about 241 K above a tropopause near 6,114 km, then a steady rise to about 477 K (204 °C) at 6,085 km, where the signal gave out. Kliore's team blamed heavy defocusing as the beam neared critical refraction, the level at which the air bends a horizontal ray as sharply as the planet curves. NASA's 1971 report puts that level at about 32 km in one chapter and near 35 km in its summary, and Fjeldbo and Eshleman wrote in 1969 that the neutral atmosphere had been probed to within 35 km of the surface. Today's picture is on the atmosphere of Venus.

A day earlier Venera 4 had lowered its capsule into the same atmosphere, and Moscow said it had measured all the way down to the ground. The two data sets agreed where they overlapped; the dispute was about where the ground was. Izvestia reported 15 to 22 atmospheres and about 550 K at the end of the capsule's descent. Extending Mariner 5's profiles to those values put them about 6,078 km from the centre, give or take 10 km, while radar put the surface at 6,056 km; a footnote in the 1969 report concedes that Mariner 5's own altitudes were quoted from 6,056 km for convenience, although their validity was questionable. In March 1968 Reese and Swan still argued in Science that Venera 4 had reached the surface. That same month, Siddiqi writes, scientists compared both missions at Kitt Peak National Observatory and concluded that neither had been entirely successful in reporting conditions at the surface. The July 1968 Journal of the Atmospheric Sciences printed the Soviet Venera 4 papers beside two papers on the radius: Kliore and Cain's left two readings open (a plateau radar had missed, or measurements that stopped short of the ground), and 'The Case for the Radar Radius of Venus' set the combined 6,085 ± 10 km against radar's 6,050 ± 5 km.

NASA's final report, published in 1971, found the two sets of profiles in excellent agreement, with Venera 4 sampling the air between 6,078 and 6,105 km from the centre, 25 to 52 km above the surface. The only tenable assumption, it said, was that the capsule's radio altimeter had failed or been misread, perhaps taking a signal at twice 26 km as meaning 26 km; a landing on a high mountain was dismissed, since radar showed nothing unusual where the capsule came down. The Russian account that epizodsspace prints under the heading 'Marov' tells the same story from the other side: Mariner 5's data and radar measurements of the radius showed the capsule had not reached the ground, and meetings of Soviet and American scientists over the next two years concluded it 'ceased to exist while still high above the surface'. As late as February 1970 Eshleman's review in Radio Science said the lowest regions of the atmosphere had yet to be probed; Venera 7 reached the surface that December.

What Mariner 5 could not reach it could extrapolate, and there the numbers drift. SP-190 draws its profiles down to a radar radius it now gave as 6,053 ± 3 km and arrives at about 75 bar and 760 K. The text beside the figure cites 60 to 120 bar and 650 to 800 K from various authors, the summary settles on about 700 K and 100 bar until measurements came from the surface itself, and Reiff's foreword to the same volume says about 600 K. Siddiqi's chronicle, repeated on NASA's mission page, gives 527 °C and 75 to 100 atmospheres. NASA's fact sheet now gives 92 bar and 737 K (464 °C) at the surface.

Higher up, the ultraviolet photometer found hydrogen glowing at the Lyman-alpha wavelength and no trace of atomic oxygen. Its fall-off with height pointed to a cold upper atmosphere: SP-190 gives about 650 K for the exosphere of Venus against about 1,500 K for Earth's. The Stanford experiment picked up a weak night-side ionosphere, and both radio experiments profiled the much stronger daytime one, which ended abruptly near 500 km, pressed down by the solar wind. How much denser the daytime peak was depends on the paper: about two orders of magnitude in the Stanford group's 1967 report, about 50 times in JPL's 1969 summary and 30 times in Fjeldbo and Eshleman's 1969 analysis.

Mariner 5 spent about two hours inside the region where Venus disturbs the solar wind. Its magnetometer and plasma probe saw a bow shock, like Earth's but much smaller, and no field belonging to the planet. The trapped radiation detector found no energetic electrons or protons down to 10,150 km from the centre, from which Van Allen's group concluded the planet's magnetic dipole moment was almost certainly below a hundredth of Earth's and probably below a thousandth; Bridge's group put its upper limit within a factor of two of a thousandth. The obstacle to the wind, the teams argued, was the electrically conducting ionosphere. The Stanford group saw in this a third kind of interaction, unlike Earth, whose magnetic field holds the wind off, or the Moon, which has neither field nor ionosphere.

Tracking weighed the planet. The 1969 report gives the gravitational parameter of Venus as 324,859.61 ± 0.49 km³/s², and Anderson's team wrote that Mariner 5 had yielded masses for the Moon and Venus more accurate than any previously reported. The value JPL lists today, from its DE440 ephemeris, is 324,858.592 km³/s², about 1 km³/s² lower, roughly twice the 1969 uncertainty.

Venus also flung Mariner 5 inwards, lowering its closest point to the Sun from 107.1 to 86.7 million km, and SP-190 notes that no man-made object had been so close to the Sun before. On 21 November 1967 controllers switched to the low-gain antenna to prepare for a long cruise and a planned reunion in 1968. One more pass came on 4 December, when a digital demodulation technique pulled 42 minutes of usable data out of a four-hour track at Goldstone's DSS 14, with attitude, power and Canopus lock all normal; JPL's 1969 volume lists it as the last tracking of Mariner 5. An extension project hoped to regain telemetry from about 22 July 1968 and use the spacecraft until January 1969 to map uncharted parts of the sky at short wavelengths and sample the solar wind close to the Sun.

Searches began on 26 April 1968, and five that assumed a healthy spacecraft found nothing. On 14 October 1968 DSS 14 locked onto a signal about 30 kHz below the predicted frequency, and on 20 October the station achieved two-way lock, but the signal carried no telemetry and commands drew no response. On 5 November 1968 the project was declared complete, its extension objectives unmet. Program manager Glenn A. Reiff's foreword to the final report says the mission cost 10 percent less than anticipated, helped by hardware already bought. Mariner 5 itself is still circling the Sun, untracked: asked for 'mariner', JPL's Horizons system returns only Mariner 2.