Pioneer Venus Orbiter

The orbiter began as one half of a cheaper idea. In January 1969 NASA's Goddard Space Flight Center published A Venus Multiple-Entry-Probe Direct-Impact Mission, a study by R. M. Goody, D. M. Hunten, V. Suomi and N. W. Spencer, and by the end of that year NASA had merged the probe carrier with a Planetary Explorer orbiter into what it called a universal bus. According to SP-461, NASA's 1983 history of the project, the idea was one spin-stabilised spacecraft on a Thor-Delta rocket that could either release entry probes into Venus or brake into orbit around it. A 1970 study by the Space Science Board and NASA's Lunar and Planetary Missions Board, the Purple Book, endorsed the idea with launches from 1975 and mission costs kept under $200 million.

Money moved the schedule before any hardware existed. In the autumn of 1970 there was no chance of funding a start in time for 1975, so the plan slipped to multiprobes in 1976 to 1977 and an orbiter in 1978. Then, SP-461 records, the Planetary Explorer programme was discontinued at Goddard in November 1971, and by January 1972 it had been transferred to Ames Research Center and renamed Pioneer Venus.

The mission Ames defined in 1972 looked different from the ones that flew: a Science Steering Group's 'Orange Book' called for two identical multiprobe spacecraft in December 1976 to January 1977 and an orbiter in 1978 designed to work for one Venus sidereal day. Two teams, led by Hughes Aircraft and TRW, won design contracts in October 1972 and were then told to compare the small Thor/Delta with the larger Atlas/Centaur; SP-461 says the extra $10 million or so for Atlas-Centaur cost no more than miniaturising both designs for Thor-Delta. Hughes's final report records that NASA selected the Atlas/Centaur in mid-April 1973 and moved the multiprobe launch to August 1978, leaving the orbiter's May to June 1978 dates unchanged. SP-461, written a decade later, dates the slip to August 1972 and blames Congress's failure to authorise a start in fiscal 1974; the Hughes report was written at the time.

For about a year the 1978 orbiter might have been European. ESRO, the European Space Research Organisation, had studied a low-cost Venus orbiter in 1971, according to the ESA historian Arturo Russo, and after an ESRO and NASA meeting in Washington on 18 April 1972 the project was tied to Pioneer Venus. NASA would provide the basic spacecraft; ESRO would add the braking motor, integrate the payload, test the spacecraft and hand it back for launch (SP-461 adds that ESRO would also supply the high-gain antenna).

It was lost in a vote. ESRO's advisory committee weighed it in early 1973 against HELOS, an X-ray astronomy satellite, and the IMP Daughter satellite, a partner for a NASA magnetosphere mission. Russo records that NASA had told ESRO it could not start Pioneer Venus in 1974 and was unsure of 1975, so a European commitment carried the risk of cancellation in Washington, while ESRO needed a project that could begin in 1974. The committee recommended the other two. At the Scientific Programme Board on 27 March 1973 France, backed by Belgium, argued for the Venus Orbiter and moved to postpone the HELOS decision. The motion lost by five votes to four, with Denmark abstaining, and the winners flew as ISEE-2 and Exosat. NASA built the orbiter alone.

Washington still had to pay for the American mission. Congress approved the project in August 1974 as a new start for fiscal 1975 (Siddiqi records it as NASA's formal approval). Hughes won the flight hardware contract in November with one bus for both missions, a spinning drum that swept its instruments across Venus, with a high-gain dish despun to stay on Earth. A last crisis came in June 1975, when the House of Representatives voted to cut $48 million tied to the mission. A slip to the 1980 window would have meant a redesign and, SP-461 estimates, as much as $50 million more; scientists and the press argued for the money, and both spacecraft were launched in 1978. NSSDCA puts the orbiter's cost at $125 million to build and operate for its first ten years.

Sources disagree on the orbiter's mass: 553 kg at launch in NSSDCA's catalogue, 517 kg in orbit on NSSDCA's older project page, and 582 kg in Siddiqi's chronicle and on NASA's mission page. An Atlas-Centaur launched it from Cape Canaveral at 13:13 UT on 20 May 1978 on a long Type II path; the Pioneer Venus Multiprobe, launched on 8 August on a shorter one, arrived five days after it. On 4 December the orbiter passed behind Venus and out of contact, and seven minutes later, at 15:58 UT, its braking rocket fired. Within two orbits the period was trimmed to 24 hours, so that periapsis came at the same time each Earth day.

Its orbit was part of the experiment: it carried the spacecraft through the ionosphere and upper atmosphere once a day and across the bow shock, where the solar wind is deflected around the planet, twice per revolution. Four of its twelve instruments looked at the planet, seven sampled what the spacecraft flew through, and a gamma-ray burst detector watched the sky, while the radio signal itself carried experiments on occultation, gravity, drag and turbulence.

At each low pass the radar mapper timed echoes from directly below. Gordon Pettengill's team reported in 1980 that the map covered 93 percent of the globe at better than 150 km resolution, and SP-461 notes that Venus's heights cluster around one level, where Earth's cluster around two. Siddiqi writes 93 percent of the band from 74° N to 63° S, at 75 km, but that band is itself about 93 percent of the planet's area (azmth's arithmetic). The map gave shape to two highland masses: Ishtar Terra in the north, about the size of Australia, and Aphrodite Terra near the equator, about the size of Africa in SP-461, while Pettengill's team described a massive equatorial region the size of South America. On Ishtar stood Maxwell Montes, 10.8 km above the mean surface in Siddiqi's figure. The mapper was switched off on 19 March 1981 and on again in 1991 to look at southern regions the early orbit could not reach, and its data outlived the spacecraft: NASA's global views of Venus from the Magellan era fill some gaps in Magellan's coverage with Pioneer Venus Orbiter data. See Mapping Venus.

Above the clouds the orbiter watched a planet change. The cloud photopolarimeter built ultraviolet pictures and followed the dark markings that circle Venus in about four days, and infrared observations implied what Siddiqi calls a clearing in the atmosphere over the north pole. The ultraviolet spectrometer started an argument that has outlived the spacecraft. L. W. Esposito reported in Science in 1984 that sulfur dioxide at the cloud tops, and the fine haze above them, had fallen by more than a factor of ten over the first five years, from values at arrival far above earlier upper limits. Because Venus had looked similar in the late 1950s, he argued for episodic injection, possibly by volcanic eruptions with an energy comparable to Krakatoa's in 1883. Emmanuel Marcq's team found another tenfold fall between 2007 and 2012 in Venus Express data and concluded in December 2012 that such injections recur on decadal timescales, but left the cause open: buoyant volcanic plumes or, with no active volcanism at all, long-period oscillations of the atmospheric circulation. See Volcanoes on Venus.

Lightning divided the team too. From December 1978, when periapsis moved into darkness, the electric field detector picked up intense, impulsive bursts of noise, and its investigators read them as radio waves from lightning, like signals attributed to lightning by the Soviet Venera 11 and 12 landers. An optical search found nothing: SP-461 reports that the orbiter's star sensor saw no statistically significant difference between the dark hemisphere and deep space, and reads this as a hint that lightning may be rare on the night side. In 1986 Harry Taylor (the orbiter's ion mass spectrometer investigator) and Paul Cloutier argued in Science that the bursts came from plasma instabilities generated locally, coinciding with troughs in the night-side ionosphere and showing no clustering over highlands. Ralph Lorenz's critical review of 2018 concluded that the balance of all observations supports some kind of frequent electrical activity on Venus, while optical searches are not consistent with lightning at Earth's rate. No observation cited here has settled it.

Space physicists used the orbiter for 14 years on one question: what happens when the solar wind hits a planet with no magnetic field of its own? It confirmed that Venus has little if any internal field, recorded crossings of the bow shock and of the ionopause, the boundary where the ionosphere holds the solar wind off, and in 1979 found flux ropes, twisted bundles of magnetic field inside the ionosphere (Russell and Elphic). Tracking of its radio signal later produced the GVM-1 gravity model of 1993 (Nerem, Bills and McNamee). In 1985 and 1986 the ultraviolet spectrometer also measured hydrogen, oxygen and carbon from comet Halley near perihelion.

After July 1980 the team stopped fighting the Sun's pull, which raised periapsis a little every orbit, to save propellant. Periapsis rose to about 2,309 km on 4 July 1986 by Strangeway's count (NSSDCA rounds it to 2,290 km), then began to fall. The long wait had a scientific reward: solar activity, high when periapsis was low in 1978 to 1980, was intermediate by the time periapsis came back down, so the same instruments could compare the upper atmosphere and ionosphere under different levels of solar activity. Magellan entered Venus orbit on 10 August 1990, and for more than two years two American orbiters worked at Venus at once.

In May 1992 the final phase began, with periapsis to be held low until the hydrazine ran out. The spacecraft was tired, its solar cells giving a spin-averaged 4 A against 13 A early in the mission. Of the original 32 kg of hydrazine (Strangeway and SP-461; NSSDCA says 32 kg per tank), about 2.3 kg was thought to remain, with a high degree of uncertainty, possibly enough to keep the orbiter flying until periapsis reached the day side. Apparently it was not. The burn before orbit 5,050 delivered less velocity than expected, and the attitude trim that followed fell short. NSSDCA and Siddiqi say periapsis was held between 150 and 250 km in this phase, but the navigation ephemeris archived in NASA's Planetary Data System shows it lower at the end: 148 km on 22 September, 131 km on 7 October. No telemetry came back after periapsis on orbit 5,056, and the last transmission reached Earth at 19:22 UT on 8 October 1992. Strangeway's plot of the final orbits shows the spacecraft had needed only three more raising manoeuvres to survive into the day-side ionosphere.

Nobody saw the end. The low passes happened behind Venus as seen from Earth, and no source found for this page gives an entry point. The last periapsis with data, on orbit 5,055, came at 19:46:28 UT on 7 October 1992 at 11.9° S, 89.2° E, 130.8 km above the 6,050 km sphere the ephemeris uses, before dawn at about 04:20 local solar time by azmth's calculation, consistent with the report by C. T. Russell and colleagues that the magnetometer sampled the post-midnight ionosphere until about 04:30. Carried forward one orbit, the steady trends of the last week put the next periapsis near 11.9° S, 90.7° E at about 128 km. That is azmth's extrapolation, and the orbiter may have survived further passes. The papers from its final weeks, collected in Geophysical Research Letters in December 1993, found the night-side ionosphere much reduced compared with solar maximum, while the night-side neutral atmosphere had barely changed. No source found reports any part of it surviving to the surface; see What we left at Venus.