Venera Landers

Don S. Montgomery, US Navy (Ret.). Public domain (US government work).

Overview

The Venera and VeGa landers are the only machines that have operated on the surface of Venus. Nine of the fourteen Soviet landers reached the surface, and those nine survived between 23 and 127 minutes before their electronics failed in the environment [1].

That range is the entire flight record of surface operation on Venus, and it has not been extended since.

Specifications

Parameter	Value
Configuration	sealed, thermally insulated pressure vessel, pre-chilled before descent
Surface endurance mechanism	insulation mass and initial internal temperature, no active cooling
Entry system class	comparable in mass to a 700 kg Venus entry vehicle in a later study
Sample acquisition	surface drill transferring soil to an internal analysis chamber

Source: [1].

Mission profile

Parameter	Value	Source
Landers that reached the surface	9 of 14 Soviet landers	[1]
Surface survival achieved	23 to 127 minutes	[1]
Derived data still in use	Venera measurements in the Venus International Reference Atmosphere	[5]

The environment they were built for

Quantity	Value	Source
Surface temperature	about 460 C	[7]
Surface pressure	about 90 bar	[7]
Reference test condition	465 C at 90 bar	[7]
Atmospheric density	65 kg/m3	[1]
Surface wind	0.3 to 1.3 m/s, average 0.6 m/s	[1]
Diurnal cycle	116 days, with night exceeding 50 Earth days	[1]
Mean zonal wind data available in the engineering model	to 80 km altitude	[5]

The atmosphere is not merely hot and dense. The gas mixture reproduced for ground testing is carbon dioxide and nitrogen carrying SO2, COS, CO, H2O, HCl, NO and HF [6]. At 460 C, common engineering materials including aluminum are unusable [1]. The engineering model of this atmosphere still carries no solid planet model and no high resolution gravity model, and takes its mean zonal wind from VIRA only to 80 km [5].

The design and its fixed lifetime

The landers were sealed, thermally insulated pressure vessels, pre-chilled before descent, which bought a fixed interval between touchdown and the moment the interior exceeded the survival temperature of the electronics. Adding protection systems is described in the successor literature as the general approach for improving electronics survival, one that increases cost and delays rather than removes the failure [1].

The scaling of that approach has been assessed since. A modern flagship Venus mission study costing 2 to 3 billion dollars found a liquid gas cooling system to be the most practical route and could still guarantee survival on the surface for less than one day [1]. Two alternatives have been studied seriously: radioisotope-powered cooling, which is highly complex and needs billions in development to cool a small chamber of electronics, and high temperature electronics, which at the time of that assessment was not close to the integration level a rover needs.

The lifetime of a sealed vessel is set by insulation mass and initial internal temperature. Nothing in that record is a repairable failure; the vessel reaches ambient temperature and the machine stops.

Entry system and surviving data

No engineering description of the Venera and VeGa landers has been published in the open literature: lander mass, dimensions, the pressure vessel design, the pre-chilling method, the imaging system, the surface drill and its transfer of sample to an internal analysis chamber, and the relay arrangement through the flyby bus are all absent. The one gross figure available is the entry system class: the Venera and VeGa entry spheres were traded against Pioneer probe aeroshells in a later Venus mission study whose 700 kg vehicle is described as similar in mass to a Venera mission [1].

What the landers demonstrably did is measurable in the derived data sets that survive. Venera probe measurements are part of the Venus International Reference Atmosphere that Venus-GRAM is built on [5], and that VIRA climatology is still what the model uses for mean zonal wind up to 80 km.

What has changed since

The Venera lifetime is a property of the architecture, not of Venus. Two things would change it.

The first is qualifying hardware against the real chemistry rather than against temperature and pressure. Reproducing temperature and pressure alone is explicitly insufficient to qualify parts for a prolonged surface mission [2], which is why the ground facility built for this work carries nine independently controlled gas streams, each fed from its own bottle through a thermal mass flow controller, rather than a furnace and a pressure vessel [6].

The second is removing the requirement for a cooled interior. Silicon carbide junction field effect transistor integrated circuits have run continuously at 500 C for 10,100 hours, 421 days, with parameters stable to under 5 percent after a 100 to 200 hour burn-in [3]. The failure mechanism at that temperature is thermal-stress cracking of the passivation stack admitting oxygen to the metallization, not the semiconductor itself [4]. In reproduced Venus conditions, a packaged SiC clock integrated circuit ran for 60 days at 465 C and 90 bar [7], and an SO2 chemical microsensor platform ran for 60 days in the same facility [2]. The demonstrated device temperature range runs from -190 C to a short-duration record of 961 C [3].

A lander built that way needs no pressure vessel and no thermal budget. The target for such a platform is 60 Earth days or longer on the surface, powered by a sodium sulfur battery of a chemistry already operated at 460 C and 92 bar [2]. Against 23 to 127 minutes, that is a change of three orders of magnitude in mission duration from the same environment [1].

Status

Complete. No lander has operated on the Venus surface since the VeGa missions.