Mission Overview
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The return of Halley's comet in 1985/86 provided a unique opportunity to
combine a mission to explore Venus, (continuing the Venera lander series)
with exploration of Halley by employing a two-element space vehicle
comprising a Venus lander (and balloon) and a carrier spacecraft which would
go on to Halley.  The mission was called VeGa, a contraction of the Russian
words 'Venera' (Venus) and 'Gallei' (Halley) and was conducted by the USSR
with scientific payloads and ground tracking support from several countries,
notably France and the USA. The Project Scientist was Roald Z. Sagdeev of
the Institute for Space Research (IKI) in Moscow.

Like many Soviet planetary missions, Vega comprised two identical spacecraft,
Vega 1 and Vega 2.  This was a standard Soviet approach to ensure the overall
reliability of the mission. If both spacecraft were successful there would be
a significant increase in the scientific return, which was particularly
valuable in the case of the Halley flybys given the variability of comet
activity.

The two spacecraft were launched by Proton rockets from the Baikonur
cosmodrome on 15 and 21 December 1984, respectively.  On 11 and 15 June
1985, the two spacecraft successfully delivered lander entry vehicles into
the Venus atmosphere. During the descent of these landers, each released a
helium-filled balloon to float near the cloud tops.  The landers each reached
and survived on the surface for about an hour, while the balloons operated
for about 2 days. The Vega 1 and Vega 2 'mother' spacecraft went on to
encounter comet Halley on 6 and 9 March 1986, respectively.

During early planning of the mission, larger French-led balloons had been
considered but the inclusion of comet Halley in the mission plan for the
flyby spacecraft forced the use of the smaller, Soviet balloons eventually
flown.

Both balloons were successful, transmitting for 46.5hrs in the nightside
atmosphere while being tracked by an international network of radio
telescopes on Earth which received the data directly from the balloons on a
very low data-rate one-way radio link (4 bits per second transmitted
initially for 270 s every 30 minutes 4.5W at 1.667 GHz).

The balloons were small (3.54m diameter) and filled with ~2kg of helium from
tanks carried on the landers. The balloon scientific return was ultimately
limited by energy, supplied by lithium primary batteries (~1 kg,  250 W-hr).
Their payload was austere (the whole gondola was 6.9kg), with pressure and
temperature sensors, a light level/lightning detector, a nephelometer (cloud
backscatter sensor) and a lightweight 'windmill' vertical anemometer.
Additional investigations included Doppler measurements to retrieve
turbulence and windspeed, and position measurements using the Differential
Very Long Baseline Interferometry (DVLBI) technique.

The spherical entry vehicles were ~1750kg in mass, including entry thermal
protection extraction parachutes, and the helium tanks for the balloons.
The landers themselves were ~750kg and took about 1 hour to reach the
surface.  The landers carried a variety of instrumentation for study of the
atmosphere and surface.

See SAGDEEV1986H and SAGDEEV1986I.