The Dawn spaceprobe was launched toward the asteroid belt a month ago now. Few people remember that the Soviet Union was also planning a multiple asteroid flyby mission in the 80's. After the successful cooperation between France and the Soviets on the VEGA Venus (and Halley comet flyby) mission in 1984-85, a new project was proposed. The Vesta mission would have consisted of two identical probes (just like earlier Soviet Venus missions), to be launched in 1991. Similar to the Vega mission, each spacecraft would deploy one or more landers or balloons into the Venusian atmosphere, and then proceed to its next target. At Venus, a French satellite dedicated to asteroid flybys would be released. It would return to us for an Earth swing-by, and then reach about 3-3.3 AUs from the Sun. There they would fly by some smaller asteroids, and Vesta, if possible, with a small probe landing there (that's why the mission was named VESTA). The exact targets depend, on the launch date, among other things. In the 1985 study, 2700 possible trajectories were analyzed for a launch date in 1991/1992. Considering all constraints, like maximum velocity increment, left about 12 candidate trajectories. Of course, the two identical spacecraft could choose different trajectories and targets. These included 5 Astraea, 53 Kalypso, 187 Lamberta, 453 Tea, 1335 Demoulina and 1858 Lobachevskij, and comet Encke.
Just as the 1988-89 Fobos missions reflect the Soviet focus shifting to Mars, around 1985 Vesta was changed to be a Mars mission, with the asteroid-part unchanged. Detailed studies indicate each probe would have visited four small bodies, including asteroids belonging to different classes - providing a representative sample of the diversity of asteroids - and probably one or two comets as well. Visiting at least one Apollo-Amor (Earth-nearing) asteroid was also given a preference. Preliminary studies call for at least the following scientific instruments to be included:
- a wide angle camera (~6.5° field of view, 512x512 pixel CCD)
- a narrow angle camera (~0.5° field of view, 512x512 pixel CCD - 3.9 arcsec/pixel)
- a near-infrared spectrometer (measuring between 0.5-5 micrometers with lambda/delta lambda = 50, 5 arcminutes per pixel)
Possible further instrumentation:
-UV spectrometer (for imaging during a comet flyby)
-radar altimeter/radiometer
-a dust detector
-ion or neutral gas detector
Onboard memory is about 240 Mbits. Images at closest approach (which is typically 500 km) have a resolution of 10 m/pixel. Worst case downlink rate is 600 bit/second (if not using Deep Space Network (DSN)). The scientific payload is about 100 kg. The spacecraft has 750 kg dry mass, and carries 750 kg propellants, and possibly a 500 kg penetrator. 20 square meters of solar panels provide 350 Watts of power.
(For a comparison, 10-15 years later, the similarly sized solar arrays on the (similarly sized) Dawn spacecraft produce 1300 W at the target distance of 3 AU from the Sun (or 10 kW at 1 AU). It's well consumed though: the ion thrusters on Dawn work at maximum thrust at the (combined) consumption of 8 kW. Another improvement: Dawn has 8 Gbyte memory. It also carries two identical, 5.5° field-of-view, 1024x1024 pixel cameras, which provide 9 meter/pixel resolution from 100 km altitude.)
If DSN support can be obtained, Doppler tracking of the Vesta spacecraft's movement can be used to accurately determine the mass of the encountered bodies. In the other case, another possibility was considered: releasing a test mass, and observing its movement near the target asteroid.
The spacecraft's structure is derived from Telecom satellites (INMARSAT), having the required mass, volume, and delta-v capabilites. It's 3-axis stabilised, with a pointing platform with 2 axes of freedom, for some of the scientific instruments.
The Mars gravity assist(s) constrain the possible trajectories (flyby altitude at Mars should't be too high, not to jeopardize the main mission, flyby speed has to be slow enough for the insertion of the Mars craft). The asteroid penetrator also imposes limits on the speed of the approach of the target asteroid (must be less than 4 km/s). Nevertheless, 3 possible trajectories were designed, with 2 Mars gravity assists. A single Mars swing-by is also possible, but the double gravity assist increases the mass budget of the spacecraft by 30%, at the cost of an additional 1.8 year in travel time to the asteroid belt. The following trajectories are for the 1994 launch window. The size and spectral(?) type of each asteroid is also shown here:
Trajectory 1:
-launch from Earth
-Mars gravity assist
-flyby of 2335 James (a 10 km X-type asteroid) (an Amor-asteroid)
-Mars gravity assist
-109 Felicitas (C-type, 76 km)
-739 Mandeville (EMP(?) type, 110 km)
-4 Vesta (V-type, or Vestoid. Has a diameter of 570 km) flyby with 3.5 km/s. A penetrator is released.
Total delta-v: 450 m/s
Trajectory 2:
-launch from Earth
-Mars gravity assist
-flyby of the P/Tritton short period comet
-Mars gravity assist
-2087 Kochera (30 km?)
-1 Ceres (flyby & releasing a penetrator)
Total delta-v: 1150 m/s
Trajectory 3:
-launch from Earth
-Mars gravity assist
-1204 Renzia (10 km?) (an Amor-asteroid)
-Mars gravity assist
-435 Ella (U type, 30 km)
-46 Hestia (F type, 165 km)
-135 Hertha (M type, 80 km)
Total delta-v: 350 m/s
In other studies, 11 Parthenope, 19 Fortuna and 20 Massalia were also considered.
Certain orbits can mean additional constraints. For example, 739 Mandeville has an inclination of 20° to the ecliptic, so you have to be lucky to catch it (when it's near the ecliptic plane). Especially if you want more than a brief rendezvous! The Dawn spacecraft is parked around Ceres and Vesta for longer periods - however, that means it first has to match their speed. This prevents it from reaching 2 Pallas, the next obvious target, because of that asteroid's high inclination.
Update: I've heard it's not ruled out that Dawn will be directed to rendezvous with 2 Pallas (for a slow flyby) in 2018, after the main mission at Vesta and Ceres is completed and enough fuel is left.
Finally, a combination of factors, probably including changing Franco-Soviet relations, the partial failure of the Fobos mission, growing financial troubles, the disbanding of the Soviet Union, put this mission on the long shelf of never realized dreams. However, the idea kept floating around (and technology kept developing), and is now resurrected (in a completely different form) as the Dawn asteroid orbiter.
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