MOSCOW, Jan. 5--The Soviet cosmic rocket Mechta lost its voice today on its plunge around the sun.

Radio signals of the 1 1/2-ton projectile faded out as it passed the 370,960-mile mark and its 62nd hour aloft, in a headlong dash from the earth into man's greatest conquest of space.

But mute or not, they expected it to streak into a solar orbit Wednesday or Thursday on a pear-shaped course that possibly--barely possible--might one day swing it back to earth.

"The program of observations and scientific investigations of the rocket has been completed," Tass, the Soviet news agency said.

It is due to enter an elliptical course between the earth and Mars, they said, taking 15 months to complete the swing around the sun that the earth completes in twelve.

The Soviets originally announced the rocket would take 447 days to circle the sun. Monday night they said there had been a minor miscalculation, that one revolution would take 450 days.

The Smithsonian Astrophysical Observatory has not been tracking the rocket, since it is "much too faint to be seen with the telescope," according to J. Allen Hynek, associate director of the Observatory. However, Hynek added that the rocket "will definitely orbit at a calculable distance."