MOSCOW, Jan. 6--The Soviet Union has launched an experimental rocket 300 kilometers (186.42) miles into the atmosphere with a human aboard, reliable sources here said tonight. The man aboard the rocket parachuted successfully to earth, these sources said.

There was no official announcement whatsoever concerning this latest Soviet venture into the atmosphere.

The successful experiment reportedly took place within a day or so after the new year, the reports said.

Some time ago Soviet scientists said they sent up dogs to a lesser height. The dogs were released and parachuted to safety, they said.

Rumors about the experiment with a human being began circulating in Moscow about a week ago.

Equality of U.S. Rockets Asserted

CAMBRIDGE, Mass., Jan. 6--A rocket similar to the German V2 now in American hands is capable of shooting a man to the 186-mile height reportedly achieved by the Russians, a scientist said tonight.

Dr. John S. Rinehart, associate director of the Smithsonian Astrophysical Observatory, said the rocket Russians reportedly used to put a man aloft did not have to be nearly as large as that required to launch a satellite.

Dr. Rinehart pointed out that putting a man to a height of 186 miles was much less a problem, in terms of the rocket power needed, than to launch a satellite of a man's weight.

The rocket with its man passenger only needed to reach the desired height, and fall back to earth by gravity.

Soviets Cut Troops

MOSCOW, Jan. 6--Soviet Russia announced Monday it is withdrawing and disbanding 58,000 troops in Hungary and East Germany as part of a slash of 300,000 men in the Soviet armed forces.

More than 41,000 will be pulled out of East Germany, the announcement said, and 17,000 from Hungary.

Deputy Foreign Minister V. V. Kuznetsov told a specially summoned news conference the cuts would start immediately and be completed this year.