It is almost impossible to state precisely were the frontier of space begins - it could be anywhere in a region between 621 miles and 1.988 miles above Earth. Within it, the random collision of atmospheric gas molecules is negligible and the particulate density of matter varies greatly. Also the region, the lighter gas molecules can escaps the gravity of Earth. From a historical viewpoint, however, the frontier can be considered as being at a much lower altitude: some 99 miles. There, the atmosphere becomes very tenuous and the laws of aerodynamics give way to those of astrodynamics.
	It is this, region that was first probed by rocket as man began his systematic exploration of space, a voyage that took him from Earth upper atmosphere, then into cislunar space, to the Moon, and finally, to the sister planets of the Solar System.
	For many years, scientists were denied the investiganion from Earth and instruments lifted by aeroplane and sounding balloon. Earth observation had obvious limitations. Only a few "windows" were open in the atmosphere for the recoding of data, generally those in the optical wavelengths. Additionally, with time, the atmosphere att lower levels above the regions accessible to such observations started to become opaque because ofboth man-made particulate pollution as well as light pollution from his expanding cities. By the end of World war 2 multi-engine aircraft had a ceiling of only 5 to 6*2 miles and the scientific sounding ballon could reach at best some 19*9 miles. Geophysical Year, instrumented sounding rockets were launched in the USA from an altitude of some 18*6 miles by utilising balloons to lift them to that altitude before launch. Such rockets carried payloads of 19*8 lb to altitudes of 62*1 miles.
	With the end of the war, both the USA and USSR found themselves with the means of penetrating to the upper reaches of the atmosphere with sizeable scientific payloads.
	It was the a4 (v2) rocket.