In a close race with the Soviets, the United States launched its first artificial satellite, Explorer 1, into a orbit on 31 January 1958. Explorer I was an long by diameter cylinder weighing , compared to Sputnik 1, a sphere which weighed . Explorer 1 carried sensors which confirmed the existence of the Van Allen belts, a major scientific discovery at the time, while Sputnik 1 carried no scientific sensors. On 17 March 1958, the US orbited its second satellite, Vanguard 1, which was about the size of a grapefruit, and remains in a orbit .