After traveling a billion kilometers, China's asteroid hunter finally arrives

What does it take to catch up with a small, tumbling rock hundreds of thousands of kilometers from Earth? For China's Tianwen-2 mission, the answer was a 400-day chase covering roughly 1 billion kilometers (621 million miles) of deep space-one that has just ended in success. The China National Space Administration has confirmed that the probe has rendezvoused with the near-Earth asteroid Kamooalewa, also known as 2016 HO3, closing to within about 20 kilometers (12 miles) and officially begin...