"It was stunning, beautiful, and scary all at the same time," one Twitter user wrote.
The object will come within around 139,500 miles of our planet.
The object, which is estimated to measure between 15-33 feet across, was discovered on September 17, 2020.
Satellites have collided only four times in the past 30 years, making it difficult to form generalizations about cause.
"I'll be in the office [Saturday] and probably early Sunday morning," Thompson said, sounding more excited than resigned. "We'll be watching it the whole way."
The harpoon could tug the 9-ton Envisat towards the atmosphere.
If we keep making satellites at the current rate, nearby planets could spot us by 2200.
Usually, space debris ends up falling to Earth over oceans and in smaller pieces, making it harder to spot.