From Wikipedia:

J002E3 is the designation given to a supposed asteroid discovered by amateur astronomer Bill Yeung on September 3, 2002. Further examination revealed the object was not a rock asteroid but instead the S-IVB third stage of the Apollo 12 Saturn V rocket (serial S-IVB-507).[1]

When it was first discovered it was quickly found that the object was in an orbit around Earth. Astronomers were surprised at this as the Moon is the only large object in orbit around the Earth[note 1] and anything else would have been ejected long ago due to perturbations with the Earth, the Moon and the Sun.

Therefore it must have entered into Earth orbit very recently, yet there was no recently-launched spacecraft that matched the orbit of J002E3. One explanation could have been that it was a 30-meter wide piece of rock, but University of Arizona astronomers found that the object's electromagnetic spectrum was consistent with white titanium dioxide paint, the same paint used by NASA for the Saturn V rockets. Back-tracing its orbit showed that the object had been orbiting the Sun for 31 years and had last been in the vicinity of the Earth in 1971. This seemed to suggest that it was a part of the Apollo 14 mission but NASA knew the whereabouts of all hardware used for this mission; the third stage, for instance, was deliberately crashed into the Moon for seismic studies.

The only other explanation was that it was the S-IVB third stage for Apollo 12. NASA had originally planned to direct the S-IVB into a solar orbit, but an extra long burn of the ullage motors meant that venting the remaining propellant in the tank of the S-IVB did not give the rocket stage enough energy to escape the Earth–Moon system, and instead the stage ended up in a semi-stable orbit around the Earth after passing by the Moon on November 18, 1969. The Apollo 12 S-IVB eventually vanished.

It is thought that J002E3 left Earth orbit in June 2003, and that it may return to orbit the Earth in about 2032.

The Apollo 12 S-IVB unit had a dry weight of about 23,000 pounds (10,000 kgf)[citation needed] and is in unstable Earth orbit, which may result in eventual collision with Earth. An object with a mass of 10,000 kg collides with earth approximately every 1-5 years.