Fifty-four years ago NASA launched the Surveyor 2, an unmanned mission to explore the surface of the moon. Sadly, the spacecraft fell en-route, after a failed course correction burn, and crashed into the lunar surface at 2.7 kilometers per second. But the rocket thruster used when it was launched has followed a different trajectory in space and has now started to orbit the Earth. It’s the conclusion astronomers who have studied 2020 SO, an unusual object first spotted last August.
NASA’s Surveyor program was designed to demonstrate the feasibility of launching, communicating, controlling, and landing an unequipped spacecraft on the moon, paving the way for subsequent manned missions. Surveyor 1, launched on May 30, 1966, was a resounding success, easily achieving its primary objectives while returning numerous images of the lunar surface and critical technical data. NASA’s hopes were therefore high to build on this success with the second mission. The intention was to Land surveyor 2 on the moon just east of its predecessor to demonstrate the feasibility of an oblique approach and landing.
After a series of minor delays, Surveyor 2 successfully launched on September 20, 1966 at 7:31 a.m. EST. The launcher was an Atlas-Centaur rocket using liquid hydrogen and liquid oxygen as propellants. After the Centaur’s engines were shut down and the spacecraft rolled for 66 seconds, Surveyor 2 left the exhausted Centaur. The rocket expelled its residual thrusters to establish a safe distance between itself and the Surveyor 2, giving it a trajectory that would miss the moon by 5,675 kilometers and send the rocket into a solar orbit. This Centaur rocket is what astronomers have confirmed to be 2020 SO.
Everything seemed to be going well for Surveyor 2. Its trajectory to the moon was almost perfectly on target, with just a slight burn needed for a course correction 16.5 hours after launch. This is where things went horribly wrong. The craft’s Vernier Engine 3 did not ignite with the others, sending Surveyor 2 tumbling into space. NASA scientists tried to start the engine 39 times before accepting defeat. It was then that they tried to glean the useful data they could from a series of engineering tests before Surveyor 2 crashed into the lunar surface on September 23, southeast of the Copernicus crater.
NASA has never officially determined the cause of the engine’s inability to ignite, although an issue with the flux of MON-10 oxidant appears to be the culprit. There were five other surveyor missions, four of which were successful, before NASA focused on crewed moon exploration.
Meanwhile, the exhausted Centaur rocket continued its merry way through space and was all but forgotten – at least until August of this year, when astronomers using the panoramic telescope and rapid response system (Pan- STARRS) in Hawaii spotted a mysterious object in a solar orbit that was supposed to pass close to Earth. The Pan-STARRS Observatory is a key instrument for detecting fast moving objects such as new asteroids. But this new object, dubbed 2020 SO, was moving too slowly to be an asteroid, and it also had a rather odd orbit.
Paul Chodas, who heads the Center for Near Earth Object Studies at JPL, was among the many avid scientists to learn more about 2020 SO. He ran computer simulations of its nearly circular trajectory, noting that its orbit was barely tilted relative to Earth. Puzzled, Chodas ran the simulation upside down and found that the object passed close enough to Earth in September 1966 to come from there. In other words, 2020 SO could be the depleted Centaur rocket that launched the doomed Surveyor 2, especially given its slightly anomalous orbit. This is proof that the object is being pushed by photons emitted from the sun, so 2020 SO should be relatively large and have low mass, just like an empty rocket thruster. (Rocky asteroids, by comparison, are relatively small and massive.)