On September 17, 2020, the Pan-STARRS1 (Panoramic Survey Telescope and Rapid Response System 1) survey at the Haleakala Observatory in Hawaii detected a slow moving object which received the official asteroid designation of “2020 SO”. Designed to survey the sky for variable or moving objects, this discovery was not unusual for Pan-STARRS1 but the orbit derived from its observations quickly showed that it was approaching Earth and would make a fairly close pass of our home world. Projected back to May 31, 2020 before the asteroid’s orbit was significantly perturbed by the Earth’s gravitational influence, 2020 SO was in a solar orbit with a semi-major axis of 1.0731 AU, an eccentricity of 0.03390, an inclination of 0.14069° and a period of 1.056 years. Based on its brightness and distance, 2020 SO was estimated to be about 8 meters across.
The small size of this object and its Earth-like orbit immediately raised red flags for Paul Chodas who is the director of the Center for Near-Earth Object Studies at JPL. With a slow approach speed of just 670 meters per second, Chodas speculated that 2020 SO could actually be a piece of space junk which escaped the Earth-Moon system at some point in the past. Projecting the object’s orbit back through time, Chodas found that 2020 SO made distant passes of the Earth in 2002 and 1984 as our planet, in its slightly faster orbit, lapped the object. But 2020 SO came no closer than about 8 million kilometers during these encounters making it too dim to detect. Projecting further back in time, the object was found to have been in the Earth-Moon system in the fall of 1966. Although his identification is still tentative, Chodas believes that 2020 SO is not an asteroid but the spent Centaur stage used to launch NASA’s Surveyor 2 to the Moon on September 20, 1966.
Surveyor 2 & Atlas-Centaur 7
The objective of NASA’s Surveyor program was to soft land a robotic spacecraft on the Moon to characterize its surface in support of the Apollo program. The launch vehicle for the metric-ton Surveyor spacecraft was the Atlas-Centaur. The 3-meter in diameter Atlas-Centaur was built by General Dynamics (whose space division is now part of Lockheed Martin) and consisted of two stages connected by an interstage adapter. The first stage was a modified Atlas D ICBM which used RP-1 and LOX as propellants for its three main engines and pair of vernier engines. With a total length of 20 meters including the interstage adapter, the modifications to the Atlas included a redesigned forward section to accommodate the Centaur which was substantially wider and heavier than upper stages used earlier with the Atlas. The MA-3 propulsion system built by Rocketdyne for the Atlas D was also replaced with a higher performance MA-5 which generated 1,730 kilonewtons of thrust at lift off.
A cutaway illustration of the Atlas-Centaur used to launch NASA’s Surveyor lunar landers. Click on image to enlarge. (NASA)
NASA’s innovative Centaur was the first rocket stage to use cryogenic liquid hydrogen and liquid oxygen (LOX) as propellants. Despite the engineering challenges, this propellant combination provides up to half again as much thrust than a like mass of conventional propellants. The new Centaur second stage had a total length of 14 meters including its nose shroud (not too different than the estimated size of asteroid 2020 SO) and a launch mass of 17 metric tons, excluding its payload. It used a pair of Pratt & Whitney RL-10 engines that generated 67 kilonewtons of thrust each. Because the types of orbits for NASA’s proposed missions required a restart capability, Centaur included an attitude control system and ullage jets powered by hydrogen peroxide to keep the stage oriented properly and its propellants settled in their tanks during the weightless coast between engine burns. Unfortunately, delays in the development of the Centaur forced NASA to adopt a direct ascent profile for the first Surveyor missions that required only a single burn of the Centaur with no stop in low orbit (see “The Launch of Atlas-Centaur 5”). Atlas-Centaur 10 (AC-10) successfully used this direct ascent profile to launch NASA’s Surveyor 1 to the Moon on May 30, 1966 for the first “operational” use of the Atlas-Centaur (see “Surveyor 1: America’s First Lunar Landing”).
Cutaway diagram of the AC-7 Centaur stage. Click on image to enlarge. (NASA)
The launch vehicle assigned to the second Surveyor mission, called “Surveyor B” before launch, was the second operational Atlas-Centaur designated AC-7. With the spacecraft targeted for a landing on Sinus Medii near the center of the visible face of the Moon, the first launch window for the Surveyor B mission extended from about 5:51 to 7:33 AM EST on September 20, 1966 with an arrival on September 22 after a 63-hour transit. If this window were missed, shorter launch windows were available during each of the following three days with later launch times on each successive day. The last chance for launch on this month was provided by a 14-minute window starting at 7:35 AM EST on September 23.
This schematic illustrates the effects of the various constraints on the launch windows of the Surveyor B mission for launch between September 20 to 23, 1966. Times are in GMT. Click on image to enlarge. (NASA)
The Flight of Atlas-Centaur 7
Following the completion of Launch Vehicle Combined System Testing at General Dynamics’ dedicated test stand in San Diego, AC-7 was destacked and shipped to Cape Kennedy for launch. The Atlas was shipped by air on June 18, followed by the nose faring and interstage adapter on June 21 and the Centaur on June 24. The Atlas first stage of AC-7 was erected on the pad at Cape Kennedy’s Launch Complex 36A on June 22, 1966 only three weeks after the launch of Surveyor 1. This was followed a week later with the mating of the Centaur upper stage and the beginning of almost six weeks of testing.
The Centaur stage being lifted into place on June 29, 1966 during the stacking of Atlas-Centaur 7 (AC-7). (General Dynamics/SDASM)
Meanwhile, the Surveyor SC-2 spacecraft to be flown on the Surveyor B mission arrived at the Cape on July 19 for the start of its preparations for launch. After SC-2 was encapsulated in its fairing, it was mated to AC-7 on August 9 for a series of integrated tests. Following the completion of these initial tests, SC-2 was demated a week later for the removal of test items and final preparations for launch. SC-2 was mated with AC-7 for a final time on September 16 for its September 20 launch.
The launch of Surveyor 2 on AC-7 from LC-36A at Cape Kennedy on September 20, 1966. (General Dynamics/SDASM)
After dealing with a series of minor issues during the countdown, 137-metric ton AC-7 finally lifted off from pad 36A at 7:31:59.8 EST (12:31:59.8 GMT) on September 20, 1966 only 8.5 seconds before the close of the first day’s launch window. With a nominal performance of both stages of the Atlas-Centaur, the Centaur’s pair of RL-10-A3-1 engines were shutdown after a total of 686.2 seconds of powered flight with the stage and what was now called Surveyor 2 travelling at a velocity of 10.53 kilometers per second.
A schematic highlighting the major milestones of the launch of AC-7. Click on image to enlarge. (NASA)
After coasting for 66 seconds, Surveyor 2 separated from the spent Centaur and started aligning itself into its cruise attitude. Surveyor 2 was now in a 165.0 by 784,170 kilometer geocentric orbit with an inclination of 33.4° that would intercept the Moon on its outbound leg. While AC-7 had succeeded in sending Surveyor 2 towards the Moon, the spacecraft itself would fail it soft landing attempt. During a routine midcourse trajectory correction performed 21½ hours after launch, one of Surveyor’s three vernier engines failed to ignite sending the spacecraft into an uncontrolled tumble. Despite the efforts of ground controllers, the failed engine could not be restarted and Surveyor 2 crashed southeast of the crater Copernicus on September 22 hitting the lunar surface at 2.7 kilometers per second (see “Surveyor 2: Things Don’t Always Go As Planned”)
Afterwards
With its mission completed, the Centaur was commanded at 12:44:38 GMT (five seconds after spacecraft separation) to turn for its retromaneuver to distance itself from Surveyor 2. Halfway through its turn, Centaur started firing its pair of 222-newton hydrogen peroxide thrusters for 20 seconds. After completing its 157° turn, the attitude was stabilized 82 seconds following the beginning of the turn. At 12:48:32 GMT, the Centaur was commanded to expel its estimated 118 kilograms of residual cryogenic propellants through the pair of RL-10 engines to further distance itself from Surveyor. The Centaur’s retromaneuver was completed 20 minutes and 42 seconds after launch and its propellant tank vents opened. Centaur’s control system was shutoff shortly afterwards to let the spacecraft free drift. Tracking of the stage’s automated C-band beacon continued until about 1¼ hours after launch. The data showed that the spent Centaur was in a 168 by 588,992 kilometer orbit which placed it a safe 730 kilometers from Surveyor 2 five hours after separation – twice the minimum requirement to avoid interfering with the spacecraft (see Table A-III from Atlas-Centaur Flight Performance for Surveyor Mission B included in the General References at the bottom of this page for detailed geocentric orbital elements). In its new trajectory, the now silent 1,594-kilogram Centaur would miss the Moon by 5,675 kilometers and be perturbed by its lunar encounter into solar orbit.
After 54 years, it now appears that the spent Centaur stage of AC-7, in the guise of asteroid 2020 SO, is now returning home. While its precise trajectory past Earth is still subject to refinement as new observations are gathered (not only because of the uncertainties in the derived orbit, but due to the trajectory’s chaotic nature which amplifies those uncertainties), 2020 SO will approach the Earth from the leading L1 Lagrange point with the eccentricity of its geocentric orbit falling below one on October 15, 2020 (indicating it has entered a closed orbit around the Earth) and entering Earth’s Hill sphere (the outer limits of geocentric orbits due to the influence of the Sun) in November.
A diagram highlighting the path of 2020 SO through the Earth-Moon system (with lunar orbit indicated by the circle). The object approaches the Earth from the left in the fall of 2020, makes one irregular loop around the Earth before exiting to the right in the spring of 2021. The details of this prediction are subject to change as the object’s orbit is refined. Click on image to enlarge. (Wikipedia)
2020 SO will then make pass about 50,000 kilometers from the Earth on December 1, 2020 when its peak visual brightness will reach an estimated magnitude of 14.1 – within the reach of larger amateur telescopes. The details of what happens next are still subject to change but, 2020 SO will make one long, irregular orbit of the Earth and pass no closer than about 150,000 kilometers on February 2, 2021 before exiting the Earth-Moon system from the trailing L2 Lagrange point. Depending on the details of its encounter with Earth, 2020 SO could return to the vicinity of our home world in the late 2030s. With luck, continued refinement of its orbit and other properties might lead to a confirmation of the identification of 2020 SO as the Centaur from AC-7 as Paul Chodas has proposed.
Follow Drew Ex Machina on Facebook.
Related Reading
“Surveyor 2: Things Don’t Always Go As Planned”, Drew Ex Machina, March 13, 2017 [Post]
“The Launch of Atlas-Centaur 5”, Drew Ex Machina, March 2, 2015 [Post]
General References
Marcia Dunn, “Fake asteroid? NASA expert IDs mystery object as old rocket”, Phys.org, October 11, 2020 [Post]
Second Surveyor Launch Set for September 20, NASA Press Release 66-248, September 14, 1966
Surveyor II Mission Report: Mission Description and Performance, JPL Technical Report 32-1086, April 1, 1967
Atlas-Centaur Flight Performance for Surveyor Mission B, NASA Technical Memorandum TM X-1616, April 1968
For those who are interested in investigating the orbit of the Centaur stage of AC-7, here is Table III-A from the last reference giving the geocentric orbital elements of Surveyor 2 and its Centaur. Please share any interesting results!
Here is Table A-III from Atlas-Centaur Flight Performance for Surveyor Mission B giving the geocentric orbital elements of the AC-7 Centaur stage. Click on image to enlarge.