Back in September, the Pan-STARRS1 survey telescope noticed an object that followed a slight but distinctly curved path in the sky, a telltale sign that it was captured by Earth’s gravity. Initially, this object was thought to be a near-Earth Asteroid (NEA) and was given a standard designation by the Minor Planet Center (2020 SO). However, the Center for Near-Earth Object Studies (CNEOS) at NASA JPL had another theory.
Based on its orbit and the way solar radiation appeared to be pushing it off course, NASA scientists have since concluded that the object might actually be the spent upper stage booster of the Centaur rocket that launched the Surveyor 2 spacecraft towards the Moon in 1966. This finding could have implications for future surveys that pick up mysterious objects near Earth (‘Oumuamua occur).
The first hints were the highly unusual orbit of the object, which was at about the same distance as Earth is to the Sun, in a similar orbital plane, and nearly circular. But the decisive clue came when astronomers at Pan-STARRS and around the world made additional observations of 2020 SO that revealed the degree to which solar radiation was altering its trajectory.
Animation of 2020 SO’s orbit on Nov. 8, 2020. Its motion has been speeded up a million times faster than real-time. Credit: NASA/JPL-Caltech
Spent rocket stages are essentially empty tubes once they discarded in orbit, that have a low density and a high surface area. As a result, they are pushed around by solar radiation pressure more than a solid, high-density object (like an asteroid). As Davide Farnocchia, a navigation engineer at NASA JPL who analyzed 2020 SO’s trajectory for CNEOS, explained:
“Solar radiation pressure is a non-gravitational force that is caused by light photons emitted by the Sun hitting a natural or artificial object. The resulting acceleration on the object depends on the so-called area-to-mass ratio, which is greater for small and light, low-density objects.”
All told, more than 170 detailed measurements were made of 2020 SO’s position over the last three months. This included observations conducted by the NASA-funded Catalina Sky Survey in Arizona and the European Space Agency’s (ESA) Optical Ground Station in Spain. These showed how much of an effect solar radiation pressure was having, which confirmed the low-density nature of 2020 SO.
The next step was to determine where the suspected rocket booster could have come from. To do this, CNEOS Director Paul Chodas ran the object’s orbit in reverse, which showed that 2020 SO had made some relatively close approaches to Earth a few times over the decades. However, the analysis showed that the object’s closest approach (in late 1966) was close enough to Earth that it might have originated here.
This 1964 photograph shows a Centaur upper-stage rocket before being mated to an Atlas booster. A similar Centaur was used during the launch of Surveyor 2 two years later. Credit: NASA
This coincides with the launch of the Surveyor 2 lunar lander, which was launched toward the Moon on Sept. 20th, 1966, on an Atlas-Centaur rocket. The mission was designed to reconnoiter the lunar surface in advance of the Apollo missions to scout possible landing sites. However, a day after it separated from its upper stage booster, one of the spacecraft’s three thrusters failed to ignite, throwing the spacecraft into a spin.
The spacecraft lost control and on Sept. 23th, 1966, it crashed into the moon just southeast of the Copernicus crater. Meanwhile, the spent Centaur upper-stage rocket sailed past the Moon and disappeared into an unknown orbit about the Sun. As Chodas said, it’s likely that it never escaped the gravity of the Earth-Moon system:
“One of the possible paths for 2020 SO brought the object very close to Earth and the Moon in late September 1966. It was like a eureka moment when a quick check of launch dates for lunar missions showed a match with the Surveyor 2 mission.”
The orbital path that 20202 SO has since brought it back to Earth for another approach. On Nov. 8th, 2020, it slowly drifted into Earth’s Hill Sphere, a region of gravitational influence that extends about 1.5 million km (930,000 miles) around Earth. This is where it will remain for another four months, making its closest approach on Dec. 1st. This will allow astronomers to get a closer look at it and a chance to confirm its identity.
Artist’s impression of the interstellar object, `Oumuamua, experiencing outgassing as it leaves our Solar System. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser
This discovery is also significant because of the way it demonstrates how low-density, high surface area objects can be accelerated by solar radiation pressure. This same principle is what informs the design of solar sails and was even observed when the interstellar object ‘Oumuamua departed our Solar System in 2017. Despite an absence of outgassing, the object accelerated from exposure to solar radiation.
This behavior and unresolved questions about its shape led Dr. Shmuel Bialy and Prof. Abraham Loeb to famously suggest in 2018 that ‘Oumuamua might be artificial in origin. Whereas Bialy is a postdoc with the Institute for Theory and Computation (ITC), Prof. Abraham Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University and the founder of the ITC. As Loeb told Universe Today via email:
“The discovery of 2020 SO by Pan STARRS in September 2020 lends credibility to the notion that artificial objects can be distinguished from natural objects based on the fact that sunlight is pushing them. `Oumuamua was detected by Pan STARRS in October 2017 and the subsequent monitoring of its orbit indicated that just like 2020 SO, it experienced an excess push away from the Sun without showing a cometary tail.”
“The fact that 2020 SO is a relic of our civilization, strengthens the case that `Oumuamua could have been artificial, made by another civilization. The situation resembles the activity I enjoy most on vacation with my family, namely walking on the beach and looking at seashells. Every now and then, we tend to find a plastic bottle that is a relic of civilization. The newly discovered object 2020 SO is such a relic from our civilization. Perhaps `Oumuamua is a relic from another civilization.”
An artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. Credit: M. Weiss/CfA
Loeb delves into the peculiar properties of `Oumuamua and the possibility that it was a “message in a bottle” from an extraterrestrial civilization in his forthcoming book “Extraterrestrial,” which will be available for purchase on January 26th, 2021.
By March 2021, 2020 SO will leave Earth’s orbit and assume a new orbit around the Sun. As NASA nears its long-awaited return to the Moon, it’s nice to be visited by an artifact from the early Space Age, a reminder of a time when humans were struggling to get to the Moon for the first time.
Further Reading: NASA