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An incoming object spotted by astronomers last August was confirmed as the upper stage of a Centaur rocket, which NASA launched in 1966 during an ill-fated mission to the moon. It’s a nice result, but it took some time to confirm.
After months of speculation, a team led by Vishnu Reddy, an associate professor at the University of Arizona’s Lunar and Planetary Laboratory, confirmed that the 2020 SO object is the upper stage of a Centaur rocket, NASA reports.
It’s a funny discovery, but as Reddy explained in a telephone interview, the ability to detect such objects is of scientific and practical importance.
“As humanity expands into space, we will see many artificial objects in heliocentric orbits [orbiting the Sun], “He said.” It is essential that we know what lies ahead, both artificial and natural. “To which he added,” The whole process shows that it is possible to identify something that was launched 54 years ago. “
Timelapse consisting of 91 frames, showing the lost rocket rocket from 1 to 2 December 2020. The flashing indicates its falling action. (Gif: Gianluca Masi, astronomer with the Virtual Telescope Project 2.0)
2020 SO was detected in August by astronomers working with the Pan-STARRS1 survey in Maui, Hawai’i. That the object was something artificial was immediately apparent, as it was traveling along an orbit thought to be unusual for asteroids. Paul Chodas, director of the Center for Near-Earth Object Studies (CNEOS) at NASA’s Jet Propulsion Laboratory, performed some clever calculations to trace its recent history, finding that the object’s 1966 overflight of Earth was close enough to may have originated from our planet.
Alerted to this possibility – that the object could be space junk returning to visit us – NASA contacted Reddy, who specializes in this type of work.
“This is totally in my wheelhouse, as I characterize asteroids for NASA and space debris in Earth’s orbit for US aviation,” he said.
NASA asked Reddy to confirm the object was an asteroid or rocket, and to do so with a ghostly signature. Object 2020 SO has very weak visual magnitude, making visual confirmations nearly impossible. A spectral signature, on the other hand, conveys the actual composition of an object, providing the evidence needed to identify it.
Using the Large Binocular Telescope in Arizona, Reddy and his colleagues collected the color observations of 2020 SO, comparing them to the most common types of asteroids. The results did not match.
“But the money is in infrared and not in visible wavelengths,” Reddy told me.
As a result, the team turned to NASA’s infrared telescope facility in Maunakea, Hawaii, but had to wait until November for the object to get bright enough. On November 17 they managed to acquire a spectrum.
Archival photos of the second stage of the Centaur rocket showed parts of it covered in white paint, so Reddy’s team contacted a paint company to obtain samples of white paint. The problem is that the spectral signature produced by these samples did not match the 2020 SO signal.
“We were really scratching our heads,” Reddy said.
The team decided to contact a NASA historian, who informed them that the white stuff on the booster wasn’t paint – it was white foam panels that had been thrown from the vehicle during the launch. Who knew?
Their next bet was to detect the stainless steel, namely 301 stainless steel, which NASA used to build the Centaur booster. Eventually, the scientists were able to create a solid match.
Reddy said he gets nervous about the spectrum data, so he wanted more spectrographic observations of the 2020 SO to be safe, so the team once again used IRTF to scan the object on November 29 and 30. This led to a new absorption feature that hadn’t appeared during scans made on November 17th. This feature actually seemed organic, as it was rich in carbon. A seemingly strange result, but scientists realized they were seeing plastic. Specifically, the plastic mounts on the aluminum mylar used to protect the electronic components located in the top and bottom ends of the Centaur repeater.
“The rocket is rolling into space,” Reddy said, “So it makes sense, since we’re seeing everything.”
But the final test, he said, was to acquire spectrographic observations of other Centaur bodies that have been in Earth orbit since the 1970s. Reddy wanted an apple-to-apple comparison.
This actually proved to be very challenging, as the IRTF, with its tiny field of view, was not designed for this purpose. On December 1, Reddy, after trying and failing to locate these old repeaters with a backyard telescope (he almost did, but a chimney obscured his view), his graduate student, Tanner Campbell, managed to acquire a centaur position, which was then forwarded to telescope operator Dave Griep of NASA’s IRTF. This particular upper stage belonged to a Centaur D launched in 1977. The team was able to locate and scan two more in visible light. These observations provided the matches they were looking for.
“You can’t get a better match,” Reddy said. “The same steel and plastic for all centaurs.”
With 2020 SO now confirmed as an upper stage booster, launched as part of the Sojourner mission to explore the surface of the Moon before the Apollo missions, Reddy said the results testify to astronomers’ ability to characterize such objects. It turned out to be a pretty great coordination exercise, he said.
“I’m very happy,” Chodas said in an email. “I also appreciate the efforts of my colleagues to confirm that this object is indeed a higher stage than the Centaur: it takes a team to solve a puzzle like this.” To which he added: “This is another demonstration of the accuracy of our CNEOS orbital analyzes and predictions, that we can link the trajectory of a new object today to a launch 54 years ago.”
Incredible stuff, but clearly the result of a lot of work. Thanks to this experience, astronomers can now look at the next artificial object, whatever it is. As for the Centaur’s upper stage, it will make two turns around the Earth before resuming a new orbit around the Sun in March 2021.
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