For the reason that koalas should not be classified as monkeys without a visible tail, spacecraft should not be classified as dark comets without a visible tail. Comets are defined as cosmic snowballs of frozen gases, rock and dust, which spew gases and dust into a glowing head and a long tail as they get warmed by the Sun. Spacecraft are vehicles for travel in space.
A recent paper identified a population of near-Earth objects that exhibit non-gravitational accelerations with no coma, and labeled them ‘’dark comets”. In a follow-up paper that I wrote with my postdoc, Richard Cloete, we showed that one of these objects, 2005 VL1, was at closest approach to Earth in late 1965, around the time when the lost Soviet spacecraft Venera 2 was launched to explore Venus.
The observed brightness of 2005 VL1 is consistent with a high reflectance from the full surface of Venera 2 including its Solar panels. As known for Venera 2, 2005 VL1 arrived within a short distance from Venus, a highly improbable coincidence (<1%) for the orbital phase of a near-Earth object that does not target a close approach to Venus. Indeed, 2005 VL1’s orbital parameters are similar to the reported values for Venera 2. Given the area-to-mass ratio of Venera 2, our paper showed that 2005 VL1’s non-gravitational acceleration and negligible transverse acceleration match the values expected from Solar radiation pressure.
Subsequently, two preprints (posted here and here) argued that the small differences between the orbital parameters of 2005 VL1 and Venera 2 imply that they are not the same object. In response, I wrote a brief research note, in which I show that the small differences in orbital parameters between these objects could have been caused by gravitational deflection and unrecorded maneuvers during the flyby of Venera 2 near Venus.
The fractional velocity shift of Venera 2 as a result of its passage at a distance of closest approach b and a velocity v relative to Venus is given by, d=(dv/v)=2GM/(bv²), where G is Newton’s constant and M is the mass of Venus. Adopting a velocity v of 30 kilometers per second and a closest distance to Venus of b=24,000 kilometers as reported for Venera 2, yields d~3%. In units of the Earth-Sun separation, the distance of closest approach to the Sun is 0.718 for Venera 2 and 0.698 for 2005 VL1. The small difference between these values of ~3% is similar in magnitude to the expected value of d. A change in inclination by 3% in radians corresponds to 1.7 degrees.
Radio communication with Venera 2 stopped on February 10, 1966, about 17 days before its closest approach to Venus from which there was no data return. If the flyby of Venera 2 near Venus was at a lower speed or distance than assumed above, then the gravitational deflection and unrecorded maneuvers of the spacecraft near Venus, as well as any additional orbital perturbations over the past 60 years, could have resulted in the shifts required to match the final orbital elements of Venera 2 to those of 2005 VL1. Following 64 successive revolutions around the Sun with non-gravitational acceleration, Venera 2 might occupy a slightly different orbit than it started with.
The distinction made by comet experts between Venera 2 and 2005 VL1 boils down to the question of whether a 60-year-old koala that looks a bit different from the way it looked close to its mother at birth must be a monkey without a tail.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
