Free Astronomy Magazine January-February 2021

15 JANUARY-FEBRUARY 2021 ASTRO PUBLISHING future orbits of Bennu and other NEOs. The broad-spectrum radiation from the Sun that is absorbed by Bennu is eventually released back into space in the form of thermal ra- diation – but not at the same time. Just as the solar radiation hitting Bennu imparts the slightest push against the pull of gravity, the release of thermal radiation from Bennu at a later time also effectively nudges Bennu in a slightly different direction, causing its orbit to drift by a few hun- dred meters per year. This phenome- non, known as the Yarkov-sky effect, is exceedingly small (for Bennu) com- pared to its 168 billion meter semi- major axis, but every phenomenon that changes an NEO’s orbit is some- thing that physicists must contend with, be that today or as the decades and centuries of this effect changes an orbit in unpredictable ways. The presence of OSIRIS-REx at Bennu enabled the detection of yet an- other active phenomenon that might have been impossible from Earth with current technologies. The ejections of small particles from Bennu’s surface have been de- tected on multi- ple occasions, pro- moting Bennu from “asteroid” to “active aster- oid.” While only approaching a few centimeters in size, the obser- vation of ejection means that Bennu is still changing as a result of such factors solar heating and meteor impacts. While the change in orbit due to these random ejections might be infinitesimal, one might argue that no phenomenon is insignificant if it in any way affects the long-term odds of collision. Our interest in Bennu goes well be- yond the many important discover- ies to be had about the chemistry and dynamics of the early solar sys- tem. We live in a time when the or- bits of Bennu and Earth set the stage for many close encounters of these two bodies, with observation O SIRIS-REx sent back a treasure trove of images reveal- ing an amazingly fragmented surface resembling a roughly spherical rock pile. In the shot on the left, captured 28 March 2020, the spacecraft provides an angled view of Bennu’s equator and northern hemisphere as seen from an altitude of about 4 kilometres (2.5 miles). The field of view is 52 metres (170 feet) across. To put the scene in perspec- tive, the largest boulder visible at upper left measures 14.5 metres (48 feet) across, or about the length of a large lorry, or tractor-trailer. The shot below, recorded 29 March 2020, shows a 48-metre-wide (157-foot-wide) field of view in Bennu’s northern hemisphere with a wide range of rock sizes. For scale, the small rock resting atop the boulder at lower left measures 2.5 metres (8 feet) wide, or about the length of a horse. [NASA/Goddard/University of Arizona] and prediction providing estimates of these encounters that are only reliable over many decades. Bennu is not just a potential colli- sion threat – it is currently the most likely object to impact with the Earth over, at least, the next few hundred years (estimates from NASA’s Planetary Defense Coordina- tion Office currently put these odds at 1/2700 in the second half of the 22 nd century). As such, we want to know as much as possible about this threat, including both its com- position and its orbit. !