An NEO over Antarctica
09 September 2016
Asteroid 2016 RB1 has hit the news because of a peculiar close passage on 7 September 2016 at 19:20 CEST. About the size of a cottage, the asteroid flew past our planet at an altitude of 34000 km, roughly the same as the so-called "geostationary ring" where most telecommunication satellites reside. Yet it posed no hazard neither to our planet nor to the satellite operators. Despite having been discovered only 24 hours before closest approach, the orbit became quickly so well constrained to ensure that the computation of the incoming flyby had the necessary accuracy to rule out any Earth impact solution. As a matter of facts already in the morning of 7 September 2016 RB1 was present in our close approaches list but not in the updated Risk List, which ranks the objects for which a non-zero impact probability is detected.
The danger to geostationary satellites would be possible (although very unlikely) only if the closest approach distance were reached near the Earth equatorial plane, where the geostationary ring is located. But 2016 RB1 was heading to the South pole, scoring the minimum altitude just over Antarctica. This had an interesting consequence: it is well known that a polar encounter geometry causes the gravitational pull of our planet to change the asteroid orbital inclination and 2016 RB1 was no exception – but with a peculiar behaviour.
The asteroid nodes were switched, meaning that while before the 7 September encounter the asteroid used to cross the ecliptic in that point coming from above, now the orbit is turned upside down after Earth gravity bended upward the asteroid trajectory. The effect is clearly shown in the diagram above, which has been computed using the NEO Coordination Centre orbit visualizer. Zooming into the Earth encounter geometry is also displayed thanks to a forthcoming new functionality, which will be soon available on our website.
The top panel shows the geometry of the 2016 RB1 Earth encounter. The bottom images show the flyby diagrams: the red arrow indicates the distance from the Earth at closest approach, the green arrow shows the direction of motion [Credits: ESA/NEOCC].
The NEOCC has timely alerted its network of collaborating telescopes to attempt follow-up observations, which were successfully carried out by the team led by Daniela Lazzaro using the OASI telescope located in Nova Itacuruba, Brazil. The excellent quality of the images obtained by Filipe Monteiro can be appreciated in the animation below showing the motion of 2016 RB1 in the 7 September morning sky.
2016 RB1 animation based on images taken at the OASI observatory [Credits: F. Monteiro, H. Medeiros, P. Arcoverde, D. Lazzaro, R.Souza, T. Rodrigues]
ExoMars, the ESA/Russia mission en route toward the Red Planet, has turned into a remarkable observing opportunity for asteroid hunters. The post launch outbound trajectory of the spacecraft mimics, in a reverse mode, the challenging detection of a small Earth impactor. The NEO Coordination Centre has therefore organized an international observational campaign with the aim of obtaining ground-based optical observations of the spacecraft. The quick imaging of a very fast moving object with poorly known ephemeris during a short time and location window is a scenario very similar to what would happen if an asteroid were discovered in an imminent impact trajectory with the Earth. Providing tracking data for navigation is also a useful by-product of the campaign. Thus the NEO Coordination Centre has activated its network of collaborating observatories located in the Southern hemisphere, from where the ExoMars spacecraft would have been observable.
A few hours after launch, on Monday 14 March, the NEOCC started receiving feedbacks: after some bad news due to local adverse weather conditions or technical problems, we received the first positive detection in the late afternoon, obtained by Alison Tripp and Sarah Roberts using a 1-meter telescope in Australia.
Here (left image) one can clearly see the two long streaks left by ExoMars and by a fuel tank jettisoned by the spacecraft a few hours earlier. Shortly afterwards, another collaborator, Grant Christie from New Zealand, reported having tracked the same two objects for almost 25 minutes from the Stardome Observatory in Auckland. His pictures could be stacked into an animation (far left image).
ExoMars was then scheduled to have a major engine burn which would send it into an Earth escape trajectory. Meanwhile the Earth was turning and just before midnight the event could be witnessed live from the OASI Observatory team (Brazil) led by Daniela Lazzaro and with Sergio Silva at the telescope. The images they obtained are astounding: the spacecraft appears as a bright object surrounded by at least six other fainter spots – possibly upper stage separation components - moving altogether in the sky (images below).
With ExoMars safely disappearing into deep space, the NEOCC observing campaign has come to an end. The remarkable results obtained have been made possible by the commitment of the SSA-NEO Team and by the enthusiastic and passionate response of the observing community, once again collaborative and ready to react even on short notice to new events and opportunities.
Update from 21 March 2016: After double-checking the orbit measurements, it
became clear that what we are seeing in these images is the Briz-M upper
stage of ExoMars. The spacecraft itself is already further ahead and not