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NEOCC Newsletter: June 2017
06 June 2017
The ESA SSA-NEO Coordination Centre has released the June newsletter summarising the most relevant data and events on asteroids and comets approaching the orbit of the Earth.
Please, feel free to forward it to potentially interested people.
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New NELIOTA project detects flashes from lunar impacts

24 May 2017

Using a system developed under an ESA contract, the Greek NELIOTA project has begun to detect flashes of light caused by small pieces of rock striking the Moon's surface. NELIOTA is the first system that can determine the temperature of these impact flashes.

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Studies such as NELIOTA are important because Earth and its Moon are constantly bombarded by natural space debris. Most of this material ranges in size from dust particles to small pebbles, although larger objects can appear, unexpectedly, from time to time. This was the case when an object almost 20 m in diameter disintegrated above the Russian city of Chelyabinsk in February 2013. The resultant explosion, caught on camera, caused considerable damage, although, fortunately, no one was killed.

Particles only millimetres across usually appear several times per hour on any clear dark night in the form of meteors or 'shooting stars'. However, the number of incoming objects in the size range from decimetres to metres is not well known. Too small to be detected directly with telescopes, they are rarely captured by cameras when they enter Earth's atmosphere.




Lunar impact flash. Credit: NELIOTA project (left panel). Kryoneri Observatory, Greece. (right panel). Credit: Theofanis Matsopoulos


One way to determine the number of larger impactors and the potential impact threat to Earth is to observe the Moon, in particular the dark area not illuminated by the Sun. When small asteroids strike the lunar surface at high speed, they burn up on impact, generating a brief flash of light, which can be visible from Earth. Assuming a typical velocity and density, the size and mass of the object can be estimated from the brightness of the event.

A new campaign to study these lunar flashes is being undertaken by the NELIOTA (Near-Earth object Lunar Impacts and Optical TrAnsients) project, which began operation on 8 March 2017. NELIOTA utilises a refurbished telescope, which is operated by the National Observatory of Athens and located close to the Greek town of Kryoneri.

The 1.2 m telescope splits incoming light into two colours and uses two advanced digital cameras to record the data at a rate of 30 frames per second. Observations of the Moon's night hemisphere are made whenever Earth's natural satellite is above the horizon and mainly dark – between New Moon and the first quarter phase, or between last quarter and New Moon.

Automated software analyses the video obtained and identifies possible impact flashes. Camera effects can be excluded by identifying events that are only visible in both cameras. The cameras operate in different colour ranges, allowing the temperature of the impact flash to be estimated – NELIOTA is the first system of this type to have the potential to determine the temperature of these flashes.

The exceptional capability of the telescope was confirmed during its pre-operational, commissioning phase, when it recorded four impact flashes in about 11 hours of observing time. The task is now to observe these flashes on the dark side of the Moon over a period of 22 months.


"Its large telescope aperture enables NELIOTA to detect fainter flashes than other lunar monitoring surveys and provides precise colour information not currently available from other projects", says Alceste Bonanos, the Principal Investigator for NELIOTA.

"Our twin camera system allows us to confirm lunar impact events with a single telescope, something that has not been done before. Once data have been collected over the 22-month long operational period, we will be able to better constrain the number of NEOs (near-Earth objects) in the decimetre to metre size range."

"The data will also help to determine the physics of impact flashes. We are analysing the flashes in collaboration with the Science Support Office of ESA, in order to measure the temperature of each flash and estimate the mass, size of the impactor and crater size created from the impact."

"These observations are very relevant for our Space Situational Awareness programme. In particular, in the size range we can observe here, the number of objects is not very well known. Performing these observations over a longer period of time will help us to better understand this number", says Detlef Koschny, co-manager of the near-Earth object segment in ESA's Space Situational Awareness programme, and a scientist in the Science Support Office.



Lunar impact flash. Credit: NELIOTA project


NELIOTA is also contributing to public outreach and education.

"We are currently training two PhD students to operate the Kryoneri telescope and conduct lunar monitoring observations", says Alceste.

"We also organise public tours of Kryoneri Observatory, during which we present the NELIOTA project, as well as talks on near-Earth asteroids for students and for the general public. This year, we plan to participate in Asteroid Day 2017, by organising a public event at Kryoneri Observatory on 30 June."


The National Observatory of Athens developed and operates NELIOTA. It is funded through a contract with ESA's Science Directorate.

The NELIOTA website ( provides the observational characteristics of the flashes (time, duration, magnitude, coordinates) within 24 hours of the observations.

Following its upgrade in 2016 for the NELIOTA project, the Kryoneri telescope is mainly used for lunar monitoring. It is also contributing to follow-up photometry of transient events, such as those detected by ESA's Gaia mission, as well as asteroid occultations.

One of the dangers humans on the Moon would face is that a small asteroid could damage their infrastructure – NELIOTA will help estimate the danger from such small asteroids. As the Moon doesn't have an atmosphere, it cannot block the smaller – but still dangerous – objects. It is likely that permanent structures on the Moon will be underground, to provide better shielding from both small asteroids or meteoroids and solar radiation.


Alceste Bonanos, NELIOTA Principal Investigator 
Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing
National Observatory of Athens
Phone: +30 210 8109177

Detlef Koschny, SSA-NEO co-manager
Directorate of Science
European Space Agency

Vicente Navarro, NELIOTA Technical Officer
Directorate of Science
European Space Agency


The 2016 WJ1 precoveries: back to the future

09 December 2016

The word "precovery" has entered the astronomical jargon rather recently but it quickly grew in popularity among asteroid hunters. It refers to the finding of an archival observation of an object which was not recognized as such because the corresponding image originally served other purposes. Hence the name "precovery", which explains why we may have direct measurements of an asteroid position at a time well ahead its discovery date. Searching for precoveries is a powerful technique for NEO impact monitoring. It allows to extend the observational arc of an object leading often to a dramatic improvement in its orbit knowledge without the need of additional telescope time. Although extremely rewarding, it is a tricky business which requires peculiar skills. The past trajectory of the asteroid (including uncertainties) must be assessed in order to start searching and retrieving candidate images from astronomical databases all over the world. A careful inspection looking for a faint spot close enough to the expected location is then carried out.

The case for 2016 WJ1 is no exception, but with a further taste of complexity: a chain of precoveries brought the NEO Coordination Centre to catch a view of the asteroid position as far back as July 2003. 2016 WJ1 was discovered on 19 October 2016 by the Catalina Sky Survey. When impact monitoring systems were run , the object turned out to be rather peculiar, with more than 200 dates within the next century when an impact could not be excluded with certainty. However, the highest probability for any of these events was quite low, about 1 in 80 000.



The left panel shows the 2003 precovery image of 2016 WJ1: the asteroid appears a faint spot between two bright stars. The right panel shows the orbit of 2016 WJ. The orbit of 2016 WJ is rather peculiar, having both nodes very close to the Earth orbital radius. The position of the Earth and of the asteroid corresponds to the 2037 encounter, formerly an impact solution.


Over the following days new observations were collected and when the impact predictions were recomputed some of the possible dates were discarded, but others became associated to a higher hazard. In particular, a possible close approach in June 2065 was now showing an impact probability of about 1 in 8000, enough to classify an object of this size at level 1 of the Torino scale (out of 10 levels).

Usually additional follow-up observations are likely to remove the possible threat, but for 2016 WJ1 this was not the case. Those collected during the subsequent week, including some from our team with ESA's OGS telescope in Tenerife, did not significantly change the impact scenario. However, they did improve our knowledge of the asteroid's orbit enough to start searching for precoveries.



The left panel shows the behaviour of the impact probability over time for two possible impact dates and its cumulative value (blue area). Note the steady increase and the almost sudden drop to a value difficult to appreciate in the scale of the plot, after the 2003 precovery observation was found. The right panel shows how the accuracy with which the 2016 WJ1 semimajor axis was known, increased with the length of the observed arc, significantly extended by precoveries.


The archives of the survey telescope Pan-STARRS revealed a promising set of images taken on 13 October 2016. An extremely faint source was indeed spotted at the expected position, but it was so barely visible to prevent deciding whether it was the footprint of 2016 WJ1 or just a fortunate alignment of noise features. Yet the information was not useless: pretending that the Pan-STARRS detection was true, the asteroid orbit was refined to the point of allowing us to push the search further back in the past - until a couple of decades ago. Were another object to be found following the new predictions, this would confirm that the Pan-STARRS faint detection was indeed correct.

The Canadian SSOIS image search system, one of the most efficient on-line precovery services available, with which the NEO Coordination Centre is collaborating, was used to this end. Two sets of images taken on 4 and 5 July 2003 with the Canada-France-Hawaii Telescope, where the object should have been visible, were retrieved. After careful inspection we were able to locate the object only 4 arcminutes away from the prediction based on the Pan-STARRS detection.Accurate astrometry was promptly extracted from these images and the outcome was lowering the probability associated to possible impact scenarios to safer values.

Following asteroid footsteps backward in time is an exciting game, to which a future development of the ESA SSA-NEO Segment is likely to provide a significant contribution: the deployment of the so-called "Fly-Eye" telescopes. Aimed at NEO discovery they will cover the whole visible sky every night thus making available a huge database which in the long run will greatly increase the chances of carrying out successful precoveries.