Focus on: ESA OGS
30 October 2014
 
The ESA Optical Ground Station has quickly become a major asset for the NEO Coordination Centre, thanks to the many nights devoted to observing asteroids. It is located at an altitude of 2400 m on the slope of a volcano, in Tenerife, hosted at the Observatorio del Teide - Instituto the Astrofisica de Canarias. The telescope has been originally committed to advanced optical communication experiments on board the ESA geostationary satellite Artemis, and it is therefore equipped with a state-of -the-art laser equipment, still operational. During the SSA social media event on 7 October a spectacular transit of the International Space Station had been organised: a laser link with the ISS was established while the telescope provided incredibly clear images of the Space Station as shown in the image below (http://www.iac.es/divulgacion.php?op1=16&id=891,  https://www.flickr.com/photos/esa_events/sets/72157648061826537/).
Once the Artemis mission was over, OGS became available for supporting other programs. The Space Debris office of ESA installed a focal reducer and a wide-field CCD camera for observing space debris and satellites. In the meantime, the telescope has turned out extremely useful for fulfilling the ESA Space Situational Awareness observing needs. Over the past few years the SSA-NEO programme has been allocated approximately four nights per month, around new moon, entirely dedicated to asteroid observations. These observations are managed by the NEOCC. As can be seen from the image, laser communication is still part of the activities at the OGS.
The main focus of these activities is to collect follow-up observations of NEOs. A significant fraction of the targets are the so-called "NEOCP objects", recently discovered asteroids whose preliminary ephemerides are posted by the IAU Minor Planet Center on the NEO Confirmation Page (http://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html). In most cases these recent discoveries have been observed only for a very short amount of time and it is therefore impossible to determine their orbits and carefully assess if they are indeed dangerous NEOs. In most cases, this same lack of knowledge results in very large positional uncertainties in the sky, thus requiring a telescope with a large field of view to be certain that the object is going to be visible in the image. The OGS, with its 47 arcminutes square field, is ideal for these searches; over the last year, about a dozen candidates per night were successfully targeted and approximately half of them were confirmed to be NEOs thanks to our observations.
The second main focus of follow-up activities is guided by the Priority List published on the NEOCC website. At any given time, the list highlights about a dozen objects in urgent need of observations, plus many lower priority ones. We therefore try to observe as many of them as possible down to at least a visual magnitude of 22, which is the practical limit of the instrument. This activity is essential in order to prevent most of these targets from being lost because of a too large orbital uncertainty at their next apparition.
At any given time some of the older, known objects also come back in the vicinity of the Earth and become visible again. These observations are often particularly challenging, because the long timespan between the last observation and the current time could result in large positional uncertainties, which need to be covered entirely to make sure the object is located.
From the beginning of 2014 we have observed almost 200 NEOs, with a success rate above 80 % (i.e. the fraction of targets that were effectively located when observed). Most of the targets are in the magnitude range between 20 and 22, which is accessible only to very few follow-up sites in the world, giving us an advantage and a very useful niche of capabilities that we can successfully exploit.

Every night a few hours are also devoted to a survey project named TOTAS (Teide Observatory Tenerife Asteroid Survey). 
So far TOTAS has discovered 10 NEOs in a total of 300 hours search time. Among them, 2014 QN266 is an interesting Virtual Impactor in an Earth-like orbit. Last February a comet now named P/2014 C1 (TOTAS) was spotted – as reported in our early news "Totas survey finds its first comet". 
 
We always stay alert for any exceptional target or event on which we may provide observations. An interesting case was the recent close fly-by of comet C/2013 A1 (Siding Spring) with Mars (to the right: our image of comet C/2013 A1 (Siding Spring) taken at 20:20 UT on 19 October 2014, with the ESA OGS telescope. The comet, marked with the red dashes, was just emerging from its closest approach and heading North. The vertical and horizontal features emerging from the saturated disk of Mars are artefacts of the telescope. The comet's tail is just barely visible in the image, pointing toward the planet.).
 
After some tests on the previous night, on October 19 we were ready to collect observations of the comet right around the time of closest approach. The weather was very uncooperative: winds up to 65 km/h, blowing right from the direction of the comet, plus humidity constantly hovering between 90 % and 100 %, forced us to keep the dome closed. However, luck assisted us for about 30 minutes, when conditions improved enough to allow us a peek at the event. And it was worth it: below is a nice picture of this unique event, taken when the objects were only 17° above the horizon.
 
It was definitely a challenging observation: in addition to the unfavourable atmospheric conditions, we had to deal with imaging a faint object sitting next to a huge source tens of thousands of times brighter. But that's the real job for asteroid hunters!