NEOShield-2 Observations at La Silla Observatory in Chile

by Marcel Popescu, Observatorie de Paris

Trip 1: October 31 – November 10, 2015

La Silla Observatory Observations

Flowering tree in Santiago. In the background the mountains can be seen lost within the clouds with some snow on top


I arrived in the afternoon at the ESO Guesthouse, located in Las Condes district in Santiago de Chile. Because the seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere, November is a month of late-spring, thus the weather is pleasant, the trees are green and the flowers are blooming all over t in this green neighbourhood.

I left from the ESO Guesthouse in the next morning, together with the colleagues which will observe on La Silla – the observer for the MPG/ESO 2.2m telescope which is operated now by the Max Planck Society, and the observer for the Swiss 1.2-metre Leonhard Euler Telescope. After a fast breakfast we were in the taxi to go to the airport for taking the flight to La Serena. The departure of this flight was at 10:25 and it flew for about one hour. In the airplane we met the observer for the ESO 3.6 m Telescope which hosts the HARPS instrument (High Accuracy Radial velocity Planet Searcher).

A small bus provided by ESO took us to the Observatory. The view on the way is majestic – the white domes appear like giant mushroom on the top of the mountain. You can easily spot the 3.6 m – which is on the highest point and the dome of the 1.54m Danish Telescope. The top of mountain has a saddle shape.

La Silla Observatory Observations

Landscape from the road that goes to La Silla. The Observatory can be seen on the furthest top of the mountain.

As I started the observations on November 4 – two days later, I spent most of the days in the room sleeping (or at least trying to sleep). I did the tour of the Observatory trying to memorize the names and to take pictures of the telescopes: the 0.6m TRAPPIST (Transiting Planets and Planetesimals Small Telescope) robotic telescope, the 1.54m Danish telescope with its big building, the MPG/ESO 2.2m telescope, further away on a crest is the New Technology Telescope or NTT, a 3.58-metre Ritchey–Chrétien telescope, and on the furthest top is the ESO 3.6 m – now dedicated to exoplanets observation. A walk from one side of the Observatory to the other takes about 30 to 45 minutes, but it’s longer when staring around.

La Silla Observatory Observations

La Silla telescopes seen from the ESO 3.6m dome cat walk.

Our observing nights were on November 4, 5, and 6 with 3.58m NTT telescope. We target to observe the smallest near – Earth asteroids that come close by Earth orbit. We want to understand what are made off, what their origin is and what they can tell about the origins of the Solar System formation. Do these objects contain material, unknown yet from the meteoritic samples? Do they contain organic molecules and how complex are them? Spectral observations in the visible and near infrared regions can provide answers to these questions. This technique – spectroscopy is a powerful one as it connects the knowledge we get from the laboratories with the physics of the celestial objects.

Spectral observations of these near-Earth objects (NEOs) is not an easy task. Their orbit is poorly known so I had to update the observing schedule in the last day in order to include their latest astrometry (i.e. position measurements), required to have the most precise orbit as well to consider  the newly  discovered ones. Their differential rates (i.e. apparent movements relative to the field of stars) are very high (in the order of ~1 to 30 arcseconds per minute) so it is difficult to know in advance the field to be observed (it depends significantly of the observing moment).  I prepared some and scripts to help me for this job.

The setup was discussed in the afternoon before the first observing night with the telescope operator. I will observe in a low spectral resolution mode (#Grism 1) with the EFOSC instrument. Around 5 PM everything was set. I had the chance to go with the telescope operator and to visit the ESO 3.6m and the NTT domes.


La Silla Observatory Observations

Asteroid identification: the telescope is set to move with the differential rates of asteroid so the stars appear as trails and the target as a point-like source. The orbit computation accuracy determine how far is the object from the expected position (as is shown here). The CCD (charge couple device) image is unprocessed.

The Control Room was moved in a building out of the telescopes. It has desks for all three big telescopes from La Silla: the ESO 3.6m, the NTT and the MPG. Although this is very practically, as observer you lose the sounds of the telescope when it points to a new object. In particular, the movement of the NTT is interesting, as the dome includes the old control room. A new target observed means discovering new things. We are looking to understand things using the physics from the laboratory, further and further into space and time.

Calibration images were obtained in each afternoon. The observations started when the sun was 15 degrees below the horizon. The atmospheric conditions were good and we started with a bright target (about ~18 apparent magnitude) to get used with the instrument. Things are automated – on my side I had to update the coordinates and the motions according to the observing moment, to correctly identify the target, to check the exposure times and to verify that the object is “in the slit”. The telescope points amazingly well and it follows with great precision the differential tracking.

I observed 26 NEOs over the three nights. The most interesting spectrum was of (52381) 1993 HA asteroid. An asteroid has a number (e.g. 52381), a temporary designation (e.g. 1993 HA) and it may have a name if its orbit is well known. The dynamic characteristics of 1993 HA (i.e very low delta-V budget, ~5km/s, required for rendez-vous with a spacecraft) makes it an important candidate for a sample return mission. The very red, featureless spectrum we obtained indicates a primitive, organic rich composition. These types of asteroids are very rare in the near Earth space, they come from the Outer belt or they could be remnants of the cometary nucleus. The only meteorite that matches these spectral characteristics is Murchison. These NEOs can hold the key to understand the origins of the life on the Earth.  An article describing our results was published in Astronomy & Astrophysics journal.

Trip 2: November 23 – December 01, 2016

La Silla Observatory Observations

Landscape near La Silla Observatory. Three llamas can be seen.

It is the beginning of the summer in Santiago: everything is green – the leaves of the trees are almost mature now and the ornamental trees are full of flowers. In particular, the garden of the Guesthouse seemed like a small corner of paradise, a gate between the huge blocks and the green flourished nature. Reading in the garden was a very good option for spending the afternoon.

There was a discussion between several ESO managers about how the management is structured the organization. They also discussed about the administrative challenges to maintain these top level instruments.

I spent the two days before the observations preparing a poster to describe our finding for D-type asteroids which are potentially targets for space missions. Using these NEOShield-2 observations we double the number of the existing candidate asteroids for a sample-return mission. These D-type objects (D – comes from “dark” – surface that reflects only 5% of the light) are very rare within the near-Earth population.

La Silla Observatory Observations

ESO 1.52-metre telescope at night.

Their origin is considered to be in the outer part of the Main Belt or they could be the remnants of comets nuclei. Their dark surface and red spectrum is associated with the presence of organics. Based on the spectral data we can infer a composition of these asteroids similar with the one of Tagish Lake meteorite which contains an abundance of organic materials, including amino acids. Based on NEOShield2 observations we discovered that they become more abundant in the near-Earth population at small sizes – bellow 300 meters, where they represent about 10% of the total number.

One night before the observations start, I went to the Control Room to see and review the work-flow and how the instrument is performing. The observers were kind and allowed me to stay. They observed with EFOSC – spectroscopy at the visible region wavelengths, the newly discovered supernova and based on these spectra they were able to classify them. Outside, the sky was amazing!


La Silla Observatory Observations

View from La Silla to Las Campanas Observatory.

Our observations were scheduled for the nights of November 28/29 and 29/30. As usual, previous to the observations start I updated the list with observable objects (to include the new discovered ones) and I did some minor changes to my scripts. The day telescope operator implemented the setup. In the evening I took the calibration images (dome flats, bias and arcs).

The observations went perfectly. We were able to observe asteroids of about ten meter size. These observations are very difficult to obtain because these objects, due to their small size will not be observable for several decades (is like a one-time opportunity). Their orbit will be difficult to recover if they will have not sufficient astrometric data (follow up observations after their discovery. There are very few asteroids of this size characterized. It is very important to understand their physical properties as these objects hit Earth with a frequency of about one hundred years (for example see the Chelyabinsk -2013 and Tunguska -1908 events) .

I observed 18 asteroids, among them some flagged as potentially hazardous asteroids and virtual impactors (like 2016 WJ1) – asteroids which have a non-zero probability of impacting the Earth and which require further study.

Trip 3: February 23 – March 03, 2017

La Silla Observatory Observations

Night sky in La Silla, February 27, 2017. Magellanic Clouds can be seen in the left side of the image. TRAPPIST telescope is the first one in the left corner.

Although it was mentioned in the other stories, it has to be outlined that ESO is organizing very well these travels: airplane tickets, taxi from the airport to Guesthouse, all the comfort in the Guesthouse. Although it was February – end of summer in Southern Hemisphere, the temperature was still agreeable in Santiago.

The beautiful corner of the moon rise, there were three days before the New Moon. In the Northern Hemisphere we say that the moon is lying: when is like a C (from French croissante –increasing) is decreasing, while when is like a D (décroissant – decreasing) is increasing. Well, in the Southern Hemisphere the Moon is telling the truth: when is like a C is increasing while when is like a D is decreasing. An annular solar eclipse was on February 26, 2017.

Our observations started on February 27 (Monday night), so there were three nights for accommodation. So, over these nights I stayed to watch the sky. The view is one of the most beautiful to be found: the whole galactic centre is rising above ESO 3.6m and NTT telescopes while the Magellanic Clouds are just above the small TRAPPIST telescope.

La Silla Observatory Observations

Twilight in La Silla. New Technology Telescope (in the centre) is ready for our observations.

The observing run went fine. About new 14 targets were observed. From these targets, seven asteroids were newly discovered once. Their estimated diameter was in the range of 20 to 70 meters.  Unfortunately, the humidity was high and the telescope operator had to close the dome for several hours. Nevertheless, the main objects for which we want to have data were observed.

This was the last observing session for the NEOShield-2 project. We observed 180 spectra for 165 objects with diameters less than 300m. This survey outlined a great and unexpected diversity within the small near-Earth object population. We found that several compositional types, which were considered rare relative to the whole asteroid population, are more abundant for the small sizes. Here it can be exemplified with the case of olivine rich asteroids. We identified and characterized new candidates for sample-return missions. As we just finished our observations, these results are under study and new conclusions are expected.

About the author: VR