Near Earth Object Confirmation at Raheny.

Main Image Credit: ESA

For many years one of the primary research at Raheny observatory is the follow up (confirmation) of newly discovered minor planets (asteroids) for which their discovery circumstances indicate might be “Near Earth Objects” (NEO).  We’re all familiar with the hollywood blockbusters such as “Armageddon” and “Deep Impact”.  However far removed the plots of these movies are, they reflect a justified fear which we will have to face some day.  We currently have no plan to send Bruce Willis or any brave soul in a spacecraft to destroy a large asteroid on a collision course with our home planet.  One of the best defense strategies revolve around forewarning us of a potential threat.  That’s where Raheny Observatory and many other observatories around the world come in.  We monitor these asteroids on a nightly basis reporting their positions to the International Astronomical Union’s, Minor Planet Center (MPC).  The MPC is then able to compute more accurate orbital parameters for these asteroids so we can be certain that they pose no threat to us in the foreseeable future.

Asteroid 2012 DA14 Racing across the Raheny Skyline (Credit: Raheny Observatory)

Most asteroids have a very well defined orbit and as such we can say with a very high level of confidence that we have nothing to fear.  Almost every night professional survey’s around the globe are discovering new objects.  Most of these objects are just harmless asteroids going about their business and are clearly never going to be a threat.  However a few of these become interesting when an initial look at their trajectory through space indicates that they might fit in a class of object whose orbit passes close to or intersects the orbit of Earth around the Sun.  These are known as Near Earth Objects (NEO’s).  An object is considered an NEO if its closest approach to the Sun is <1.3 astronomical Units.  The next parameter which is looked at is it’s Mean Orbital Intersection Distance (MOID).  This is how close it actually comes to Earth’s orbit.  If this is less that 0.05astronomical units and the object is not too small (less than 150m) they are considered Potentially Hazardous Objects (PHO’s).

When data is received by the MPC for a newly discovered object and the initial indication suggests that it might be an NEO, details are posted on a special webpage, the Near Earth Object Confirmation Page (NEOCP).  The problem is that the discovery observations for these objects have been taken over a short period of time and this is not sufficient to predict the path of the object into the future with great certainty.

The following illustration explains this concept.

In the diagram we illustrate the actual path of an object through the sky over a period of a period of two days.  The perfect (actual) orbit of the object is illustrated by the black line.  Let’s now look at the effect of tiny errors in the observations make to the predicted position of the object at day 2.  In part A we introduce a very small error into the measured position of the object (red and blue) and we can see that at day 2 this has introduced a significant error into the predicted position for the object at that time.  We can see in part B, that if we increase the time between the observations (nown as increasing the observation arc) the error at day 2 is significantly reduced.

Next we look at the effect of increasing the number of measured observation points for an object and how this has a great effect on reducing the impact of observation errors (known as observed/computed residuals) on the calculated orbit.

See the following illustration;

In this illustration we plot a set of 16 observations taken over 4 days.  Some of these have greater or lesser residual errors.  However when many observations are made over a longer period of time the impact of any single residual error is almost entirely removed from the computed orbit.  The computed orbit is the one which “best fits” the observations received.  There are other factors which infuence the orbit which is compuyted based on observations.  For example we can visualise space in 3d but our observations are of an effective 2d plane and do not take into account if the object is moving toward or away from us.  We won’t dwell too much on this area (although I personally find this fascinating).  In short the more observations we get for an object, the more accurate its computed orbit will be and thus the more accurately we can predict its path into the distant future.  We can also see from our earlier illustrations that getting more observations as early as possible will make it more likely that the predicted path is more accurate and thus there is a greater prospect of finding the object again in the future.

This is where observatories like Raheny come in.  Our job is to capture these objects and measure their positions as accurately as possible.  Thus we contribute in a very direct way to assisting the computation of an accurate orbital solutin for the object.

So thats the theory over, now how does this work in practice?  In the next section we will look at a the procedure and processes involved in measuring the position of an object and examine some of the techniques involved.

The main telescope at Raheny is a 0.36m (14″) newtonian reflector.  In combination with a sensitive CCD camera, it means that the system is capable of detecting some very faint objects indeed.  However many of the objects which are listed on the NEOCP are incredibly distant and faint.  Normally the solution to capturing fainter objects is to keep the shutter of the camera open for longer so that more of the faint light from the target object can fall and accumulate of the camera detector.  However asteroids are moving relative to the background stars and as such exposing the camera for too long means that the image of the asteroid will trail on the image and therefore become hard to measure its position precisely.  Also the motion of a faint asteroid will mean that not all of the light from the asteroid will fall and accumulate on the same point of the camera detector.  So does this mean that there is a limit on how faint an asteroid can be detected by the telescope at Raheny.  Yes and No.  Yes, of course all systems have a limit however by using some advanced techniques it is possible to detect minor planets which would normally be out of range of the telescope system.

The brightness of an astronomical object is given by a value called “magnitude”.  The larger the number the fainter the object.  Just for reference the Sun has a magnitude of -27 the full Moon -13.  Vega, one of the brightest stars in the sky is magnitude 0.  Rarely will we see an object on the NEOCP brighter than magnitude +17.  Most are in the range +19 to +21.  The faintest object ever captured at Raheny was magnitude +23.  However this took 5 hours of exposure.  So we can see that in theory most objects on the NEOCP are within the range of the telescope at Raheny but in practice taking 3 observations of objects at the fainter end of the scale would simply take too long.  Therefore it is only practicable for the telescope at Raheny to image asteroids down to approximately magnitude +20 in a reasonable time frame.

To overcome the motion (and faintness) of these objects a technique called “Track and Stack” is employed.  Rather than taking one long exposure of an object, multiple exposures are taken.  The length of these exposures are carefully chosen to ensure the trailing of the asteroid does not affect measurement.  The quantity of these is also carefully selected to ensure enough light is collected across all of the exposures.  These are then stacked on top of each other taking into account the motion of the asteroid so that in the final image the asteroid remains fixed on the image and it is the background stars which appears to trail.

First lets look at a sequence of images which have been aligned and stacked without taking into account the motion of the asteroid.  This sequence of 20 x 10 second images was taken for NEOCP object YSAC392.

The actual position of YSAC392 is marked with the pink circle.  However the asteroid is barely visible as a faint streak from the circle to the 2oclock position.  It would certainly not be possible to get an accurate measurement using this image.

Now lets restack these same images taking into account the motion of YSAC392.

Here the asteroid is clearly visible and is easily measurable.

The track and stack technique is made possible by a wonderful piece of software called “Astrometrica”. Not only does astrometerica allow the automatic alignment of a set of images along the path of motion of an asteroid, but it also allows the precise measurement of the asteroid and prepares the observations in a format suitable for submission to the MPC.

When we click on the object in question we see this;

This allows us to verify that we have selected the correct object and also see its exact co-ordinates (RA and DEC) and also the measured magnitude of the object.

Astrometrica then prepares a properly formatted observation suitable for submission to the MPC as follows;

YSAC392 KC2017 09 22.99539 00 51 04.96 -06 06 02.3 17.9 V J41

This means (in english) object YSAC392 was measured in a (K) stacked image on 2017 09 22.99539 (the exact time of the midpoint of the observation in UT).  It’s co-ordinates were RA = 00 51 04.96 DEC = -06 06 02.3.  Its magnitude (visual) is 17.9 and the observatory reporting is J41 (Raheny). Once three such observations are obtained.  The report is submitted to the MPC in a plain text email as follows;

CON D. Grennan., Address []
OBS D. Grennan.
MEA D. Grennan.
TEL 0.35-m f/3.8 reflector + CCD
ACK Batch 1702 – Minor Planet Astrometry
AC2 []

YSAC392 KC2017 09 22.99539 00 51 04.96 -06 06 02.3          17.9 V      J41
YSAC392 KC2017 09 23.00189 00 51 14.68 -06 07 12.0          18.0 V      J41
YSAC392 KC2017 09 23.00823 00 51 24.14 -06 08 20.3          17.9 V      J41

Once the MPC has received sufficient observations to calculate a reasonably precise orbit the obect is the given a formal designation.  In this case YSAC392 is given the designation 2017 SD12.  The discovery of this monor planet is then formally announced by way of a minor planet electronic circular (MPEC).  All contributors are formally credited.  In the case of 2017 SD12, its MOID (mean orbital intersection distance is well within the range that qualifies it as a potentially hazardous object.  It’s size of between 20-50metres means is is not classed as a PHA.

You can read the full MPEC here.