Tuesday 17 February 2015

OT: the discovery of Near Earth Asteroid 2015 CA40 (updated)

Satellites is not the only thing I dabble with: as some long-time readers of this blog know, I am also involved in asteroid searches.

Since 2012 I am part of a small team that searches for asteroids with the 60-cm Schmidt telescope of Piszkéstető (MPC 461, Konkoly obs, Szeged university) in Hungary. The project is run by Dr Krisztián Sárneczky from the Szeged university.

My task in this project is to visually inspect the images for objects that have been missed by the automated (computerized) moving object detection routines. Typically, Krisztián sends the images to me via Dropbox within hours of the observing session. I then inspect them on my pc at home here in the Netherlands and measure any unidentified objects I encounter on the images. Over the years I have fished out a number of new main belt asteroids from our imagery.

This weekend, I found a Near Earth Asteroid in the imagery, my first NEA find in this project and my second in total (10 years ago I found NEA 2005 GG81 when I was a plate reviewer with the Spacewatch FMO project).

Part of one of the discovery images from Feb 15. Note the faint trail.

We had a run of several nights with the Piszkéstető Schmidt telescope last week. On Monday around lunchtime I was inspecting images taken Sunday-on-Monday night by Krisztián at high declination (+56 degrees) in Ursa Major. Usually, images at this high a declination are devoid of asteroids. But this time I noted a small moving streak in the images near RA 14h 22m 32.6s, dec. +56 16' 37". See above for (a part of) one of the images, and the animation below. Each frame in the animation below is a 5-minute exposure.

Animation of the discovery images.

Initially I was a bit cautious. As can be seen in the animation above, the object was very faint in the first two frames and brighter in the last two. This is a bit unusual (it can be due to rapid rotation of the object, or -most likely in this case- to changing sky conditions). My first thought therefore was a high altitude slowly flaring satellite: but checking the image times it was clear that this object moved much too slow for a satellite. So: a Near Earth Asteroid?!

I mailed Krisztián the positions noting that it looked like an FMO, a fast moving NEA. Krisztián remeasured the images (measuring is difficult with trailing objects, and certainly faint trails) and sent the observations to the Minor Planet Center (MPC) of the IAU in Harvard, under our temporary object designation "SaLa122".

It was then posted on the MPC's "NEOCP" page, a webpage that lists potential Near Earth Asteroid discoveries with a request to other observatories for confirmation. Due to a mistake it initially appeared as "SaLa123" there (see below) with only 50% of our data: this was however quickly corrected and soon it was on under the correct designation "SaLa122".

SaLa122 (under the erroneous designation SaLa123) on the NEOCP

At that moment we had a 30-minute observational arc, which is very short. It was vital that the object should be recovered over the next day, otherwise the object would be regarded as "lost" and would not count as a discovery.

Luckily, that recovery happened! The next night (16-17 Feb) Krisztián managed to relocate the object with the 60-cm Schmidt (see image below) and could follow it for several hours. In addition, astronomers at the Czech Klét observatory and British amateur astronomer Peter Birtwhistle at his private Great Shefford Observatory in the UK looked for the object too and could confirm it. This expanded the observational arc to 29 hours, enough for a preliminary orbit determination.

Stacked follow-up images from MPC 461 in the night of Feb 16-17

In the late afternoon of Feb 17 the MPC made the official discovery announcement in MPEC 2015-D10: the object now has the official designation 2015 CA40.

2015 CA40 is a borderline Amor/Apollo asteroid with [updated 22 Feb 2015] a semi-major axis of 1.1049538 AU, an eccentricity of 0.0910145 and an orbital inclination of 15.04 degrees. The perihelion is just outside the orbit of the earth at 1.004 AU. The aphelion is at 1.20 AU, well within the orbit of Mars. The orbital period of the asteroid is 1.16 years. With H=24.5 the asteroid is estimated to be about 45 meters in diameter.

Orbit of 2015 CA40

[Updated] 2015 CA40 orbital elements (MPC, from MPEC 2015-D47)

Epoch 2014 Dec. 9.0   TT = JDT 2457000.5 
M 298.04901 (2000.0) 
n 0.84857047     Peri. 176.17310     T = 2457073.50630 JDT 
a 1.1049538      Node 334.93131      q = 1.0043870 
e 0.0910145      Incl. 15.04278      Earth MOID = 0.01551 AU
P 1.16           H 24.5 

From 98 observations 2015 Feb. 15-21, mean residual 0".51. 

The theoretical minimum distance (MOID) of the asteroid's orbit  to the orbit of the Earth is 0.0155 AU or about 6 times the Earth-Moon distance. Closest actual approach of the asteroid to Earth this year, to about 6.3 times the lunar distance, is in the night of Feb 23-24 when it might reach mag. +16.6 and will be moving at a speed of 42" per minute.

Objects in this kind of orbit with a semi-major axis of ~1.0 AU (similar to the orbit of the Earth) are objects that already must have had one or more close encounters with the Earth and/or Mars.

We plan to follow the object over the coming nights, to expand the observational arc as much as possible, in order to increase the chances of it being found back during the next similarly close approach, which will be on 23 February 2066. There are some earlier dates at which the asteroid comes near Earth too (indicated in the diagram below: e.g. 2022, 2029, 2037, 2044, 2051 and 2058), but at a clearly larger distance than in 2015 and 2066. It will be much fainter and hence harder (but not impossible, given a big enough telescope) to detect during those years.

click diagram to enlarge: distance (in AU) of 2015 CA40 to earth over the coming century

Earlier close approaches to less than 0.1 AU over the past 200 years were in 1813 (0.0161 AU);  1849 (0.0429 AU); 1863 (0.0245 AU); 1899 (0.0773 AU); 1928 (0.0469 AU); 1950 (0.0503 AU); and 1979 (0.0665 AU).

2015 CA40 is  the 7th Near Earth Asteroid discovered by the Konkoly survey and my second NEA discovery (and my first in the Konkoly project).

More on my other asteroid discoveries here.

Update (21 Feb 2015): we are still following this object and the arc now includes observations from early Feb 21.

Acknowledgement: we thank Peter Birtwhistle and the people of Klet observatory for their follow-up observations.

No comments: