In my last post, I described a utility called OrbBroswer I’ve developed to help identify minor planets that might be useful or interesting to observe. From time to time, I download the Near Earth Asteroid orbit elements file (NEA.dat) from the Minor Planet Center data page and run it through the utility to look for objects that may be in need of follow up observations.
One approach I’ve tried is to select from all NEAs those having an Uncertainty value of 2 through 5. This helps find NEOs that may need additional observations to improve their orbit, while being good enough not to require an extensive search. This filter brings the number of candidates from around 15,000 to 5,000. Next these are loaded into the planetarium program C2A and filtered to identify observable objects at an apparent magnitude of 20, reducing the count further down to about 150.
These can be viewed in C2A at their estimated positions from a given observing location and time. Checking the sky at the onset of (full) darkness and again before the sky starts to lighten should show all objects that will be observable during the night. Leaving out objects right on the horizon or in the Milky Way, and having a sufficient magnitude to detect, should indicate all of the reachable targets from this set.
Since I only have the name of the objects displayed, I then look up each one in the MPC Observation Database to see the type of object, it’s uncertainty value and the time of last orbit update and observation. Then I can select minor planets that might be useful to observe that have not been seen for a while or have a small number of recent observations.
When I tried this a while back in September, I noticed a numbered minor planet, 418265, belonging to the Atira or Inner Earth Object (IEO) class of minor planet. It had an uncertainty value of 4 and was last observed on 10 December, 2014.
The Atira-class family of minor planets has their orbits entirely within that of Earth’s and is relatively recently known. The first known object in this class, 163693 Atira, was discovered in 2003 at the Lincoln Laboratory Experimental Test Site observatory in New Mexico. As of today, there are only 26 of these known out of nearly half a million asteroids. Six of these have been observed enough to receive a number designation, but at least two have sparse observations and are effectively lost. So these are pretty rare!
Most of the planets and minor planets have orbits outside of the earth’s, in the “Outer” solar system. When these are on the same side of the sun as the Earth, they are visible in the night sky. And when they are located directly behind us (relative to the Sun), they will rise in the evening, transit around mid night and set at dawn – just like the Full Moon. In this orientation they are said to be in “opposition”. They are also nearly fully illuminated and at their brightest at this time.
In contrast, the inner planets are only observed around the time of sunset or dawn. Mercury is quite close and never gets too far from the Sun in the sky, so it stays quite close to the horizon. But since it is a good-sized planet, it is possible to see it before the sky is fully dark, especially with a pair of binoculars – provided the Sun is safely below the horizon of course! Venus is the Morning or Evening “Star” and can range fairly high in the sky because it is further away from the Sun and can get closer to us. Currently it is near its greatest elongation and is high and bright at around 30 degrees from the horizon in the evening sky.
Ephemerides of Atira-class 418265 indicated that it would be low in the sky at 10-20 degrees above the horizon before dawn from the Slooh observatories in the Canary Islands. I was able to get a time slot around dawn on one night and give it a try. A moving object was visible around the expected position of the IEO but it was quite faint. Checking the predicted position on future nights, it looked like it would be a bit higher up in the sky later in September, so I tried again. This time I was able to get good positions on two nights and submitted them to the Minor Planet Center.
The observations were accepted and published and these stand as the only positions on the body since 2014. The orbital uncertainly remains at 4, but the positions I obtained agreed very closely to the expected positions determined by the find_orb package, so 418265 was right where it was expected. And since it has an estimated size of 2200 meters across, that’s a good thing to know!
Atira-class 418265 has an orbit that ranges from 0.4 AU to 0.8 AU. Its closest approach to Earth is a very safe 0.18 AU. But since it is fairly far from the Sun and gets somewhat close to us, it will can range fairly high up in the sky when it is in our neighborhood. That and it’s large size make it fairly easy to catch when it is nearby. It will have a fairly close approach in 2026 but hopefully it will be visible before then for more orbit updates.
A little later in the Fall, I noticed that Atira itself might be observable from the Southern Hemisphere, so I tried it from the Slooh observatory in Chile. I was able to get images at around 30 minutes before the end of complete darkness but was not able to see any moving objects. The estimated magnitude was 17.5 but this was probably not bright enough to pick it up with this telescope so close to the horizon. I also tried from the iTelescope observatory in Australia but Atira was below the horizon limits of the scope I used. So these can be hard to get!
Looking over candidates again in November, I noticed another Atira-class object, 2008 UL90, deep in the southern sky at a declination of -60 and moving further south. I was able to capture some images at the expected location and then realized the object was in the southern end of the Milky Way and was not discernible in images containing many hundreds of stars. The object continued to move south as it passed close to the Earth and then started to move back quickly north in December after it’s closest approach on the 12th. I was not able to get observations for a few nights but then was able to catch it on the nights of the 20th and 21st from the iTelescope.net site in New Mexico.
A number of observatories from the Northern Hemisphere have also observed 2008 UL90, so it has had quite a few positions reported recently. This object will be visible well into January and is getting lots of additional observations on this pass. I think will be a candidate to receive a numeric designation since it has been seen on 5 “oppositions”.
Atira 2008 UL90 is considered an Potentially Hazardous Asteroid (PHA) because it has a considerable size of 800 meters, and also because it’s aphelion is in our neighborhood at 0.96 AU, with an estimated closest approach of 0.026 AU based upon it’s current known orbital elements. So this one is certainly worth keeping an eye on! As can be seen from the diagram below, the object ranges close to our distance from the Sun but is significantly inclined:
So I was able to get a look at and report on two Atira’s this past year. Since there are only 26 I think that’s pretty fortunate. Looking over all of the 26 known, it looks like (163693) Atira should be visible in March around a close approach. Another numbered Atira, 413563, may be visible around then too at mag 18.5. All of the others look to be hard to see or extremely faint over the coming year. But since two new Atira’s were discovered in 2016, so there’s always the chance of catching a new one!
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