PHA 2016 WJ1

[Updated 3 Dec with new data available – see below]

As mentioned previously, the Spaceguard Priority List at the ESA is a great resource for finding Near Earth Objects (NEOs or NEAs) that are useful to observe – even by amateurs! The site assigns priorities to observable NEOs ranging from “Urgent” down to “Low Priority” in order to help triage candidates for the larger observatories (and others) for follow up, based upon a number of criteria.

One reason an NEA might be assigned to the Urgent category is having a narrow time window for observation. The surveys for minor planets often pick up smaller, bright objects that are only detectable because they are passing close by. These will tend to be moving fast and will only be visible for a relatively short time. So it’s important to get observations on these while they are still visible to determine a reasonably accurate orbit so they can be found again later. Also, some of these may even pass close enough to become targets for radar observation. In this case, priority visual observations can be made to help locate an asteroid well enough to point a radar at it. Radar observations can provide highly accurate locations and a variety of physical data for general research.

The fast-moving NEOs can be hard to accurately observe with the modest telescopes I have access to, so I usually don’t try for things in the Urgent category. Observations on the “Needed” and “Useful” objects are often worthwhile, and even the Low Priority objects can be useful to observe for orbit improvement. But I was interested to see an object in the Urgent category this past week that was in a brighter magnitude range (around 18) and moving at the relatively modest rate of around 2 arcseconds per minute: 2016 WJ1. Certainly doable!

2016 WJ1 is likely listed in the Urgent category because it is identified as having a potential for a close approach or impact to Earth in the distant future and is of significant size to be designated a Potentially Hazardous Asteroid or PHA. It is not uncommon for newly discovered asteroids to be placed on the JPL and ESA “watch lists” for potentially hazardous NEOs. An orbit can be estimated for a solar system object after only 1 or 2 nights observation, but this will only be an approximate orbit until the it has been observed for an extended period of time. So newly discovered minor planets are sometimes found to have potential close approaches to Earth, based on a rough orbit, and so are placed on the watch list upon initial discovery. Sort of like a quarantine! These are often removed after more data comes in and more accurate tracks can be projected. For example, in the Sentry Risk Table maintained for NASA by the JPL, one can see that dozens of minor planets discovered in 2016 have been removed from the list after follow up observations, out of the 1700 or so that have been discovered this year in total. So it’s important to get data on these objects while they are still observable to help identify, or rule out, a close approacher as being a potential threat.

2016 WJ1 was in the northern sky this past week at with a declination of around +15. This would be best observed from the site in the Canary Islands, but they were closed due to an extended spell of bad weather. The object was lower in the sky from their observatory in the southern hemisphere near Santiago, Chile but still observable. And the weather has been very nice there of late, with sunny summer skies and warm temperatures, so this looked to be worth a try.

So I scheduled 4 “missions” on the night of Nov 27th from Santiago and got nice images back the next morning. After solving the images in Astrometrica, I could see a very faint moving object around the expected position for 2016 WJ1. To improve the signal and get a more accurate position I was able to stack the images from each set, giving 3-4 images taken about 1 minute apart for each stack. Here’s a portion of one of these stacks showing the presumed target, circled in purple:


In this image stack, Astrometrica is shifting each image by the expected movement of our target object. The oval-shaped objects are stars that are offset between frames because they are moving relative to the asteroid. But the asteroid lines up between the images, giving a  nice circular shape and a better defined peak. (And the fact that it does also confirms we are seeing something moving at the expected rate and direction of our target). The red box marks the area where Astrometrica expected 2016 WJ1 to be. We are somewhat off (about 1.8 arc minutes in RA) but in the neighborhood. And this offset is consistent in each image, helping to confirm we are in fact seeing an object agreeing with the expected track of 2016 WJ1.

Now Astrometrica can give us the position in the sky at each time point for each stack, so we have 4 “observations”. I then ran these through the findOrb package along with other published observations to calculate the best orbit solutions for the combined set. This gives the position at each time point plus the delta or “residual” between the observed location and the location predicted for the orbit:

16 11 28.21566 W88 04 03 52.25 +14 38 35.3 .29+ .36+ .15613 1.1416
16 11 28.22933 W88 04 03 48.37 +14 38 24.5 .41- .34- .15603 1.1415
16 11 28.25381 W88 04 03 41.58 +14 38 06.4 .51+ .13- .15583 1.1413
16 11 28.27146 W88 04 03 36.61 +14 37 53.3 .06- .17+ .15570 1.1411

In the first line, the observed and predicted positions differ by around 0.3 arc minutes in RA and Dec respectively. This is pretty good accuracy for a stacked image and should be useful data. The second and third timepoints were a little more off but the 4th is good, so I submitted the first and last lines to the Minor Planet Center through their email submission mailbox.

The observations were accepted, but I missed that publication deadline for that day. A few more positions were obtained the next night and sent in on 29 November.  Both sets were published that later day in the MPC Minor Planet Electronic Circular (MPEC) 2016-W117.

These publications are rather technical and daunting to read. A more user-friendly form can be see in the Tracking News section of the Earth’s Busiest Neighborhood site here.

What we see in the first section in the link above are observations of 6 different minor planets that have been recently discovered and being followed up as potential risks, including our 2016 WJ1. My observations from Slooh station W88 program code 7 are listed for both nights, along with data from other observatories from England, Germany, the Czech Republic and an ESA observatory in the Canary Islands. Adding in a few positions from the Southern Hemisphere is likely helpful to estimating the minor planet’s orbit as a balance to all of the other submissions from the North.

The Daily Orbit Update on the next date (MPEC 2016-W127) shows observations on 2016 WJ1 from over 10 observatories, so it’s certainly getting a lot of attention. I also submitted a third set on 30 Nov which was published in 2016-X07 on 1 Dec. As of today, there are 220 observations over a mere 12 days, so there are certainly a number of eyes on this NEO.

2016 WJ1 is still on the risk list, but certainly needs to be observed for a longer period of time to really understand how concerned we need to be about it. It’s discovery was confirmed and then published just recently on 22 November, so it’s still very “new”. It was one of 18 new designations published that day, ranging in estimated sizes from 10 meters up to 1000 meters, so it is of moderate size at around 180 meters. Fifty meters is considered to be a significance threshold in terms of potential damage, so the object is certainly worth tracking – but is not a monster! An object’s speed relative to the Earth is also highly important to consider when assessing risk.

Another factor is the number of potential encounters, which depends upon the orbit type. 2016 WJ1 is an Apollo-class minor planet which means that is primarily located outside of the Earth’s orbit but crosses it. It’s orbit is rather elongated and ranges from inside of Venus’ orbit to beyond that of Mars. In fact, it seems to make close passes to Venus, Earth and Mars on a fairly regular basis in an orbit close to the solar plane. We are seeing it now because it is in our neighborhood and will pass by at a very safe 0.05 AU (~ 5 million miles) around mid December.

It is projected to come back around in 2023 at an even safer distance but will observable. So we’ll get to check up on it again in a few years, which is re-assuring! Then it will continue to come around every 7-10 years or so for more viewing chances too, and can likely be seen in between those visits by the larger guns. Each of these approaches will help refine the orbit’s accuracy further and check against course changes caused by encounters with the 3 planets it interacts with.

A pass in 2065 looks like a close shave and that is currently contributing to much of the estimated risk. But, again, this is a very long extrapolation of a 3 week old orbit to really worry about just yet. And that fact that it is expected to make a series of close passes could well be an advantage. One could imagine launching missions to encounter it and study if further. And it could well be a good test or subject for risk mitigation – such as steering it away from us. I even wonder if somebody somewhere is thinking it might be a good candidate for asteroid mining. One could imagine valuable ore being offloaded each time it comes back in the vicinity perhaps.

2016 WJ1 is expected to have a much more rapid motion in the sky as it gets closer and then passes by over the next few weeks, so further observations are probably best left to the professional observatories. And there are always other candidates to follow up on. If the weather clears in the Canary Islands some time, I’d like to try another newly discovered ‘risk’ candidate – 2016 TF94. Unless there’s something more interesting on the list!

Update: new observations dated from 2003 were published yesterday from the observatory at Mauna Kea in MPEC 2016-X21. I’m guessing these are “pre-covery” data. When a new object of interest is discovered, observatories often go back through their data to see if was observed before. Perhaps they had seen it in 2003 but it was never confirmed and was not published as a new object at that time. But now, the older observations are found to be consistent with those of 2016 WJ1 and are now added to its history.

Now the known arc spans for about 13 years instead of a month, greatly reducing the long-term uncertainty of the minor planet’s trajectory. 2016 WJ1 was removed from the JPL Sentry list yesterday. The NEODyS close approach table shows that close approaches in the next 100 years will be just that – approaches that should safely pass us by.

More observations during this and subsequent passes will tighten this up further, but this is certainly good news!

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