# Why did the distribution of asteroids discovered in 2010 have a radial modulation?

This answer links to one of Scott Manley's excellent asteroid videos Asteroid Discovery - 1970-2015 - 8K resolution. The animation highlights the positions of the meteors at the moment of their discovery, and by watching one can see the technology improve and notice patterns as instruments are pointed in different directions to avoid the light from the Sun and (at least sometimes) the Moon. (there is music, adjust volume accordingly)

There are often fan-shaped patterns showing the directions that more sensitive telescopes with modest fields of view are pointed.

However, I noticed that only during the year 2010, roughly between asteroid numbers 500,000 and 520,000 there are radial striations at certain distances from the Sun. I don't see this happen at any other time during the video.

Is this just a rendering artifact, or is it real? If real, what would cause the periodic radial modulation of sensitivity, and only in 2010?

note 1: YouTube allows for playback rates between 25% and 200%, and variable video resolutions. I found 25% and 1080p optimal for my current internet connection and screen.

note 2: For those with GIFs disabled, one image is a GIF.

note 3: 2nd image contains several cropped screenshots highlighting the "periodic radial modulation of sensitivity" in asteroid detection during 2010, for clarification purposes.

• The gif you've got there is also pretty much the only time there were lots of discoveries at around 5 o'clock. That seemed to be another long lasting pattern. – curiousdannii Apr 4 '18 at 3:48
• One of the first versions of that video (I think 1970-2012) includes a voice over by scott explaining what is going on. – PlasmaHH Apr 4 '18 at 13:26

I'm pretty sure that the radial pattern found in the data is a result of WISE's approximately 90 minute sampling cadence (dictated by the satellite's orbit), astrometric precision (about 0.2 arcseconds in the stacked images around launch, see Wright et al. 2010), and the number of free parameters in fitting the asteroid orbits based on that data. See, in the actual images the asteroids appear as points of light that shift appreciably between frames. IIRC, they expected 7 to 12 observations per asteroid. So you have 10-ish observations spanning 15 hours or so to fix the asteroid's orbital parameters around the sun. As you can imagine, there will be more parameters than can be perfectly fit with a single pass in this data set alone.

At a guess: it's related to quantization in uncertainty estimates and how that feeds forward into the orbit fitting algorithm.

I don't know the details behind the striping, but I'd bet it's related to the numerical precision used in the early processing of the data. They've either refined the orbits since using observations from passes separated by ~6 months, or modified how they're handling the numerical precision in their astrometric measurements since. More likely the former, but I'm sure if you asked Amy Mainzer (PI of NEOWISE, and head of the asteroid hunting part of the mission), Roc Cutri (head of the database creation and data processing part of the team), or any of the people on Mainzer's team, they could tell you more.

Relevant background: I was Ned Wright's grad student (original PI of WISE), and he had me design and test an asteroid hunting algorithm in the run-up to launch (we ultimately didn't use it - it scaled like $N^2$, IIRC, and existing efforts in the literature scaled like $\log(N)$ or $N\log(N)$). I ended up working with the extragalctic part of the team, hence my uncertainty on the precise details of what the solar system team did. I think I asked them about the striping during a presentation, but I don't recall the answer, so I'm pretty sure the answer was mundane and unavoidable.

• So if I'm understanding you right, it's a visual artifact of precision/rounding errors in the orbital estimates? – BradC Apr 3 '18 at 19:58
• I understand your explanation, thank you very much! If one could fix eccentricity to zero, then 15 hours in this geometry and 0.2 arcsec could get you to roughly one part per thousand in distance to the Sun. But of course one can not, so it doesn't. Interestingly, the spacing of the structures in the video is about 0.1 AU. I'm sure it's not as simple as rounding, but it's fun to think so. – uhoh Apr 3 '18 at 19:59
• I had thought that the video displayed the timing of the object appearance to its discovery, but the orbital motion would be that of more established elements once it was verified and assigned an official identity. But this is a video for popular consumption rather than for a purely scientific audience, and so the orbits could be based on the first estimates from the initial observation. Thank you very much for your answer and excellent explanation! – uhoh Apr 3 '18 at 20:02
• @uhoh the first video was made before the orbital elements could be refined. – Sean Lake Apr 3 '18 at 20:19
• @uhoh the 0.2 arcsecond figure is for stars (primarily observed by the shortest wavelength channel, 3.4 micron W1) with positions fixed in a coadded stack. I don't know what the single frame precision was, but it as almost certainly worse, and asteroid observations were primarily driven by the 12 micron images, W3, which was little less precise (not by as much as diffraction limit ratios would suggest). – Sean Lake Apr 3 '18 at 20:24

Answer based on a misunderstanding of the question, left here because it contains some useful background on WISE.

The pie-shaped patterns starting in 2010 are results of the WISE mission (see the video description). The radial pattern within those pie shapes is not explained by my answer.

NASA's Wide-field Infrared Survey Explorer (WISE) is a space telescope launched in 2009 to map the entire sky in infrared wavelengths.

WISE imaged the entire sky twice before running out of coolant in 2010. It then did a brief mission called NEOWISE, to look at near-Earth objects (NEOs) such as asteroids and comets, for four months before being placed into hibernation in February 2011. Less than three years later, in December 2013, the telescope was revived to continue its NEOWISE mission. That work continues today.

Looking at WISE:

you can see the telescope is perpendicular to the solar panels, so it'll tend to look at objects perpendicular to the Earth-Sun line, which causes the bright bands you see in the video.

https://www.space.com/33659-wise-space-telescope.html

• Thanks for identifying the instrument, but this does not explain the pattern yet. If you look, the lines (the fine structure) of "the periodic radial modulation" are parallel to the direction of the orbits of the asteroids. I've added note 3 and an image to highlight this structure. It seems to be referenced to the distance to the Sun for each asteroid. – uhoh Apr 3 '18 at 10:56
• I see what you mean. Nasa has a visualization of the WISE data only, and that doesn't show this pattern: jpl.nasa.gov/news/news.php?feature=6864 and if you really want to get to the bottom of this, here's the original data: cneos.jpl.nasa.gov/stats/wise.html – Hobbes Apr 3 '18 at 11:54
• Hey thanks for the links! OK I'll (try to) take a look. – uhoh Apr 3 '18 at 12:31

The effect is already visible in the first video released in 2010: https://www.youtube.com/watch?v=S_d-gs0WoUw

For what it's worth, I tried to download the newest astorb.dat and plot it, but couldn't see the effect there. So it might very well be that the 2010 asteroids were based on preliminary WISE data and weren't all that accurate, and the later videos have not updated the old animations.

And indeed, from the comments of the 2010 video:

odysseus9672: @szyzyg I spoke with Prof Wright, the project PI, and he explained that the striping is due to the Minor Planet Center using an approximate fitting technique to the WISE data. The reasoning for this, I think (we've left Ned approved commentary here), is that the WISE data has a relatively short time baseline (~24 to 48 hrs), so the errors on the orbital fits will be quite large anyway, so there's no point in refining it much past what the error bars will support.﻿

Scott Manley: @odysseus9672 Glad to hear an explanation, this pretty much confirms my suspicions and makes me ask how many of the objects are likely to be lost again in the future. Most of the objects discovered in the leading edge scan in January will have passed through opposition with the Earth and most orbits haven't been refined. The second pass of the WISE survey will hopefully start getting longer arcs on some of these, but WISE will run out of coolant before it can observe every asteroid twice.﻿

• Ah, that's it. I'm odysseus9672 there. :) – Sean Lake Apr 3 '18 at 21:13
• Thanks for digging in and posting this information! What is the source for the image? – uhoh Apr 3 '18 at 21:13
• Also, after the "left Ned approved commentary" is my own speculation. – Sean Lake Apr 3 '18 at 21:16
• @uhoh The image is my own plot with gnuplot from the data here ftp.lowell.edu/pub/elgb/astorb.html . It's just line number vs. semimajor axis so could very well be too simplified to show the effect, or the orbital parameters could be updated (many of the asteroids originally discovered in 2010 have a later update date for the orbit in that file). – jpa Apr 4 '18 at 4:41