Asteroid Sleuths Go Back to the Future

Asteroid 2016 WJ1 (Credit: Canada-France-Hawaii Telescope)
Asteroid 2016 WJ1 (Credit: Canada-France-Hawaii Telescope)

PARIS (ESA PR) — Careful sleuthing through decade-old images has enabled ESA’s asteroid team to decide that a newly discovered space rock poses little threat of hitting Earth any time soon.

Spotting a previously unknown asteroid for the first time always raises the big question: is there a risk it will impact Earth?

Yet, upon discovery, analysts often have very little to go on. The initial image from the observatory, survey team or individual backyard astronomer who spotted the rock typically gives only basic information – its location in the sky and its brightness – and sometimes these aren’t known terribly accurately.

The most crucial information needed to determine with any degree of confidence whether it is a ‘near-Earth object’ (NEO) – and that it will miss Earth (or not) – is the new object’s path. And determining that requires a series images acquired over a period of days or even months.

“We need multiple follow-on images to compute the trajectory and make a risk estimate, but even then the uncertainty can be very large. It really takes many months of observations to get a good, reliable impact risk estimate, and in the meantime, there can be reason to worry,” says Ettore Perozzi of the NEO Coordination Centre at ESA’s facility in Italy.

Spotted From Arizona

This is precisely what happened on 19 October, when asteroid 2016 WJ1 was discovered by the Catalina Sky Survey.

Additional images were taken by observers worldwide over the next few weeks, including by a team working at ESA’s own observatory on Tenerife in the Canary Islands, but uncertainty of the path meant that a possible close approach in June 2065 – with a worrying impact probability of about 1 in 8000 – could not be excluded.

“The additional images allowed us to refine our knowledge of the trajectory sufficiently to begin searching astronomical archives, to see if anyone had previously imaged this asteroid without having recognised it as such,” says Marco Micheli, observer at the NEO centre.

If any were found, the team would score what astronomers call a ‘precovery’ – short for pre-discovery.


The investigation quickly bore fruit: images found online from the Pan-STARRS survey taken earlier in October showed what might be the target asteroid.

While these were inconclusive, the team assumed they were, in fact, accurate and then used these to call up additional, highly accurate images from a Canadian astronomical image search system.

On 11 October 2015, just 12 hours after its discovery, WT1190F was first confirmed by ESA’s NEO Coordination Centre using observations from the Agency’s Optical Ground Station, Tenerife, Spain.

Bingo: two sets of images from 4 and 5 July 2003 with the Canada–France–Hawaii Telescope were found.

“After careful inspection we were able to pinpoint the object, and the team were able to perform some very accurate determinations,” says Detlef Koschny, responsible for the NEO portion of ESA’s Space Situational Awareness programme.

“The result was that we could preclude any risk of Earth impact from asteroid 2016 WJ1 anytime soon or well into the future.”

ESA is now developing a new set of automated, wide-field-of-view ‘Fly-Eye’ telescopes that will conduct nightly sky surveys, creating a large future archive of images that will make critical precovery confirmations more efficient in future.

  • Andrew Tubbiolo

    “Space is big, space is really big. If you think the walk to the corner chemist is long, that’s peanuts compared to space……”. In comparison, Earth is small, really small, it’s so small that walk to the pharmacy is not so bad at all. As such impacts are really rare, so rare that as we recover asteroids for this purpose me and my co-workers call it “knocking it off the list.”.

  • I am just fascinated by what you astronomers can do with discovery/recovery/precovery/etc. The fact that we can scan photo plates that are 100+ years old and make new discoveries by combining new and old data is just incredible. While “the singularity” still isn’t living up to the hype, the “astronomy singularity” keeps paying off in tangible ways!

  • Andrew Tubbiolo

    We still use observations taken in the 1700’s. Astronomical data never dies. Not to mention the nova data from the Chinese going back over a thousand years. If you count the C-14 spikes in tree ring data, we have super-nova observations going back on the order of 10,000 years. We don’t know where in the sky the nova was, but we know the radiation from one hit the Earth down to the month.

  • It’s amazing. All we’ve done is keep adding more types of observations of the same things: more wavelengths, more particle types, now adding gravity observations to the mix! The straw is becoming Niagara Falls.

  • Kapitalist

    I recently saw a Japanese heliophysicist explain that they draw the sunspots by hand from a simple projection, just like Galileo Galileo and his successor astronomers did, in order to trim the interpretation of historic sunspot counts. Novas have been dated thanks to ancient Chinese astronomers. When Uranus was discovered to be a planet, older observations of it (then believing it was a star) quickly helped determine its orbit. Haley could make the first prediction of a comet’s arrival thanks to historic observations. History is a resource. Some things take time.

  • ajp

    It’s this sort of stuff that makes me wish I knew more/anything about astronomy. It’s so far detached from the human scale that when I learn a tidbit I get it, from a numbers standpoint, but I don’t really GET it.

  • Andrew Tubbiolo

    Try this, run the basic numbers of what you see when you fly in an airplane. What’s the scale of what you see out of the window at 1km then 4km then 8km altitude? Then when, you’ve correlated that view with your numbers, start playing with extrapolating that, to 30km altitude, then 200km altitude. And compare that photos taken from high altitude balloons and orbiting spacecraft. Then really extrapolate it, by running the numbers for what you see from the Clarke Belt, and beyond. The effect is knockout, it’s positively psychedelic.