We know about 95% of all near-Earth objects larger than 1 km in size – the huge, mass-extinction-sized space rocks that roam the Solar System.
However, when it comes to near-Earth objects larger than 30 metres in size, only about 18 000 of an estimated 2 million have been detected so far. This is because they are very faint and only become observable as they get close to Earth.
In July this year, an unknown asteroid passed Earth at a distance of 10 million km, about 26 times the distance to the Moon. Now named 2020 OM3, it is estimated to be 40 m in size – about the diameter of the Tunguska asteroid that exploded in Siberia on 30 June 1908, flattening 2000 square kilometres of forest.
Using its 1-m telescope in Tenerife, Spain, ESA typically spends four nights per month observing 'NEO's. The focus is particularly on follow up observations – getting a better look at objects once they have been discovered, to better understand their size, orbit and chance of collision.
Once regular follow-up targets have been observed, ESA's Near-Earth Object Coordination Centre runs a search programme to look for new objects. Images are taken of certain sections of the sky, and repeated four times.
On 24 July, asteroid 2020 OM3 showed up as a faint speck of light in four images taken between 2:46 UT and 3:36 UT. During these 50 minutes it had crossed quite a significant part of the telescope's field of view, making it quite a challenge to spot.
This animation is a crop of the telescope's larger view. Nonetheless, three moving objects are visible, including 2020 OM3. The four exposures are repeated in an endless loop and tiny dots can be seen hopping in a straight line.
The challenge is to discover the moving objects:
• If you find any, send us the approximate start and end positions of each asteroid using the numbers on the horizontal and vertical axes to create coordinates.
• Bonus question: can you give us the official names of the objects that you've found?
Hint: 2020 OM3 is the fastest of all moving objects, and it’s also the faintest.
Good luck asteroid hunting!
You can download a high resolution version of the image at the top of the page or on the Near-Earth Object Coordination Centre's asteroid riddle homepage. Send your responses to firstname.lastname@example.org before 25 September. Use as subject of your email: “Riddle #4 – solution”.
Please let us know if you would prefer not to have your name included in the list of correct replies.