Asteroid Watch

Hi All,

Its been an exciting time as asteroid 2024 YR4 caught the worlds media attention for a while and gathered several of us for comment on this resident space object, initially rated as a potential Earth impactor on 22 Dec 2032.

2024 YR was discovered just 2 days after Christmas Day 27 Dec 2024. Initial data indicated it as an impact risk to Earth rising to 1% on 27 January 2025 thus triggering a note from the International Asteroid Warning Network (IAWN) to the United Nations Office of Outer Space Affairs (UNOOSA). At this point the worlds media took an interest and from late January to 18 February 2025, the asteroids risk level rose from 1% to 3.6%, the highest risk rating ever of an asteroid measured so far.

Although a probability of 3.6% is relatively high, it was important to note that the probability of a miss was 96.4% but being an object of diameter between 40-90m (larger than a Jumbo Jet), the likelihood vs consequence matrix gave it a Torino level of 3 or equivalent to an object that could inflict significant regional damage if it were to hit. With an energy release equivalent to several megatons of TNT, the media labelled it as a City killer.

With 2024 YR4 already enroute away from Earth on its highly elliptical 4 yr orbital path, gathering new astrometric data to define its approach in 2032 was becoming more difficult by the day for this locally large but cosmically small dark object.

After its discovery by the Asteroid Terrestrial Impact Last Alert System (ATLAS) Chile on 27 Dec 2024, it was initially classed as Torino 1 then updated to Torino 3 soon after. Feverish observations were completed by 1.5m (Danish), 4.3m (Lowell), 8.2m (VLT), 1.5m (Loina), 2.56m (Nordic ) telescopes plus other telescopes around the world. Observatories also scoured their previous data bases where calculations indicated that 2024YR4 should have been visible in 2012, 2016 and 2018 however searches through previous datasets, found no trusted evidence of the asteroid.

The January and February 2025 observations did however indicate that the asteroid had a 4yr orbital period, an elongated semi major axis of 2.4Au, a rotational period of 19.4min, an elliptical orbit a/b ratio >1.4 and spectral data indicating it as an S-type asteroid mainly composed of iron and magnesium silicates.

?It was difficult to differentiate whether 2024 YR4's composition was monolithic or of loose aggregate (like a bag of sand) held together by gravity. Yes cohesion apparently was still possible even with a 19.4m rotational rate. Composition is important to know because a loosely held asteroid impacting the atmosphere would likely release energy as an airburst of heat and shock waves rather than reaching the ground as an impact crater if it were a solid cohesive object.

By late February ground-based telescopes of 2-4m aperture could just observe 2024 YR4 and by March it was predicted that only the biggest 8-10m telescopes were suitable. NASA did task the James Webb Telescope class to adds its observations. By 2028 on its next pass on 19th December more data could be taken however the point now remains moot as recent observation on the 20th February 2025 has downgraded the impact risk to 0.0027%. The risk to a Lunar impact however has increased to 1.7%.

The activity of the IAWN and the sister organisation Space Mission Advisory Planning Group (SMPAG) were a show piece of international collaboration, exchange and support. Its pleasing to note that these organisations are running “what if” table top exercises using 2024 YR4 as their threat example in preparation for perhaps the next time.

Asteroid 99942 Apophis once also christened an impactor in 2004, reaching Torino level 4, will make its closest approach on Friday 13th April 2029 and should be an interesting site in the night sky passing just 31,860km above our heads. In recognition, the UN has endorsed 2029 as the International Year of the Asteroid.

Whilst there are no known asteroids representing an impact threat to earth within the next 100yrs, it’s the rogue surprises that we need to be watching and ready for. Australia has an important part to play.