Why is asteroid 2024 YR4 important?
Asteroid 2024 YR4 was discovered on 27 December 2024. As of 10 February 2025, it has an approximately 98% chance of safely passing Earth on 22 December 2032. Astronomers are working to reduce our uncertainty about the asteroid’s orbit and rule out any impact risk, but it will fade from view from Earth in a few months’ time, and a small chance of impact may persist until it becomes visible again in 2028.
The chance of impact is very slim, and the asteroid is small enough that the effects of any potential impact would be on a local scale, but the situation is significant enough to warrant the attention of the global planetary defence community.
What do we hope to learn by studying the asteroid with Webb?
Astronomers around the world are using powerful telescopes to measure the asteroid’s orbit as accurately as possible. But knowing its orbit will only tell us if the asteroid could impact Earth, not how significant an impact could be.
To accurately assess the hazard posed by asteroid 2024 YR4, we need a more precise estimate of its size. Our current estimate, 40—90 m, has not changed much since the asteroid was first discovered in December 2024, despite many follow-up observations.
This is because astronomers are currently limited to studying the asteroid via the visible light it reflects from the Sun. In general, the brighter the asteroid, the larger it is, but this relationship strongly depends on how reflective the asteroid’s surface is. 2024 YR4 could be 40 m across and very reflective, or 90 m across and not very reflective.
It is very important that we improve our size estimate for 2024 YR4: the hazard represented by a 40 m asteroid is very different from that of a 90 m asteroid.
Webb is able to study the infrared light (heat) that 2024 YR4 emits, rather than the visible light it reflects. Infrared observations can offer a much better estimate of an asteroid’s size, as explained in an article recently published in the journal Nature, co-authored by members of ESA’s Planetary Defence Office.
Astronomers will use Webb’s MIRI instrument to get a much more precise estimate of the asteroid’s size. This, in turn, will be used by ESA, NASA, and other organisations to more confidently assess the hazard and determine any necessary response.
Observations made using Webb’s NIRCam instrument will complement MIRI’s thermal data and will also provide additional measurements of the asteroid’s position once it is beyond the reach of Earth-based telescopes.
When will the observations of 2024 YR4 take place?
The first round of observations will take place in early March, just as the asteroid becomes observable by Webb and is at its brightest. The second round of observations will take place in May. Astronomers will use these later observations to study how the temperature of 2024 YR4 has changed as it has moved further away from the Sun and to provide the final measurements of the asteroid’s orbit until it returns into view in 2028.
Who requested the observations?
Each year, a small amount of Webb’s observational time is reserved for ‘Director’s Discretionary Time’. This time is set aside for time-critical or new discoveries made after the annual Webb proposal deadline that cannot wait for the next proposal cycle.
An international team of astronomers from institutions including ESA’s Planetary Defence Office submitted a proposal to use some of this time to study 2024 YR4. This proposal has now been accepted. The total observation time will amount to around four hours. The resulting data will be publicly available.
About Webb
Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.
Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).
Many thanks to the team at ESA’s Planetary Defence Office and to Andrew Rivkin of the Johns Hopkins University Applied Physics Laboratory, Principal Investigator for the James Webb Space Telescope observations of 2024 YR4, for their input to this post.
Discussion: 8 comments
It is obvious this asteroid will be a close encounter, and it gives the opportunity to study it in close view. What then?
Apophis is larger and passing close on April 13, 2029. What will we do? Measure in by radar at least.
Is it worth launching a mission to intercept and land, or sample? Maybe! Write your proposal!
The awarenes about near earth space and NEOs that was created in the general public is an oportunity to show why it is important to support investment in this areas. Great to see the JWST involved in this.
There is a saying , blow on the cold ..suggests not to focus so much on the observation in detail only now send already on it satellites that change its movement in orbit.Greetings
Will it just become observable to Webb in March because it’s currently farther from Webb than from Earth, or because the Sun or Earth is in the way/too close? The asteroid is getting dimmer from Earth, but it will be brightest to Webb right around the time it first is able to see it?
JWST can’t turn towards the sun to avoid heating, so the asteroid has to move a critical angle to see.
It will be much dimmer, but it’ll give a longer orbit arc to refine its path in 8 years.
Will the James Webb be able to refine the orbit in May by a substantial amount? As the asteroid now has about 2% chance of hitting earth, if Earth is at the edges of the impact ellipse, how much smaller can the ellipse be after JWST observations? Say 50% smaller? Also if I understand correctly the March observation will only focus on sizing while the orbit will be refined in May? The reason I ask is that I foresee a possible scenario were JWST refines the orbit just enough for Earth to still be inside the 2032 impact area, but simultaneously greatly raising the perceived threat probability to say e.g. 10% or more (but more likely to reduce it to zero). Is this within the possibilities?
Thank you.
JWST proposal DD 9239 gives details which are very well summarized on this blog.
This project is for planetary defense.
It is public info. Very helpful for delving into project details.
https://www.stsci.edu/jwst/science-execution/program-information?id=9239
DD 9239
Principal Investigator: Andrew Rivkin
PI Institution: The Johns Hopkins University Applied Physics Laboratory
Title: Size Measurements of a Potential Earth-Impacting Asteroid with JWST MIRI and NIRCAM
It provides contact and coordinator info, a link to status information with an outline of the observation visits, a link to the Public PDF file which contains an abstract and observing description and identifies the names and institutions of the Principal Investigator and Coinvestigators.
Hoping for huge success.
What a well put together team of professionals!!
They have thought this out very well.