ScienceAlert reports that an asteroid is accelerating in a way gravity alone does not account for, and that the usual explanations are not working. The known causes are sunlight-driven heating, hidden outgassing and radiation pressure.
Key takeaways
- ScienceAlert reports that an asteroid has been observed accelerating in a way that gravity alone does not appear to explain.
- Astronomers call any push on a small body that is not caused by gravity a non-gravitational acceleration, and they have a short list of known causes for it.
- The best-known causes are uneven heating by sunlight, gas escaping from the surface, and the pressure of sunlight itself.
- According to the report, the standard explanations are not fitting well in this case, so the object’s behaviour is still an open question.
The reported acceleration
The source material says only that an asteroid is accelerating through space and that researchers are running out of ways to explain it. Which object it is, how large the acceleration is, how it was measured and who made the measurement are not given in the material available here. This article does not try to fill those gaps. It explains what an unexplained acceleration means and why the familiar explanations might fail.
An asteroid’s orbit is normally predicted with great precision. Its position is calculated from the gravity of the Sun, the planets and other large bodies, and then compared with telescope observations over months or years. If the object keeps turning up slightly away from where gravity says it should be, and the difference cannot be put down to measurement error, astronomers add a small extra force to their orbital model. This is the non-gravitational acceleration. Usually it is tiny, but sustained over many orbits it can move an object a long way from its predicted path.
When such a signal is found, the next step is to work out what is causing it. Each possible cause leaves a different signature, depending on the object’s size, surface, rotation and distance from the Sun. The headline claim amounts to saying that none of these signatures fits the observed motion well.
The Yarkovsky effect
The most common explanation for small changes in an asteroid’s orbit is the Yarkovsky effect. A rotating asteroid absorbs sunlight on its day side and gives the heat back off as infrared radiation. Because the surface takes time to warm up and cool down, most of that heat leaves in a slightly different direction from the one the sunlight came from. Each escaping photon carries a little momentum, so the asteroid gets a very weak but steady push.
The size and direction of that push depend on how fast the asteroid spins, which way its spin axis points, what its surface is made of and, above all, how big it is. Small objects are affected far more than large ones, because the force grows with surface area while the mass to be moved grows with volume. Astronomers know roughly what Yarkovsky drift to expect for an object of a given size and type. If the measured acceleration is much stronger than that, or points the wrong way, the effect cannot account for it on its own.
A related process, known as the YORP effect, changes an asteroid’s spin rather than its orbit. Over long periods it can alter the conditions that set the Yarkovsky push, but it does not produce a sudden or large acceleration by itself.
Outgassing and the idea of dark comets
The second main explanation is outgassing. Comets speed up and slow down partly because ice on or below the surface turns to gas as they approach the Sun. The escaping gas works like a weak rocket engine. In a typical comet this activity is easy to see, because it lifts dust off the surface and forms a fuzzy coma and a tail.
The difficulty comes when an object behaves as if it is outgassing but shows no visible coma or tail. Astronomers have described a group of such bodies, sometimes called dark comets: objects that look like ordinary asteroids in telescope images but whose motion suggests gas is escaping. One proposed explanation is that the gas comes off without carrying enough dust to be seen, or that the activity is too weak for current instruments to pick up directly.
If the asteroid in the ScienceAlert report showed a visible coma, the puzzle would mostly be solved. The point of the report seems to be that no such sign has been confirmed, which leaves outgassing possible but unproven.
Radiation pressure from sunlight
Sunlight also pushes directly on any surface it hits. This radiation pressure is extremely weak, and for most asteroids it is negligible compared with gravity. It only matters for objects that are very light for their size, such as thin fragments, highly porous rubble or bodies with unusual shapes.
The interstellar object ‘Oumuamua is the best-known example. It showed a non-gravitational acceleration away from the Sun, with no clear sign of the outgassing that would normally be expected. Researchers suggested a range of explanations, from exotic ices that leave little visible trace to an unusually thin, low-density structure pushed by sunlight. None of these was settled to everyone’s satisfaction, and ‘Oumuamua is now often used as the reference case for objects whose motion stays unexplained.
For an ordinary asteroid, explaining an acceleration through radiation pressure would require it to be much less dense than expected. That would be a notable finding in itself.
Other possibilities
Besides these three main mechanisms, researchers usually look at more mundane explanations. Errors in early position measurements, mistakes in matching observations to the right object or incomplete modelling of the gravity from other bodies can all produce an apparent acceleration that later disappears. A small body could also be shedding material through rapid rotation or collisions, or it could be a binary system whose wobbling centre was mistaken for a change in orbit.
Which of these, if any, has been ruled out for the asteroid in question is not stated in the material available here.
What the case shows about tracking small bodies
Taken together, these explanations show that an asteroid’s motion is a test of what is known about its physical make-up, not just a matter of gravity. Every candidate cause depends on properties that are hard to measure from Earth: spin, surface heat behaviour, density, and whether there is hidden ice. When the observed motion fits none of the standard models, at least one assumption about the object is probably wrong.
There are practical reasons why this matters. Planetary defence work relies on predicting where near-Earth objects will be decades ahead, and small non-gravitational forces are one of the main sources of uncertainty in those forecasts. An object that speeds up for reasons nobody understands is harder to forecast.
The case also points to a blurring of the old line between asteroids and comets. Objects that look rocky but act as though they contain volatile material suggest the two categories overlap more than textbook descriptions imply. Each unexplained case adds to the evidence that small bodies are physically more varied than the simple labels suggest.
For now, according to ScienceAlert’s report, this asteroid joins a short list of objects whose motion has outrun the explanations available. More observations of its brightness, spin and surroundings would probably be needed before any one explanation can be preferred.
Sources and further reading
- ScienceAlert: the report describing the asteroid’s unexplained acceleration, which prompted this explainer.
- NASA Center for Near-Earth Object Studies: background on how asteroid orbits are calculated and monitored.
- European Space Agency Planetary Defence Office: material on near-Earth object tracking and orbital uncertainty.
- Peer-reviewed planetary science journals: general literature on the Yarkovsky effect, cometary outgassing and non-gravitational forces.
Surfaced from the google:GB signal “unexplained asteroid acceleration”. AI-assisted draft, editorially reviewed.
