Asteroids that spin really fast

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.

– Hamlet

One of the great privileges of astronomy is that there’s always something new and surprising to discover. It’s a big universe. There’s a lot in it.

In appreciation of this simple joy, I thought I’d share a nifty discovery made by the Rubin Observatory*. This is a big ground-based telescope with an absolutely mammoth camera that is conducting a decade-long survey to unprecedented depth. It basically images the sky over and over again every night. Since it is such a large telescope with a huge camera, each snapshot is a deep image of a large chunk of sky. That lets us see lots in co-added images that we couldn’t otherwise see. But it also adds a new dimension: time. By imaging the sky deeply every clear night, night after night, we see things that vary. That includes variable stars and things that move, like asteroids.

Most asteroids are in the main belt between Mars and Jupiter. The size distribution is a power law with lots of little asteroids for every big one. The small ones are faint, so hard to see, but exactly the sort of thing that is now made easy by the Rubin Observatory. Practically as soon as serious survey work began, well over a thousand new asteroids were discovered. Right away, in the first week of observation. So many new objects, so many orbits to trace. That’s pretty cool, especially if you’re a solar system geek. I am not; I am a jaded old man unamused to be witnessing a repeat of the decline and fall of both science and society. It takes more than a bunch of rocks in space to grab my attention.

Nevertheless, the thing about finding lots of new objects is that this often reveals some unexpected extremes. As Luca Rizzi at NSF put it, the “Rubin Observatory will find things that no one even knew to look for.” Amongst the new asteroids are some truly weird objects. In particular, Greenstreet et al. (2026) report the discovery of a number of rapidly rotating asteroids. As they state in their abstract,

We model lightcurves and derive rotation periods and colors for ∼2000 objects. We find 75 main-belt asteroids and one near-Earth object with reliable rotation periods spanning 0.031–21.3 hr … We find 19 superfast rotators with periods shorter than the 2.2 hr spin barrier. Rubin-discovered MBA 2025 MN₄₅ is the fastest-rotating d > 0.5 km known asteroid with a rotation period of 1.9 minutes; along with NEO 2025 MJ₇₁ (1.9 minutes) and Rubin-discovered MBAs 2025 MK₄₁ (3.8 minutes), 2025 MV₇₁ (13 minutes), and 2025 MG₅₆ (16 minutes), these five super- to ultrafast rotators join a couple of NEOs as the fastest-spinning subkilometer asteroids known.

Here is an example: the light curve of their fastest rotator, MN₄₅:

Every once in a while, a result comes along that make stop and remember that heavenly objects can be pretty darn cool. This is an asteroid about half a kilometer in diameter that is spinning once every 1.88 minutes. That’s 112 seconds. That’s crazy talk.

Imagine standing on the surface of this object. The horizon is visibly curved. You could walk around its diameter (if you could walk at all) in less than an hour. But it would be a hike that traverses ~15 soccer pitches, so not nothing. So let’s just stand still, eh? Still relative to the surface, of course – you could not run fast enough to keep up with the rotation at the equator (about 14 m/s or 30 mph).

You see a sky full of stars. The sun is the brightest by far, but not quite as bright as we’re used to here on Earth, being much closer in at 1 AU than the asteroid belt (mostly between 2 -3 AU). More importantly for our eyes, there is no atmosphere (hope you remembered your spacesuit), so there is no scattered light obscuring the other stars when the sun is above the horizon. You can see all the stars all the time, better than the darkest night on Earth.

That sky full of stars is rising and setting at a furious pace. You can watch it happen, no need for the horizon to make clear that the sun is setting. It takes just under a minute for a star to traverse 180^o from rising on one horizon to setting on the opposite side of the sky. I imagine that’d be really cool to watch: the entire Galaxy of stars spinning over your head, rising, passing through the meridian, setting, then coming ’round again in less time than it takes to read this post. I wonder how fast the awe gets replaced by nausea (hope your space suit is equipped with a barf bag).

While it is fun to imagine circumnavigating an asteroid this small, it would be hard to walk as it has hardly any gravity. It’s mass is about a hundred million metric tonnes, depending on the density (more on that below). That sounds like a lot, but it ain’t – not for gravity, at any rate. The surface gravity would be less than 5 cm/s/s, a factor of 200 times less than we feel here on Earth. You could play superman and leap tall buildings with a single bound, but you’d never come down: it would be easy to achieve escape velocity with your Terran muscles. Simply walking would be hard: your first natural-strength step would be your last one.

Maybe it would be safer to stand at the pole than at the equator. Indeed, it is the only place you’d have a hope of standing. This thing is a giant merry-go-round in space. At the equator you’d get flung into space by the centrifugal force if you didn’t have a death grip on the surface. So how does this thing hold together?

That is something Greenstreet et al. (2026) wondered as well. They note that most main belt asteroids have periods longer than 2.2 hours (not minutes!). This is a “spin barrier” that most objects don’t cross. If they did, they would fly apart.

Binary asteroids are surprisingly common given their mutual lack of gravity. A possible origin of such systems is angular momentum-induced fissionning after a gradual spin-up by the YORP effect. The cohesion limit depends on the composition of the object:

Most asteroids and comets are rubble piles. That’s the technical term. Rather than being monolithic boulders in space, the typical asteroid is an agglomeration of many smaller chunks. We know this from a variety of lines of evidence, but a key one is that in the cases where their sizes and masses have been well measured, their densities are lower than that of solid rock. A typical rock has a density of ~3 g/cc while asteroids are half that, typically ~1.7 g/cc. There’s a fair amount of nothing mixed in with the solid material. That’s why they can sustain the impacts that make relatively large craters: they don’t shatter because seismic waves can’t propagate through the empty interstitial spaces. They’re basically big sandbags^$.

MN₄₅ is not a rubble pile. A rubble pile has no cohesive strength; it would fly apart if spinning so fast. We’ve observed rubble piles be shredded by tidal forces. None of the obvious space substances fit the bill to hold MN₄₅ together; even terrestrial clay would not hold together. MN₄₅ needs to be at least as cohesive as solid rock. The same holds for MK₄₁. So maybe there are big boulders?

I wonder what the surfaces of these super-fast rotators look like. The standard-issue asteroid surface is heavily cratered and covered with dust, regolith, small rocks, and the occasional boulder. Such loose material should get flung off into space when the spin rate is to high. Leaving what, pray tell?

There is a good reason asteroids are not, in general, solid rocks. They do run into each other on occasion. Not often; the asteroid belt is pretty empty, it is nothing like the dense hazard to navigation that is beloved of science fiction movies. But they spend billions of years orbiting around the sun, so their paths do sometimes cross. When they do, their relative speeds are typically several km/s, so they do not bump gently in the night – hence the heavily cratered surfaces they typically display. A big, monolithic rock would likely shatter as a result of the larger collisions. So I’m skeptical that MN₄₅ is a literal boulder. What is it then? Since it isn’t my specialty, I feel free to engage in some wild speculation.

Most of the meteorites that reach the Earth’s surface are stony, but a few percent are irons, composed of metal (mostly iron and nickel). It is quite an experience to pick one up, as they are quite dense (iron has a density of 7.9 g/cc). They feel much heavier than the eye expects for a rock that size. I love to pass examples from our meteorite collection around in class – the irons never fail to raise eyebrows.

Things in space have a size distribution. Small things are common and hit the earth all the time. Look up at a clear, dark sky for ten minutes and there’s a good chance you’ll see^ a shooting star. That’s typically the size of a grain of sand – it hits the atmosphere at high speed and gets incinerated as it zips across. Larger objects also burn up in the atmosphere; something has to be pretty hardy and/or large for a chunk of it to reach the ground as a meteorite. Big things are rare, but they’re out there.

So maybe MN₄₅ is a big iron? Just a giant nickel-iron nugget in the sky.

I wondered what might be known about such a hypothetical object, but there does not seem to be much in the literature. It is always hard to be sure one isn’t missing something important in a literature search, but perhaps this is a case of discovering a thing we didn’t know to look for. Though some things may just be forgotten: trying a search again, I find this gem from Chapman (1974) discussing the size distribution of asteroids that might be the sources of stony-iron and iron meteorites:

A class of apparently stony-iron asteroids shows a relative excess frequency between diameters of 100 to 200 km. It is proposed that the latter objects are predominantly metallic cores of a group of chemically differentiated planetesimals all of which (except Vesta) have suffered catastrophic collisions.

That sounds a bit like the Phaeton hypothesis in which a protoplanet formed where the asteroid belt is now but subsequently shattered into what we now see as asteroids. This idea fell out of favor because Jupiter’s gravity precludes things in the asteroid belt from clumping up; it acts for the sun like shepherd moons act for Saturn to maintain structure in its rings. But the rings had to come from somewhere, and the iron meteorites appear for have come from an object that had chemically differentiated into an iron core and a rocky mantle. It is now believed that Jupiter might have migrated in the very early days of the solar system when there was still a lot of the solar nebula to exchange angular momentum with, so maybe some protoplanets formed, collided, and shattered before Jupiter arrived to nail them down?

The curious thing I found in my first search is a report that main-belt asteroid (16) Psyche has a large metal content on its surface. That’s not the same as being one giant iron, and one always has to take these things with a grain of salt. There is a cool image at the top of that link that looks like a metallic (16) Psyche, but it is obviously fake: we have no picture of that asteroid at that resolution. Scroll down to the bottom to see the real data, which I would have found more compelling without the teaser slop image.

So maybe there are large nuggets of metal in the sky? I’m sure there are other possibilities. We should also bear in mind that this is astronomy. Things go wrong; maybe these results will change after months and years of repeated observation. These data were obtained over the course of a mere nine nights of observing; Rubin’s survey will continue for a decade. That means lots more data, so it should be come clear if something more mundane explains these extreme spin rates. Or something even more outrageous. Maybe to mess with us aliens painted black & white polka dots on MN₄₅ and its compatriots to make it look like they are spinning faster than they are: what we measure is the light curve, we don’t resolve the structure of the tiny asteroid. Of course, invoking aliens would be even more insane.

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*I hear people refer to the telescope as “the Rubin.” Having known Vera Rubin personally, I can’t write that. Rubin was a vital, dynamic scientist and marvelous human being. It’s great to name⁺ it after her, but Rubin is a person, not an object.

⁺As an scientist, Vera was all about how things on the sky moved. As an observer, she made extensive use of spectroscopy to trace motion via the Doppler effect. The Rubin Observatory is strictly an imaging survey, which I find it a little ironic, as it operates in an observing mode that’s about as far removed from what she did as you can get with a ground-based telescope.

^$We learned a great deal from the Deep Impact mission, which collided with the nucleus of Comet 9P/Tempel 1 on July 4, 2005. Mike A’Hearn was the PI of that project; we were both at the University of Maryland when it happened. The mission had a normal space probe component and also an impactor – a copper bullet (easy to distinguish from cometary material) to collide with the comet. People involved with the project had bets all over the place as to what would happen, ranging from a big explosion that excavated a new crater to a clean penetration like a bullet through talcum powder. We got the former, consistent with a rubble pile. A surprising amount of tarry, carbon-rich material was ejected along with the expected ice and rocks. So this “dirty snowball” was a tarry mess.

The orbit of the mission put the spacecraft with its bullet in front of the comet, so Mike would say we didn’t crash into the comet; the comet ran over our spacecraft. Motion is relative, yes, but intent matters. As part of the outreach for the mission, people could record a message that was carried on the bullet to the comet on a CD. Knowing full well it was our intent to violently crash into the comet, I couldn’t resist contributing “We come in peace.”

^These days you’ll also see lots of satellites. Lots and lots of satellites. So many satellites messing up the sky. They look like points of light that move slowly but steadily across the sky. Slower than airplanes without the bright, blinking lights. Note the direction of travel: most commercial satellites travel east-west, following the economics of launching with the angular momentum granted by the planet’s spin. Spy satellites often pay extra to launch into polar orbits, moving north-south to get a peek at the whole globe as it rotates underneath.

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For those who don’t recognize it, the image under the title is from a recreation of the classic early-days video game Asteroids, which I recall playing as a teenager. One flew a little ship around and shot asteroids before they ran into you. They break apart like monolithic boulders, a common misconception for what asteroids are like that I shared at the time. Being more like giant sandbags, you could shoot these things as much as you could, even with nukes, and the thing would still run you over.

It was gratifying to find the recreation of the classic game on the web, so had to play it some. I notice that this version encourages another misconception. After firing the rocket to start into motion, that motion does not persist: one grinds to a halt like there is friction in space. I don’t recall that mistake being made in the original. It has been more than a few decades since I played it, but my recollection is that you would just keep going once started, in accordance with Newton’s first law. Indeed, I liked to see how high I could the ramp up speed and still dodge asteroids. Not very fast. I’m more forgiving of the periodic boundary conditions. These rather undersell the immensity of space, but one can think of it as a mere re-centering of the coordinate system. The basic fact of Newton’s first law would be easier to get right in the code than the wrong that this realization implements.