What happens when a comet flies past the Sun, slows its spin to a dead stop, and then starts spinning backward?
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Today we're telling you the story of Comet 41P/Tuttle-Giacobini-Kresak — a tiny, icy wanderer that did something astronomers had never witnessed before. Its rotation reversed. And the physics behind that reversal is as dramatic as it sounds. Stick with us to the end. We promise the ride is worth it.
A Tiny Comet Stopped Spinning and Reversed Direction — Here's the Science
What Exactly Is Comet 41P/Tuttle-Giacobini-Kresak?
Comet 41P/Tuttle-Giacobini-Kresak — let's call it TGK for short — belongs to the Jupiter family of comets. These are short-period comets whose orbits are strongly shaped by Jupiter's gravity. TGK's orbit carries it from a perihelion (closest point to the Sun) of 1.045 au out to an aphelion of 5.124 au, completing one lap every 5.4 years .
Its likely birthplace? The Kuiper Belt, that cold reservoir of icy bodies beyond Neptune. Roughly 1,500 years ago, a close brush with Jupiter flung TGK into its current orbit, and numerical simulations suggest it'll stay on a similar track for the next 10,000 years — assuming the nucleus holds together.
TGK has a colourful history of outbursts. In 1973, it brightened by a staggering 9 magnitudes — that's about 4,000 times brighter practically overnight. And in 2017, a smaller 0.6-magnitude flare-up was recorded. These events signal a surface that's unstable, constantly reshaped by the Sun's heat.
How Did Astronomers Discover the Spin Reversal?
Here's where the story gets genuinely strange.
When observers tracked TGK during its 2017 perihelion passage (April 12, 2017), they watched the nucleus spin period more than double — climbing from about 20 hours in March to roughly 53 hours by May. That slowdown alone set records. As astronomer Dennis Bodewits of the University of Maryland put it:
Then came the truly unexpected twist. David Jewitt of UCLA analysed archival Hubble Space Telescope images taken in December 2017 — eight months after perihelion. The nucleus was now spinning with a period of 14.4 hours (0.599 day), which is shorter than either the March or May readings.
A shorter period after all that slowing? The simplest explanation: the spin slowed to zero around June 2017, and then the comet started rotating in the opposite direction. Lightcurve data alone can't tell us which way a comet spins. But when Jewitt plotted every measurement on a timeline, the data only lined up smoothly if the rotation had passed through zero and flipped.
Why Do Comets Change Their Rotation Near the Sun?
Comets are clumpy mixtures of rock, dust, and frozen gases. For most of their orbit they cruise quietly through cold space. When they swing close to the Sun, though, the ice in their bodies transforms directly into gas — a process called sublimation.
This escaping gas doesn't always leave evenly. It erupts in jets and geysers, each one pushing on the nucleus like a tiny rocket engine. Each jet applies a torque — a twisting force. If those jets aren't balanced, they either speed up or slow down the comet's rotation.
Think of it like this: imagine you're sitting on a swivel chair and you blow through a straw off to one side. You'd start to spin. Now imagine dozens of straws, all pointing in slightly different directions, switching on and off as the chair heats unevenly. That's roughly what happens to a comet near the Sun.
Some comets spin faster and faster until they literally tear themselves apart. Others, like TGK, slow down — and if the torque persists long enough, they reverse [[2]].
How Small Is This Comet's Nucleus — and Why Does That Matter?
Size matters enormously here. A smaller nucleus responds to outgassing torques far more dramatically than a large one — the same way a light canoe rocks more than a cargo ship when a wave hits.
Jewitt combined three independent methods to pin down TGK's size:
1. Optical Photometry (Hubble)
Using the WFC3 camera aboard Hubble with the ultra-wide F350LP filter, Jewitt measured the brightness of the nucleus after carefully subtracting the faint surrounding coma. The result: an absolute magnitude of H = 18.89 ± 0.03, which translates to an effective radius of about 560 m (assuming a visual geometric albedo of 0.04).
2. Radar Observations
Radar measurements at 12.6 cm wavelength set a lower limit of rn ≳ 450 m. These results remain unpublished beyond an abstract, and they depend on an assumed radar albedo, so there's some uncertainty.
3. Non-Gravitational Acceleration
When outgassing pushes a comet off a purely gravitational trajectory, we can measure that extra acceleration. JPL's orbital solution gives a total non-gravitational acceleration of about 1.3 × 10−7 m s−2 for TGK, almost entirely directed away from the Sun. Plugging that into a recoil model yields rn ≈ 440 m.
All three methods agree: the nucleus is a sub-kilometre object with a working radius of rn = 500 ± 100 m — roughly 10 football fields across.
| Method | Radius Estimate | Key Assumption |
|---|---|---|
| HST Optical Photometry | ≈ 560 m | Albedo pV = 0.04 |
| Radar (12.6 cm) | ≳ 450 m | Assumed radar albedo |
| Non-Gravitational Acceleration | ≈ 440 m | kR = 0.5, ρn = 500 kg m−3 |
| Adopted Working Value | 500 ± 100 m | — |
The Math Behind the Torque: How Outgassing Flips a Comet
Let's walk through the physics — step by step and in plain language. If equations aren't your thing, don't worry; we'll explain every symbol.
Angular Momentum of the Nucleus
A spinning nucleus of radius rn, density ρn, and rotation frequency Ω carries angular momentum:
This assumes a uniform-density sphere with moment-of-inertia coefficient kI = 2/5.
The Outgassing Torque
Gas escaping the surface at speed Vth and mass rate Ṁ exerts a torque:
Here kT is the dimensionless moment arm — the fraction of outflow momentum that pushes sideways rather than straight out. For TGK, Jewitt finds kT = 0.013, about twice the median for short-period comets.
Rate of Spin Change
Because torque equals the rate of change of angular momentum (T = dL/dt), we get the spin-change equation:
Notice the rn4 in the denominator. A nucleus half the size responds 16 times more to the same outgassing rate. That single detail explains why TGK's rotation changed so fast: it's tiny.
Estimating the Nucleus Radius from Recoil Acceleration
Jewitt also used the non-gravitational acceleration (ζ) to independently calculate the radius:
With kR = 0.5, Ṁ = 90 kg s−1, Vth = 500 m s−1, and ρn = 500 kg m−3, this gives rn ≈ 440 m — consistent with the optical measurement.
| Parameter | Value | Source / Note |
|---|---|---|
| Semi-major axis (a) | 3.085 au | Orbital elements |
| Eccentricity (e) | 0.661 | Orbital elements |
| Inclination (i) | 9.2° | Orbital elements |
| Perihelion distance | 1.045 au | Orbital elements |
| Aphelion distance | 5.124 au | Orbital elements |
| Orbital period | ≈ 5.4 years | Derived |
| Nucleus effective radius (rn) | 500 ± 100 m | HST + radar + non-grav. |
| Assumed density (ρn) | 500 kg m−3 | Groussin et al. 2019 |
| Assumed albedo (pV) | 0.04 | Typical cometary value |
| Perihelion water production (2017) | ≈ 90 kg s−1 | Combi et al. 2020 |
| Dimensionless moment arm (kT) | 0.013 | Jewitt 2026 |
| Projected axis ratio | ≥ 1.4 : 1 | Lightcurve range 0.4 mag |
| Rotation period (Dec 2017) | 0.599 day (14.4 h) | HST lightcurve |
| Time in current orbit | ≈ 1,500 years | Pozuelos et al. 2018 |
| Spin-up lifetime (τs) | ~ 13–25 years | Jewitt 2026 |
What Does the Shrinking Active Fraction Reveal?
Not every square metre of a comet's surface sublimates. The active fraction (fA) measures how much of the surface actually produces gas compared to what a bare ice sphere would yield.
In 2001, TGK's water production rate was a whopping ~1,500 kg s−1, giving an active fraction of about 2.4. An active fraction above 1 means the nucleus was "hyper-active" — likely boosted by icy grains sublimating in the coma itself, not just on the surface.
By 2017, production had dropped tenfold to ~90 kg s−1, and fA fell to approximately 0.14. That's a dramatic decline in just two orbits. The surface was sealing itself under a growing refractory mantle — a crust of dust left behind as ice evaporated.
This orbit-to-orbit change tells us something profound: a comet's personality isn't fixed. The strength and direction of outgassing torques can vary wildly over time, making long-term predictions tricky.
Can a Comet This Fragile Survive?
Here's a puzzle that kept us thinking long after we read the paper.
If the spin-up timescale is only about 13 to 25 years — a handful of orbits — how has TGK managed to exist for 1,500 years in its current orbit [[1]]? A body that reaches the critical breakup spin in a few decades should have shattered centuries ago.
The breakup spin for a strengthless sphere of density 500 kg m−3 corresponds to a rotation period of about 4.7 hours [[1]]. At the 2017 deceleration rate, TGK would reach that limit in roughly 13 years — about 2.5 orbits.
Jewitt offers two possible explanations:
Possibility 1 — Unusually high current activity. Maybe TGK is going through a phase of exceptional outgassing right now. If the surface is normally more heavily mantled, mass loss rates would be lower on average, and the torques gentler. We know the water production dropped by an order of magnitude between 2001 and 2017, so wild swings in activity clearly happen.
Possibility 2 — Remnant of a larger body. TGK may have entered its current orbit as a bigger object — perhaps a kilometre across — and has been whittled down by sublimation or partial breakup events. A larger nucleus resists torque changes far better (remember that rn4 in the denominator). So its ancestors could have survived comfortably for millennia.
Both scenarios may operate at the same time. We simply don't have enough data yet to choose between them.
What's Next for Comet 41P?
TGK's next perihelion is predicted for 2028 February 16. The viewing geometry won't be as good as in 2017, but new observations should still give us a fresh spin measurement. Comparing 2028 data with the 2017 record will reveal whether the torque pattern repeated, strengthened, or changed direction entirely.
No published spin rates exist from the September 2022 perihelion, so we're missing one data point in the timeline. That gap is frustrating — but it also means any future ground-based or space-telescope campaign targeting TGK will be doing genuinely new science.
Jewitt's 2026 analysis is currently a preprint — it hasn't passed through peer review yet. That doesn't mean it's wrong; it means the scientific community is still weighing the evidence. And that openness is exactly how good science works.
| Date (2017) | Rotation Period | Observation |
|---|---|---|
| March | ≈ 20 hours | Ground-based photometry |
| April 12 (Perihelion) | Rapidly increasing | Jets & lightcurves |
| May | ≈ 53 hours | Ground-based observations |
| ≈ June 9 (estimated) | ∞ (spin = 0) | Polynomial fit |
| December 11–14 | 14.4 hours (reversed) | HST archival data |
Final Thoughts
Comet 41P/Tuttle-Giacobini-Kresak is, by any measure, a small and fragile world — half a kilometre across, leaking gas into space, and spinning at the mercy of its own exhaust. Yet it has taught us something extraordinary: outgassing torques can completely reverse the rotation of a cometary nucleus in a matter of months.
That single fact rewrites our intuition about how stable these icy bodies really are. It also raises urgent questions. Will TGK spin itself apart before 2028? Is it a relic of a once-larger parent? How many other small comets are doing the same thing right now, unobserved?
We don't have all the answers yet. And honestly, that's the part we love most. Science isn't a textbook of finished truths — it's a living conversation between observation and imagination.
This article was crafted for you by FreeAstroScience.com, where we explain complex scientific principles in simple terms — because knowledge shouldn't be locked behind jargon. We believe education isn't something that ends when you close a book. Keep your mind active at all times, because the sleep of reason breeds monsters.
Come back to FreeAstroScience whenever you want to sharpen your understanding of the cosmos. We'll be here — asking questions, chasing answers, and sharing the wonder with you.
Sources
- Jewitt, D. (2026). "Reversal of Spin: Comet 41P/Tuttle–Giacobini–Kresak." arXiv preprint, arXiv:2602.06403v1 [astro-ph.EP]. Department of Earth, Planetary and Space Sciences, UCLA.
- Starr, M. (2026, February 20). "This Comet Mysteriously Reversed Its Spin After Passing The Sun, But Why?" ScienceAlert.
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