iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
Comet 67P/Churyumov–Gerasimenko is smaller and returns far more often than 1P/Halley. Rosetta measured 67P’s unusual two-lobed nucleus in detail, finding a very low-density, highly porous mixture of ice and dust. Halley’s nucleus is substantially larger on the dimensions reported by NASA, follows a much longer retrograde orbit, and was examined during a brief 1986 flyby that concentrated on its nucleus, coma and dust rather than an extended interior survey.
The comparison below keeps those measurements in context: a comet’s solid nucleus is not the same thing as its coma, and the two missions did not collect equivalent data.
At a glance: the main differences
| Characteristic | 67P/Churyumov–Gerasimenko | 1P/Halley |
|---|---|---|
| Nucleus shape and scale | Two joined lobes. Small lobe about 2.6 × 2.3 × 1.8 km; large lobe about 4.1 × 3.3 × 1.8 km. Earlier Hubble estimates were roughly 5 × 3 km. | Approximately 15 × 8 km in NASA’s current description; a NASA Giotto image record gives about 16 × 8 × 8 km. |
| Orbital period | 6.57 years in NASA’s comet table; ESA commonly rounds this to about 6.5 years. | Average 76.1 years. The exact interval changes because planetary gravity perturbs the orbit. |
| Orbit | Elliptical path from about 1.24 AU at perihelion to 5.68 AU at aphelion. | Very elongated, retrograde orbit tilted about 18 degrees to Earth’s orbital plane; aphelion lies beyond Neptune. |
| Best spacecraft evidence | ESA’s Rosetta orbited the comet for an extended mission and deployed the Philae lander. | ESA’s Giotto made a fast flyby in March 1986, imaging the nucleus and sampling the coma and dust. |
| Interior evidence | Volume 21.4 km³, mass about 10 billion tonnes, bulk density 470 kg/m³ and an estimated 70–80% porosity under an ice-and-dust density assumption. | The cited mission results describe volatile material, dust composition and gas/dust properties, but do not provide a matching consolidated porosity estimate. |
Using the rounded periods, Halley takes more than eleven times as long as 67P to complete an orbit. That ratio is a calculation from 76.1 and 6.57 years, not a separately measured comet property.
How big is Halley’s Comet?
NASA describes Halley’s solid nucleus as approximately 15 × 8 kilometres. A NASA record for the Giotto image gives a closely related estimate of about 16 × 8 × 8 kilometres. Those values use different dimensional conventions and rounding; they are not evidence that the comet changed size between descriptions.
Popular on Amazon
The nucleus is the durable solid body made of frozen volatile material mixed with dust. It is much smaller than the bright coma and tail seen when Halley is active. Sunlight heats the nucleus, causing ice to turn directly into gas and carry dust away. The escaping material forms the coma around the nucleus and, when solar radiation and the solar wind sweep it outward, the tail. Comparing nucleus dimensions therefore gives a meaningful physical size comparison, while photographs of the surrounding coma do not.
Halley compared with 67P’s lobes
Halley’s reported length of roughly 15–16 kilometres is several times the longest dimension of either 67P lobe. Rosetta resolved 67P as two connected, irregular lobes rather than a single rounded object. The small lobe measures about 2.6 × 2.3 × 1.8 km and the large lobe about 4.1 × 3.3 × 1.8 km. Earlier Hubble observations, made before a spacecraft reached the comet, characterized the whole object more simply as roughly 5 × 3 km.
These figures should be read as approximate bounding dimensions, not as the diameter of a sphere. Neither comet is spherical, and the neck joining 67P’s lobes makes a single “diameter” especially misleading. Halley’s 15-by-8-kilometre description likewise summarizes an irregular nucleus rather than a perfect ellipsoid.
Recommended Free Tools
How often does each comet come around?
67P’s 6.57-year cycle
NASA’s comet table lists 67P’s orbital period as 6.57 years; ESA rounds it to about 6.5 years. After passing near the Sun, 67P travels outward to the colder part of its orbit and then returns on this comparatively short cycle. Its perihelion, the point nearest the Sun, is about 1.24 astronomical units (AU), while its aphelion, the most distant point, is about 5.68 AU.
An AU is the average distance between Earth and the Sun. Expressing a comet’s orbit in AU makes clear that 67P’s path reaches beyond Mars’ orbit and into the outer Solar System, but not as far as Halley’s extreme aphelion.
Halley’s roughly 76-year average
NASA gives Halley an average orbital period of 76.1 years. “Average” matters: each return does not occur after exactly the same number of years. Close approaches to planets, especially Jupiter and Saturn, alter the comet’s speed and trajectory. NASA expects the next perihelion return in 2061, but the date and interval are refined as observations and gravitational calculations improve.
Also popular on Amazon
Halley’s long interval explains why a person may see it only once in a lifetime, if at all. The comet appeared in 1910 and 1986, but the 1986 return was the first opportunity for spacecraft to examine it directly. The next expected return, in 2061, is not a sign that the comet has “disappeared” in the meantime; for most of its orbit it is too faint and distant for casual observation.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
How do their orbits differ?
67P: an elliptical, prograde orbit
67P follows an ellipse that carries it from about 1.24 AU at perihelion to 5.68 AU at aphelion. It travels around the Sun in the same general direction as Earth, the usual sense for most planets and many comets. Its comparatively modest period reflects a smaller orbital path and a lower average distance from the Sun than Halley’s.
Halley: long, tilted and retrograde
Halley’s orbit is much more elongated. NASA describes it as retrograde and tilted about 18 degrees to Earth’s orbital plane, with aphelion beyond Neptune. Retrograde means Halley moves around the Sun opposite Earth’s direction of travel. The tilt and extreme elongation take it far from the planetary region for most of its 76-year cycle, then bring it back through the inner Solar System for a short active season.
Retrograde does not mean Halley is orbiting the Sun backward in a way that violates gravity. It is a geometric description of direction relative to the planets. A retrograde path can still be a stable, predictable ellipse; its orientation simply differs from the prograde orbits followed by Earth and most major planets.
Why periods and paths change
Comet orbits are not isolated, fixed tracks. Planetary gravity nudges a comet whenever it passes near a massive body. Those perturbations can change the period, orientation or timing of a future perihelion. Consequently, 6.57 years for 67P and 76.1 years for Halley are useful current reference values, not guarantees that every return will be separated by exactly the same interval.
Quick wins for a faster PC:
Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →What is Comet 67P made of?
Rosetta’s measurements support a nucleus dominated by water ice and dust with an exceptionally open internal structure. ESA reported a total volume of 21.4 km³, a mass of about 10 billion tonnes and a measured bulk density of 470 kg/m³. Typical constituent ice-and-dust material is denser—about 1,500–2,000 kg/m³ in the assumption used for the estimate. Reconciling that material density with 67P’s much lower bulk density led scientists to estimate that 70–80% of the nucleus is empty space, or porosity.
Also popular on Amazon
That 70–80% figure is an inference from bulk properties and an assumed density range, not a direct count of every void inside the comet. It means that most of the nucleus’s volume is estimated to be pore space between solid particles and ice-rich material. The result helps explain why 67P has such a low average density and why its surface and cliffs can respond dramatically to heating and activity.
Why Rosetta could make this assessment
Rosetta did not merely pass 67P once. It entered orbit around the comet, watched it approach the Sun, and deployed the Philae lander. The long campaign allowed scientists to relate changes in outgassing and surface activity to the comet’s position in its orbit. ESA Rosetta project scientist Matt Taylor summarized the strategy: “Rosetta is essentially living with the comet as it moves towards the Sun along its orbit, learning how its behaviour changes on a daily basis and, over longer timescales, how its activity increases, how its surface may evolve, and how it interacts with the solar wind.”
This time-resolved, close-range coverage is why 67P’s structure can be discussed with measurements of volume, mass, density and inferred porosity rather than only with a broad description of its bright coma.
What do we know about Halley’s composition?
Giotto’s 1986 encounter established that Halley has a dark, solid nucleus surrounded by an active coma and provided measurements of volatile components, dust elemental and isotopic composition, and gas and dust properties. The spacecraft also imaged the nucleus during its high-speed flyby.
Those results are valuable, but they do not constitute the same kind of interior survey Rosetta performed at 67P. Giotto passed Halley once in March 1986, crossing the coma at high speed. Its instruments sampled material released from the surface and observed the nucleus from the flyby trajectory. The cited Halley sources do not provide a consolidated porosity estimate comparable to 67P’s 70–80% model-based value.
It is therefore accurate to say that both comets contain volatile ices and dust, as expected for comets, while avoiding the stronger claim that their internal composition or void fraction has been measured in equivalent detail. Halley’s coma measurements describe material escaping from the nucleus; they are not a complete density map of the solid body.
Also popular on Amazon
Why the mission designs matter
| Mission | Encounter | What it enabled | What it could not establish by itself |
|---|---|---|---|
| Rosetta and Philae | ESA spacecraft orbited 67P and followed it over an extended period. | Repeated imaging, changing activity measurements, close study of the surface and global properties, plus lander observations. | A complete, direct inventory of every internal pore or every layer was not obtained; porosity remains an inference from measured bulk properties and assumed material density. |
| Giotto | ESA flyby of Halley in March 1986. | Imaging of the nucleus and in-situ sampling of coma gas and dust, including elemental and isotopic information. | A single fast passage did not provide a Rosetta-style, long-duration survey of Halley’s interior or seasonal evolution. |
The apparent imbalance in composition detail is a consequence of observation strategy, not proof that one comet is intrinsically more complex. Rosetta had time to track 67P’s changing behavior; Giotto had to collect as much as possible during one rapid crossing of Halley’s active environment.
Which comet is larger, more frequent and more extreme?
- Larger nucleus: Halley, at approximately 15 × 8 km (or about 16 × 8 × 8 km in the Giotto image record), is larger on the reported dimensions than 67P’s two lobes.
- More frequent return: 67P, with a 6.57-year period, comes back more than eleven times as often as Halley’s 76.1-year average.
- More unusual shape: 67P’s connected “rubber-duck” form is a defining feature of its nucleus; Halley’s published dimensions describe a larger, elongated body without the same two-lobe presentation.
- More extreme orbit: Halley is retrograde, tilted about 18 degrees and reaches beyond Neptune at aphelion. 67P’s quoted orbit remains within about 5.68 AU at its farthest point.
- Better-constrained bulk structure: 67P has measured volume, mass and bulk density from Rosetta, supporting a 70–80% porosity estimate under stated assumptions. A directly comparable Halley porosity value is not established in the cited material.
How to interpret the numbers without being misled
Do not treat every dimension as a diameter
The dimensions are length-by-width-by-height estimates for irregular bodies. A comet’s apparent size can also vary with viewing angle, lighting, active jets and the method used to reconstruct its shape. That is why 67P’s lobe measurements, earlier Hubble estimate and Halley’s two NASA descriptions should be presented as compatible approximations rather than as precision engineering drawings.
Separate nucleus, coma and tail
The nucleus is the solid object being compared for size and bulk density. The coma is a temporary envelope of gas and dust produced when solar heating activates the surface. The tail is shaped by sunlight and the solar wind. A large coma does not mean a larger nucleus, and coma spectra cannot automatically be converted into a complete recipe for the interior.
Keep measured values separate from inferred values
Rosetta measured 67P’s volume, mass and bulk density. The 70–80% porosity is calculated by comparing that bulk density with an assumed 1,500–2,000 kg/m³ density for constituent ice and dust. It is scientifically useful, but it is not the same type of direct measurement as the spacecraft’s mass estimate. Halley’s flyby composition results likewise describe sampled gas and dust rather than a whole-nucleus porosity measurement.
What this comparison says about comet science
Putting 67P and Halley side by side shows why “What is a comet made of?” has no single level of answer. A flyby can reveal the chemistry and physical behavior of material escaping into the coma. An orbiter can add seasonal context, map changing activity and constrain the bulk properties of the nucleus. Neither approach alone provides every detail.
Free tools Windows power users keep installed
One-click scans. No signup required.
For 67P, the strongest conclusion is a very low-density, highly porous, ice-and-dust nucleus with two distinct lobes and a short-period orbit. For Halley, the strongest conclusions in the cited measurements are a substantially larger nucleus, a long and retrograde orbit, and rich observations of coma volatiles and dust during the 1986 Giotto encounter. The evidence supports comparison, but not a claim that the two interiors have been characterized to the same precision.
Also popular on Amazon
Frequently asked questions
Is 67P bigger than Halley’s Comet?
No. Halley’s nucleus is reported at roughly 15 × 8 km, while 67P’s lobes measure about 2.6 × 2.3 × 1.8 km and 4.1 × 3.3 × 1.8 km. The bodies are irregular, so these are approximate bounding dimensions rather than spherical diameters.
Which comet returns more often?
67P returns more often. Its orbital period is 6.57 years, compared with Halley’s average of 76.1 years.
Is Halley’s Comet bigger because it has a longer orbit?
Not necessarily. Orbital period depends mainly on the size and shape of the path around the Sun, while nucleus size is a separate physical property. Halley is both larger on the cited dimensions and much farther-ranging in its orbit, but one does not cause the other.
Does Halley have the same 70–80% porosity as 67P?
There is no matching value established in the cited material. The 70–80% estimate applies specifically to 67P and depends on Rosetta’s measured bulk density plus an assumed density for ice and dust.
When will Halley’s Comet return?
NASA expects Halley to return in 2061. The exact timing of any return is subject to gravitational perturbations and continuing orbital refinement.
Why did scientists learn more about 67P’s structure than Halley’s?
Rosetta orbited 67P and observed it over an extended mission, while Giotto made a single high-speed flyby of Halley in March 1986. The longer encounter provided more opportunities to measure 67P’s changing activity and constrain its bulk properties.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
