<p>If I say &quot;Orbital mechanics is easy&quot; some people who know a lot about this will get very angry.</p>
<p>But for the purposes of this question: orbital mechanics are easy.</p>
<ul>
<li>we know where the comet is</li>
<li>we know how fast it is going</li>
<li>we know which direction it is going.</li>
<li>we know about the various objects that might have a large infulence, such as the planets.</li>
</ul>
<p>so with all this information we can calculate the orbit of the comet. The saying goes &quot;what goes up must come down&quot; but with orbits: what's coming down must have gone up at some point in history.</p>
<p>So with the information about the orbit we can spool back the timeline, and check where the planets were at various points in time. And apparently, 50.000 years ago the orbit of the comet and that of earth got reasonably close together.</p>

CC BY-SA 4.0
2023-01-13T11:49:00.797Z
Well summarized. It's not easy. But the principle is which you describe here - and that's how this number is derived.
planetmaker
CC BY-SA 4.0
2023-01-13T13:14:45.520Z
We don't know where the comet is. We know measurements of azimuth and elevation, but not range (unless we've pinged it with radar). Unless we've pinged it with radar, we do not know velocity, and even if we did, that would just give range rate (the component of velocity toward the Earth). If you consider a graduate level class on statistical orbit determination easy, then yes, "orbital mechanics is easy".
David Hammen
CC BY-SA 4.0
2023-01-14T04:10:20.640Z
@uhoh My answer does not say what you read it as saying. Propagating comets, and this is particularly so for long-period comets, is highly nontrivial and not very accurate. There are now articles saying the comet will never come back. We do not know that. We also do not know when the last time the comet visited the inner solar system, if it ever did. What we do know is that it appears to be a long-period comet as opposed to an invader from outside the solar system.
David Hammen
CC BY-SA 4.0
2023-01-14T04:42:57.477Z
@ScienceAJ Yes n-body orbital mechanics can be chaotic but it doesn't mean it's *never predictable*. Chaos theory is fun & important concept is [bifurcation](https://en.wikipedia.org/wiki/Bifurcation_theory). DH's answer suggests there simply hasn't been observations widely-enough spaced over time to extract a current trajectory of sufficient accuracy to extract orbital parameters, or back-propagate to a previous inner solar system pass, much less to know if any major bifurcations may have occurred due to  very close approaches where even small uncertainties can result in big errors
uhoh
CC BY-SA 4.0
2023-01-14T04:53:09.827Z
@DavidHammen yes you are right, thanks. I've revised the comment. Nice edit too! Oh the current Horizons output includes the following: `data arc: 2021-07-10 to 2023-01-06` and `EC= 1.0002694255512`
uhoh
CC BY-SA 4.0
2023-01-13T10:59:18.393Z
2023-01-13T10:59:18.393Z
Borgh
<p>50000 years is the comet's <strong>estimated</strong> orbital period. That does not necessarily mean that the comet was naked-eye visible from Earth 50000 years ago. That also does not necessarily mean that the comet last came close to the Earth's orbit (as opposed to the Earth) 50000 years ago. The 50000 years is an estimate based on nine months or so of observation time. In addition, the comet's orbit might have been perturbed in the time between its last perihelion passage and the current one. This might even be the comet's first visit to the inner solar system.</p>
<p>From NPR's <a href="https://www.npr.org/2023/01/08/1147685424/comet-green-january-e3-ztf-nasa" rel="noreferrer">A bright green comet may be visible with the naked eye starting later this month</a> (admittedly yet another pop-sci article),</p>
<blockquote>
<p>&quot;If C/2022 E3 has ever passed through the solar system before, it would have last been seen in the sky more than 10,000 years ago,&quot; says Jon Giorgini, a senior analyst at NASA's Jet Propulsion Laboratory.</p>
</blockquote>
<p>Note well: This admittedly is yet another pop-sci article. However, NPR has a JPL expert who says it is it least 10000 years ago (if ever) that the comet last visited the inner solar system rather than 50000 years ago. This article is fairly recent. NPR did their research well; they went to an expert from JPL.</p>
<p>Determining the orbit of a long-period comet is highly non-trivial. We have nine months worth of partial observations (mostly azimuth and elevation, which have significant measurement errors, even from the best observatories) of a comet with a suppose 50000 year period. If true, that 9 month interval is 15 millionths of the comet's orbit. That simply is not a long enough of an arc to perform precise orbit determination.</p>
<p>To make matters worse, those long-period comets necessarily travel well beyond Pluto's orbit. At those distances, the entire inner solar system out to Neptune gravitationally act essentially as a single body located at the solar system barycenter. Inside Neptune's orbit, it's better (from an orbital element perspective) to look at objects as orbiting the Sun with the planets as perturbations.</p>
<p>There are now articles saying the comet will never return. That's because using JPL's Horizons to provide osculating orbital elements yields an eccentricity slightly greater than one -- in heliocentric coordinates. Osculating elements can be deceiving, particularly so for long-period comets.</p>
<p>Bottom line: There is no telling if the 50000 year value is anywhere close to correct.</p>
<p>Take popular science articles with a grain of salt.</p>

CC BY-SA 4.0
2023-01-13T10:56:33.203Z
But also the probabilities of other objects having interacted with it significantly over that time are very, very small, so the assumption of its orbit not having been disturbed is by far the most likely situation.  A grain of salt is fair, but not more than that. :)
Graham
CC BY-SA 4.0
2023-01-13T14:20:49.307Z
The comet is currently in a parabolic trajectory. I'm not even sure what the period of such a trajectory is supposed to mean. (Has it transitioned *from* an elliptical orbit in the last 50,000 years?)
chepner
CC BY-SA 4.0
2023-01-13T15:24:01.410Z
@chepner I guess they define the period as 2x the time it takes to go from perihelion to aphelion.
Barmar
CC BY-SA 4.0
2023-01-13T15:25:15.617Z
Aphelion isn't really defined for a parabola; the distance grows without limit as the object moves away from the son.
chepner
CC BY-SA 4.0
2023-01-13T17:18:59.857Z
what are you talking about? are you saying that our ancestors 50000 years ago didn't have binoculars to watch this comet? o0
BЈовић
CC BY-SA 4.0
2023-01-14T03:13:23.093Z
The article specifically discusses the orbit perihelion and aphelion, and has no mention of "parabolic" nor "parabola"
Mooing Duck
CC BY-SA 4.0
2023-01-14T03:14:56.037Z
https://www.space.com/comet-c2022-e3-ztf-visible-naked-eye-january-2023 mentions "But this may very well be the last time that C/2022 E3 comes our way again. The latest orbital elements suggest that the comet is currently traveling on an orbital path with an eccentricity of 1.00027, or in other words, a parabolic orbit. Such an orbit is not closed, so after it sweeps around the sun C/2022 E3 will move back out into deep space, never to return again", so I'm curious how they found the orbit then.
Mooing Duck
CC BY-SA 4.0
2023-01-14T03:25:35.787Z
@MooingDuck it's amusing (to me) that the wikipedia article on this comet uses osculating orbital elements from JPL Horizons as their citation for the 50K year orbit. Clicking on the link shows a period of more than 9e99 years. Determining the orbital period of a long-period comet is highly nontrivial. When the object is well outside the orbit of Pluto, the inner solar system (out to Neptune) gravitational ly acts as a single body. But inside Neptune orbit, it's better to look at an orbit as about the Sun, why the planets as perturbing objects. Aside: space.com articles are oftentimes garbage.
David Hammen
CC BY-SA 4.0
2023-01-14T03:58:44.843Z
@MooingDuck The error bounds on that 1.00027 eccentricity value are large enough to make the right answer "we don't know". I also suspect space.com (which far too often writes garbage articles; ChatGBT could do better) used JPL Horizons, which provides osculating elements because the calculation is easy. Some sort of mean elements would be better, but that calculation is nontrivial. Some sort of mean elements with respect to the barycenter would be even better, but that calculation is highly nontrivial, and it requires data that spans a lot more than 15 millionths of an orbit.
David Hammen
CC BY-SA 4.0
2023-01-15T15:37:19.287Z
Giacomo1968, I have rejected your edit. Adding commas to those numbers would hurt rather than help. Various parts of Europe use the comma to denote the decimal point and the period to split numbers into thousands. It's better to leave the number as 50000 as opposed to 50,000 or 50.000.
David Hammen
CC BY-SA 4.0
2023-01-12T15:48:23.453Z
2023-01-14T04:00:42.137Z
2023-01-14T04:00:42.137Z
David Hammen
<p>This article (<a href="https://gothamist.com/news/a-green-comet-is-sailing-over-new-york-and-earth-for-the-first-time-in-50000-years" rel="noreferrer">https://gothamist.com/news/a-green-comet-is-sailing-over-new-york-and-earth-for-the-first-time-in-50000-years</a>) claims that a comet will pass by earth soon for the first time in 50,000 years. How do we know that this exact comet passed earth 50,000 years ago? It obviously was not a recorded event at the time so how can such a claim be made?</p>

CC BY-SA 4.0
2023-01-13T14:33:26.340Z
"This fossil is 70 million and 2 years old." "That's really precise, how do you know that?" "Well I was here two years ago and they told me it was 70 million years old then."
user3067860
CC BY-SA 4.0
2023-01-15T06:37:47.317Z
SSD Small Body Database: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=C%2F2022%20E3 Horizons: https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=90004676
PM 2Ring
<comets>
How do we know that a comet passed by earth 50,000 years ago?
coco
# How do we know that a comet passed by earth 50,000 years ago?

(Asked by: username_0 on 2023-01-12)

This article (<https://gothamist.com/news/a-green-comet-is-sailing-over-new-york-and-earth-for-the-first-time-in-50000-years>) claims that a comet will pass by earth soon for the first time in 50,000 years. How do we know that this exact comet passed earth 50,000 years ago? It obviously was not a recorded event at the time so how can such a claim be made?

**Tags:** comets

> **Comments:**
> **username_1**: "This fossil is 70 million and 2 years old." "That's really precise, how do you know that?" "Well I was here two years ago and they told me it was 70 million years old then."  
> **username_2**: SSD Small Body Database: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=C%2F2022%20E3 Horizons: https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=90004676  

---

## 2 answers

### Answer by username_5 on 2023-01-12. Score: 45 | Accepted answer

50000 years is the comet's **estimated** orbital period. That does not necessarily mean that the comet was naked-eye visible from Earth 50000 years ago. That also does not necessarily mean that the comet last came close to the Earth's orbit (as opposed to the Earth) 50000 years ago. The 50000 years is an estimate based on nine months or so of observation time. In addition, the comet's orbit might have been perturbed in the time between its last perihelion passage and the current one. This might even be the comet's first visit to the inner solar system.

From NPR's [A bright green comet may be visible with the naked eye starting later this month](https://www.npr.org/2023/01/08/1147685424/comet-green-january-e3-ztf-nasa) (admittedly yet another pop-sci article),

> "If C/2022 E3 has ever passed through the solar system before, it would have last been seen in the sky more than 10,000 years ago," says Jon Giorgini, a senior analyst at NASA's Jet Propulsion Laboratory.

Note well: This admittedly is yet another pop-sci article. However, NPR has a JPL expert who says it is it least 10000 years ago (if ever) that the comet last visited the inner solar system rather than 50000 years ago. This article is fairly recent. NPR did their research well; they went to an expert from JPL.

Determining the orbit of a long-period comet is highly non-trivial. We have nine months worth of partial observations (mostly azimuth and elevation, which have significant measurement errors, even from the best observatories) of a comet with a suppose 50000 year period. If true, that 9 month interval is 15 millionths of the comet's orbit. That simply is not a long enough of an arc to perform precise orbit determination.

To make matters worse, those long-period comets necessarily travel well beyond Pluto's orbit. At those distances, the entire inner solar system out to Neptune gravitationally act essentially as a single body located at the solar system barycenter. Inside Neptune's orbit, it's better (from an orbital element perspective) to look at objects as orbiting the Sun with the planets as perturbations.

There are now articles saying the comet will never return. That's because using JPL's Horizons to provide osculating orbital elements yields an eccentricity slightly greater than one -- in heliocentric coordinates. Osculating elements can be deceiving, particularly so for long-period comets.

Bottom line: There is no telling if the 50000 year value is anywhere close to correct.

Take popular science articles with a grain of salt.

> **Comments:**
> **username_7**: But also the probabilities of other objects having interacted with it significantly over that time are very, very small, so the assumption of its orbit not having been disturbed is by far the most likely situation.  A grain of salt is fair, but not more than that. :)  
> **username_8**: The comet is currently in a parabolic trajectory. I'm not even sure what the period of such a trajectory is supposed to mean. (Has it transitioned *from* an elliptical orbit in the last 50,000 years?)  
> **username_9**: @username_8 I guess they define the period as 2x the time it takes to go from perihelion to aphelion.  
> **username_8**: Aphelion isn't really defined for a parabola; the distance grows without limit as the object moves away from the son.  
> **username_10**: what are you talking about? are you saying that our ancestors 50000 years ago didn't have binoculars to watch this comet? o0  
> **username_11**: The article specifically discusses the orbit perihelion and aphelion, and has no mention of "parabolic" nor "parabola"  
> **username_11**: https://www.space.com/comet-c2022-e3-ztf-visible-naked-eye-january-2023 mentions "But this may very well be the last time that C/2022 E3 comes our way again. The latest orbital elements suggest that the comet is currently traveling on an orbital path with an eccentricity of 1.00027, or in other words, a parabolic orbit. Such an orbit is not closed, so after it sweeps around the sun C/2022 E3 will move back out into deep space, never to return again", so I'm curious how they found the orbit then.  
> **username_5**: @username_11 it's amusing (to me) that the wikipedia article on this comet uses osculating orbital elements from JPL Horizons as their citation for the 50K year orbit. Clicking on the link shows a period of more than 9e99 years. Determining the orbital period of a long-period comet is highly nontrivial. When the object is well outside the orbit of Pluto, the inner solar system (out to Neptune) gravitational ly acts as a single body. But inside Neptune orbit, it's better to look at an orbit as about the Sun, why the planets as perturbing objects. Aside: space.com articles are oftentimes garbage.  
> **username_5**: @username_11 The error bounds on that 1.00027 eccentricity value are large enough to make the right answer "we don't know". I also suspect space.com (which far too often writes garbage articles; ChatGBT could do better) used JPL Horizons, which provides osculating elements because the calculation is easy. Some sort of mean elements would be better, but that calculation is nontrivial. Some sort of mean elements with respect to the barycenter would be even better, but that calculation is highly nontrivial, and it requires data that spans a lot more than 15 millionths of an orbit.  
> **username_5**: Giacomo1968, I have rejected your edit. Adding commas to those numbers would hurt rather than help. Various parts of Europe use the comma to denote the decimal point and the period to split numbers into thousands. It's better to leave the number as 50000 as opposed to 50,000 or 50.000.  

---

### Answer by username_3 on 2023-01-13. Score: 12

If I say "Orbital mechanics is easy" some people who know a lot about this will get very angry.

But for the purposes of this question: orbital mechanics are easy.

* we know where the comet is
* we know how fast it is going
* we know which direction it is going.
* we know about the various objects that might have a large infulence, such as the planets.

so with all this information we can calculate the orbit of the comet. The saying goes "what goes up must come down" but with orbits: what's coming down must have gone up at some point in history.

So with the information about the orbit we can spool back the timeline, and check where the planets were at various points in time. And apparently, 50.000 years ago the orbit of the comet and that of earth got reasonably close together.

> **Comments:**
> **username_4**: Well summarized. It's not easy. But the principle is which you describe here - and that's how this number is derived.  
> **username_5**: We don't know where the comet is. We know measurements of azimuth and elevation, but not range (unless we've pinged it with radar). Unless we've pinged it with radar, we do not know velocity, and even if we did, that would just give range rate (the component of velocity toward the Earth). If you consider a graduate level class on statistical orbit determination easy, then yes, "orbital mechanics is easy".  
> **username_5**: @username_6 My answer does not say what you read it as saying. Propagating comets, and this is particularly so for long-period comets, is highly nontrivial and not very accurate. There are now articles saying the comet will never come back. We do not know that. We also do not know when the last time the comet visited the inner solar system, if it ever did. What we do know is that it appears to be a long-period comet as opposed to an invader from outside the solar system.  
> **username_6**: @ScienceAJ Yes n-body orbital mechanics can be chaotic but it doesn't mean it's *never predictable*. Chaos theory is fun & important concept is [bifurcation](https://en.wikipedia.org/wiki/Bifurcation_theory). DH's answer suggests there simply hasn't been observations widely-enough spaced over time to extract a current trajectory of sufficient accuracy to extract orbital parameters, or back-propagate to a previous inner solar system pass, much less to know if any major bifurcations may have occurred due to  very close approaches where even small uncertainties can result in big errors  
> **username_6**: @username_5 yes you are right, thanks. I've revised the comment. Nice edit too! Oh the current Horizons output includes the following: `data arc: 2021-07-10 to 2023-01-06` and `EC= 1.0002694255512`  

---

