<p>Using the follow settings:</p>
<ul>
<li>Target body name: Voyager 1</li>
<li>Center body name: Solar System Barycenter</li>
<li>Reference frame : ICRF</li>
</ul>
<p>I get the following from Horizons, in km/sec, at 2024 Oct-20 at 00:00 TDB:<br />
VX=-2.062 VY=-16.407 VZ= 3.610</p>
<p>If I switch the central body to the Sun, I get<br />
VX=-2.074 VY=-16.400 VZ= 3.614</p>
<p>This is almost the same as the barycentric results.</p>
<p>If on the other hand I switch the central body to the Earth, I get<br />
VX= 11.808 VY=-40.698 VZ=-6.918</p>
<p>This is not anywhere close to the same as the barycentric results (which is not surprising as most of the relative motion comes from the motion of the Earth relative to the Sun or the solar system barycenter), but also is not close to the same as the values in the question, even after switching coordinates to make Y rather than Z be up.</p>
<p>Finally, if I switch the reference frame from ICRF to ecliptic of J2000.0, and keep the central body to the Earth, I get<br />
VX= 11.1808 VY=-40.092 VZ= 9.841</p>
<p>Also note that</p>
<ul>
<li>The Z position of Voyager 1 is positive (strongly positive) in all of the queries. Voyager 1 is above the equator and above the ecliptic.</li>
<li>If you want to determine whether an object is above or below the ecliptic you should be asking for ecliptic coordinates rather than Earth equatorial coordinates.</li>
</ul>
<p>I suspect the problem is with two items:</p>
<ul>
<li>You did not specify the time of the query in the question; I used a time close to the time of my answer; or</li>
<li>Universe Sandbox is goofy. That it uses Y as &quot;up&quot; is a big sign of that goofiness. That the plus Z axis is &quot;up&quot; is pretty much standard.</li>
</ul>
<p>That JPL can still communicate with the Voyager satellites is a huge indicator that JPL's trajectories are close to spot on. The antennae JPL uses to capture telemetry from the Voyagers have a rather narrow beam width.</p>

CC BY-SA 4.0
2024-10-21T17:01:22.000Z
Thank-you for the answer. I've been using 1 Jan 2000, 12:00:00 as the time for the results, since the model's based on that epoch.
Kazon
CC BY-SA 4.0
2024-10-20T05:29:28.543Z
2024-10-20T05:29:28.543Z
David Hammen
<p>I tried using Horizon but the values seem wrong, consistently taking Voyager beneath the ecliptic plane in my model (and to the right of Earth, if Voyager's position is straight ahead) rather than above it. Forgive me if the question's not easy to understand. I admit I'm not familiar with all the terminology surrounding motion.</p>
<p>EDIT: Sorry for the confusion. I meant to ask which speed, in km/s, V1 is travelling on the X, Y, and Z planes (VX, VY, VZ in Horizons). For full context, I'm doing this in Universe Sandbox, with a model wherein X is 0 hours (right ascension), Y is up (90 deg declination), and Z is 18 hours (RA).
My goal is to make a model of various objects V1 will encounter on its current trajectory.</p>
<p>As John Holtz requested, the values I'm getting in Horizons are:
VX=-6.725345450054260 VY=-12.27899493136756 VZ= 10.13492593876307.
The values I'm getting from the sim, by pointing the velocity of an object around Earth toward where Voyager 1 should be, are: VX -48.7, VY 13.4, VZ 36.1.</p>

CC BY-SA 4.0
2024-10-19T21:58:52.903Z
If JPL Horizons has it wrong, I honestly would not expect anyone else to have it correct.
Greg Miller
CC BY-SA 4.0
2024-10-19T22:53:40.457Z
It's probably myself who has it wrong, but I don't know how to make it right.
Kazon
CC BY-SA 4.0
2024-10-20T00:01:23.083Z
Please post the values you are getting. Then someone can explain why they are correct or what is wrong.
JohnHoltz
CC BY-SA 4.0
2024-10-20T00:10:59.770Z
Yes, your question *is* confusing. ;) In the title, you ask about velocity vectors, but in the question body you seem to be talking about position vectors. Several months ago, [I said](https://astronomy.stackexchange.com/a/55927/16685): "The current speeds of Voyager 1 and 2 relative to the Sun are roughly 17 km/s and 15 km/s, respectively. Earth's orbital speed is around 30 km/s, so there are periods each year when the Voyagers have negative speed relative to Earth".
PM 2Ring
CC BY-SA 4.0
2024-10-20T02:32:29.420Z
@PM2Ring Sorry, I meant to ask which speed, in km/s, V1 is travelling on the X, Y, and Z planes (VX, VY, VZ in Horizons). For full context, I'm doing this in Universe Sandbox, with a model wherein X is 0 hours (right ascension), Y is up (90 deg declination), and Z is 18 hours (RA). You make a good point with motion versus Earth. Maybe using the Sun as a reference is better? I'd still prefer its motion to be relative to Earth in J2K, but I can go with the ecliptic plane instead if necessary. I'll edit this into the question.
Kazon
CC BY-SA 4.0
2024-10-20T02:32:33.200Z
@JohnHoltz I will, thank you.
Kazon
CC BY-SA 4.0
2024-10-20T02:46:02.037Z
Yes, I think motion with respect to the Sun would be less confusing, since Earth's speed wrt to the Sun is so much greater than the Voyagers'. FWIW, here's a plot of Voyager 1 Astrometric RA & Dec in the ICRF frame (Horizons observer quantity #1), covering 1 year, with a 5 day timestep. https://i.sstatic.net/0kLs70JC.png
PM 2Ring
CC BY-SA 4.0
2024-10-20T02:52:23.930Z
Can you post your Horizons settings (in a code block)? If you're using the app, there's a link towards the bottom of the page: *"If you plan to use one of the "batch" modes to access Horizons, the batch-file corresponding to the settings above can be viewed by using this link."*
PM 2Ring
CC BY-SA 4.0
2024-10-20T02:57:48.910Z
Ok. I just noticed one source of confusion. The Universe Sandbox has its Z axis in the equatorial plane. But in ICRF & Horizons "the ICRF pole direction (+Z) and right ascension origin (+X), along with the orthogonal +Y direction."
PM 2Ring
CC BY-SA 4.0
2024-10-20T03:24:03.003Z
As I mention in https://space.stackexchange.com/a/67041/38535 the Horizons trajectory data for the Voyagers only goes upto the year 2100. You can extrapolate beyond that date using the conservation of energy equation https://en.wikipedia.org/wiki/Specific_orbital_energy but they won't encounter bodies outside the Solar System for a *long* time, and to predict those encounters you need to have a vaguely realistic model of gravity and proper motions in the galactic neighbourhood.
PM 2Ring
CC BY-SA 4.0
2024-10-21T17:05:07.533Z
@PM2Ring Here you go:

!$$SOF
MAKE_EPHEM=YES
COMMAND=-31
EPHEM_TYPE=VECTORS
CENTER='500@10'
START_TIME='2000-01-01 11:59:59'
STOP_TIME='2000-01-01 12:00:00'
STEP_SIZE='1 DAYS'
VEC_TABLE='5'
REF_SYSTEM='ICRF'
REF_PLANE='ECLIPTIC'
VEC_CORR='NONE'
CAL_TYPE='G'
OUT_UNITS='KM-S'
VEC_LABELS='YES'
VEC_DELTA_T='NO'
CSV_FORMAT='NO'
OBJ_DATA='NO'
Kazon
CC BY-SA 4.0
2024-10-22T02:35:28.100Z
Ok. That gives the velocity relative to the Sun, using the ecliptic as the XY plane. But the numbers don't match what you gave in the question. `$$SOE
2451544.999988426 = A.D. 2000-Jan-01 11:59:59.0000 TDB 
 VX=-2.157875824230245E+00 VY=-1.389610168160854E+01 VZ= 1.004801680496838E+01
$$EOE` And if I change the CENTER to Earth `500@399` those numbers don't match either...
PM 2Ring
CC BY-SA 4.0
2024-10-22T11:41:28.810Z
@PM2Ring I get the same values as you, and these do indeed not match the values in the question. Pilot error?
David Hammen
<proper-motion>
What are the equatorial XYZ velocity vectors of Voyager 1, relative to J2000 Earth?
Kazon
# What are the equatorial XYZ velocity vectors of Voyager 1, relative to J2000 Earth?

(Asked by: username_0 on 2024-10-19)

I tried using Horizon but the values seem wrong, consistently taking Voyager beneath the ecliptic plane in my model (and to the right of Earth, if Voyager's position is straight ahead) rather than above it. Forgive me if the question's not easy to understand. I admit I'm not familiar with all the terminology surrounding motion.

EDIT: Sorry for the confusion. I meant to ask which speed, in km/s, V1 is travelling on the X, Y, and Z planes (VX, VY, VZ in Horizons). For full context, I'm doing this in Universe Sandbox, with a model wherein X is 0 hours (right ascension), Y is up (90 deg declination), and Z is 18 hours (RA).
My goal is to make a model of various objects V1 will encounter on its current trajectory.

As John Holtz requested, the values I'm getting in Horizons are:
VX=-6.725345450054260 VY=-12.27899493136756 VZ= 10.13492593876307.
The values I'm getting from the sim, by pointing the velocity of an object around Earth toward where Voyager 1 should be, are: VX -48.7, VY 13.4, VZ 36.1.

**Tags:** proper-motion
> **Comments:**
> **username_1**: If JPL Horizons has it wrong, I honestly would not expect anyone else to have it correct.  
> **username_0**: It's probably myself who has it wrong, but I don't know how to make it right.  
> **username_2**: Please post the values you are getting. Then someone can explain why they are correct or what is wrong.  
> **username_3**: Yes, your question *is* confusing. ;) In the title, you ask about velocity vectors, but in the question body you seem to be talking about position vectors. Several months ago, [I said](https://astronomy.stackexchange.com/a/55927/16685): "The current speeds of Voyager 1 and 2 relative to the Sun are roughly 17 km/s and 15 km/s, respectively. Earth's orbital speed is around 30 km/s, so there are periods each year when the Voyagers have negative speed relative to Earth".  
> **username_0**: @PM2Ring Sorry, I meant to ask which speed, in km/s, V1 is travelling on the X, Y, and Z planes (VX, VY, VZ in Horizons). For full context, I'm doing this in Universe Sandbox, with a model wherein X is 0 hours (right ascension), Y is up (90 deg declination), and Z is 18 hours (RA). You make a good point with motion versus Earth. Maybe using the Sun as a reference is better? I'd still prefer its motion to be relative to Earth in J2K, but I can go with the ecliptic plane instead if necessary. I'll edit this into the question.  
> **username_0**: @JohnHoltz I will, thank you.  
> **username_3**: Yes, I think motion with respect to the Sun would be less confusing, since Earth's speed wrt to the Sun is so much greater than the Voyagers'. FWIW, here's a plot of Voyager 1 Astrometric RA & Dec in the ICRF frame (Horizons observer quantity #1), covering 1 year, with a 5 day timestep. https://i.sstatic.net/0kLs70JC.png  
> **username_3**: Can you post your Horizons settings (in a code block)? If you're using the app, there's a link towards the bottom of the page: *"If you plan to use one of the "batch" modes to access Horizons, the batch-file corresponding to the settings above can be viewed by using this link."*  
> **username_3**: Ok. I just noticed one source of confusion. The Universe Sandbox has its Z axis in the equatorial plane. But in ICRF & Horizons "the ICRF pole direction (+Z) and right ascension origin (+X), along with the orthogonal +Y direction."  
> **username_3**: As I mention in https://space.stackexchange.com/a/67041/38535 the Horizons trajectory data for the Voyagers only goes upto the year 2100. You can extrapolate beyond that date using the conservation of energy equation https://en.wikipedia.org/wiki/Specific_orbital_energy but they won't encounter bodies outside the Solar System for a *long* time, and to predict those encounters you need to have a vaguely realistic model of gravity and proper motions in the galactic neighbourhood.  
> **username_0**: @PM2Ring Here you go:  !$$SOF MAKE_EPHEM=YES COMMAND=-31 EPHEM_TYPE=VECTORS CENTER='500@10' START_TIME='2000-01-01 11:59:59' STOP_TIME='2000-01-01 12:00:00' STEP_SIZE='1 DAYS' VEC_TABLE='5' REF_SYSTEM='ICRF' REF_PLANE='ECLIPTIC' VEC_CORR='NONE' CAL_TYPE='G' OUT_UNITS='KM-S' VEC_LABELS='YES' VEC_DELTA_T='NO' CSV_FORMAT='NO' OBJ_DATA='NO'  
> **username_3**: Ok. That gives the velocity relative to the Sun, using the ecliptic as the XY plane. But the numbers don't match what you gave in the question. `$$SOE 2451544.999988426 = A.D. 2000-Jan-01 11:59:59.0000 TDB   VX=-2.157875824230245E+00 VY=-1.389610168160854E+01 VZ= 1.004801680496838E+01 $$EOE` And if I change the CENTER to Earth `500@399` those numbers don't match either...  
> **username_4**: @PM2Ring I get the same values as you, and these do indeed not match the values in the question. Pilot error?  

---

## 1 answers

### Answer by username_4 on 2024-10-20. Score: 2

Using the follow settings:

* Target body name: Voyager 1
* Center body name: Solar System Barycenter
* Reference frame : ICRF

I get the following from Horizons, in km/sec, at 2024 Oct-20 at 00:00 TDB:  
VX=-2.062 VY=-16.407 VZ= 3.610

If I switch the central body to the Sun, I get  
VX=-2.074 VY=-16.400 VZ= 3.614

This is almost the same as the barycentric results.

If on the other hand I switch the central body to the Earth, I get  
VX= 11.808 VY=-40.698 VZ=-6.918

This is not anywhere close to the same as the barycentric results (which is not surprising as most of the relative motion comes from the motion of the Earth relative to the Sun or the solar system barycenter), but also is not close to the same as the values in the question, even after switching coordinates to make Y rather than Z be up.

Finally, if I switch the reference frame from ICRF to ecliptic of J2000.0, and keep the central body to the Earth, I get  
VX= 11.1808 VY=-40.092 VZ= 9.841

Also note that

* The Z position of Voyager 1 is positive (strongly positive) in all of the queries. Voyager 1 is above the equator and above the ecliptic.
* If you want to determine whether an object is above or below the ecliptic you should be asking for ecliptic coordinates rather than Earth equatorial coordinates.

I suspect the problem is with two items:

* You did not specify the time of the query in the question; I used a time close to the time of my answer; or
* Universe Sandbox is goofy. That it uses Y as "up" is a big sign of that goofiness. That the plus Z axis is "up" is pretty much standard.

That JPL can still communicate with the Voyager satellites is a huge indicator that JPL's trajectories are close to spot on. The antennae JPL uses to capture telemetry from the Voyagers have a rather narrow beam width.

> **Comments:**
> **username_0**: Thank-you for the answer. I've been using 1 Jan 2000, 12:00:00 as the time for the results, since the model's based on that epoch.  

---

