<p>Answer posted under the wire during closure. Others may be able to write an additional and/or different answer as soon as two more reopen votes are cast.</p>
<blockquote>
<p>Would it have been possible to send a radio signal towards ʻOumuamua?</p>
</blockquote>
<p>Yes. It is certainly true that at any time, a large radio telescope or even a small transmitter can always sent a signal <em>toward</em> something. <a href="https://en.wikipedia.org/wiki/New_Horizons" rel="nofollow noreferrer">New Horizons</a> can receive our signals now even though it's in the Kuiper belt because it knows when and where to listen and at what frequency.</p>
<blockquote>
<p>As we could not get any radio signals from asteroid <a href="https://en.wikipedia.org/wiki/%CA%BBOumuamua" rel="nofollow noreferrer">ʻOumuamua</a>, couldn't we have sent a powerful radio signal to it and then check if we can get any radio signal in response?</p>
</blockquote>
<p>The asteroid was quite close to us in the 2nd half of 2017, but now it's much farther than New Horizons. However if it had advanced technology it would know we were very active in the electromagnetic radio spectrum and it might be listening. If so, we certainly could have sent it a signal that it could receive.</p>
<p>Checking for a response is also hard because we'd need to know what frequency and when. We'd have to allocate resources, etc.</p>
<p>For the frequency, see also</p>
<ul>
<li><a href="https://scifi.stackexchange.com/q/197905/51174">https://scifi.stackexchange.com/q/197905/51174</a></li>
<li><a href="https://scifi.stackexchange.com/q/16348/51174">https://scifi.stackexchange.com/q/16348/51174</a></li>
</ul>
<hr />
<p><strong>Below:</strong> Plots of the distance of ʻOumuamua (bumpy, blue) and the New Horizons spacecraft (straight, green) from Earth as a function of years since 2017-01-01.</p>
<p><a href="https://i.sstatic.net/nxadL.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/nxadL.png" alt="distance from earth after 2017-01-01 for New Horizions (straight green) and ʻOumuamua (bumpy blue)" /></a></p>
<p><a href="https://i.sstatic.net/QvjWS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QvjWS.png" alt="JPL Horizons" /></a></p>
<p><a href="https://i.sstatic.net/QEuIm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QEuIm.png" alt="JPL Horizons" /></a></p>

CC BY-SA 4.0
2020-09-29T04:59:22.717Z
It takes some time for closures to propagate through the system. There may even be a grace period to allow in-progress answers to be posted.
Mick
CC BY-SA 4.0
2020-09-29T05:37:17.970Z
@uhoh Thanks for the answer. I do agree with "Checking for a response is also hard because we'd need to know what frequency and when".
Mahen
CC BY-SA 4.0
2020-09-29T06:29:10.927Z
@Mahen for questions about radio communication and radar measurements within the solar system, Space Exploration SE might be a better site. They can be asked in both places perhaps, but maybe Space is better.
uhoh
CC BY-SA 4.0
2020-09-29T06:30:04.297Z
@Mahen *However* if you were to post a follow-up question here about all the kinds of measurements that contributed to our understanding of ʻOumuamua's distance and speed (visible, infrared, radar, etc.) that would be an excellent question in either site, and maybe best here in Astronomy since all of those are astronomical observations. You can ask how quickly each one gets weaker as it moves farther away, which is a really interesting question I think.
uhoh
CC BY-SA 4.0
2020-09-29T06:33:50.317Z
@Mahen have a look at [Why is Saturn invisible in this radar image of its rings?](https://astronomy.stackexchange.com/q/33060/7982) and [What is the farthest object we've been able to bounce signals off of to date?](https://astronomy.stackexchange.com/q/4749/7982) and [How did Arecibo detect methane lakes on Titan, and image Saturn's rings?](https://astronomy.stackexchange.com/q/33023/7982) and [What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence?](https://astronomy.stackexchange.com/q/22480/7982) for example
uhoh
CC BY-SA 4.0
2020-09-29T06:49:24.510Z
@uhoh Sure uhoh. Thanks for the interesting links.
Mahen
CC BY-SA 4.0
2020-09-29T12:44:13.180Z
@uhoh Mick is correct. There is a grace period for four hours after the question is closed where it still accepts answers that were started prior to closure: https://meta.stackexchange.com/a/316139/228367
called2voyage
CC BY-SA 4.0
2020-09-29T04:27:59.837Z
2020-09-29T08:24:52.630Z
2020-09-29T08:24:52.630Z
uhoh
<p>As we could not get any radio signals from asteroid <a href="https://en.wikipedia.org/wiki/%CA%BBOumuamua" rel="nofollow noreferrer">ʻOumuamua</a>, couldn't we have sent a powerful radio signal to it and then check if we can get any radio signal in response?</p>
<p>This way we can at least check if it is just a rock or any alien spaceship. If it were an alien spacecraft, then they should definitely give a radio signal in response I believe.</p>

CC BY-SA 4.0
2020-09-28T12:29:35.110Z
what do you mean by "fast"? All radio signals travel the same speed
Rory Alsop
CC BY-SA 4.0
2020-09-28T12:43:18.643Z
If I were an alien and could decode signals from Earth in 2017, I'd keep radio silence and try to look like an asteroid.
Mike G
CC BY-SA 4.0
2020-09-28T15:49:27.107Z
@Mahen are you sure that radar signals have been intentionally sent to it? Can you add a link or reference for that? The distance was always quite far and the size relatively small, it's hard to imagine that there could have been a detectable reflection.
uhoh
CC BY-SA 4.0
2020-09-28T18:17:46.023Z
@uhoh scientists have given its location and speed, and I believe Radar system is the only possible way to find such details for a very far and fast moving object. That way I presume along with Mick's info that Radar system was used. I am not very sure if there is any other way to locate such a far object.
Mahen
CC BY-SA 4.0
2020-09-29T00:43:59.377Z
@Mick are you sure that radar signals have been intentionally sent to it? Can you add a link or reference for that? The distance was always quite far and the size relatively small, it's hard to imagine that there could have been a detectable reflection.
uhoh
CC BY-SA 4.0
2020-09-29T03:31:47.460Z
@uhoh I read that the Canadian Meteor Orbit Radar was involved, but apparently that was used to search for meteor showers from Oumuamua's radiant.
Mick
<radio-astronomy><extra-terrestrial><trojan-asteroids><fast-radio-bursts>
Would it have been possible to send a radio signal towards ʻOumuamua?
Mahen
# Would it have been possible to send a radio signal towards ʻOumuamua?

(Asked by: username_0 on 2020-09-28)

As we could not get any radio signals from asteroid [ʻOumuamua](https://en.wikipedia.org/wiki/%CA%BBOumuamua), couldn't we have sent a powerful radio signal to it and then check if we can get any radio signal in response?

This way we can at least check if it is just a rock or any alien spaceship. If it were an alien spacecraft, then they should definitely give a radio signal in response I believe.

**Tags:** radio-astronomy, extra-terrestrial, trojan-asteroids, fast-radio-bursts
> **Comments:**
> **username_1**: what do you mean by "fast"? All radio signals travel the same speed  
> **username_2**: If I were an alien and could decode signals from Earth in 2017, I'd keep radio silence and try to look like an asteroid.  
> **username_3**: @Mahen are you sure that radar signals have been intentionally sent to it? Can you add a link or reference for that? The distance was always quite far and the size relatively small, it's hard to imagine that there could have been a detectable reflection.  
> **username_0**: @uhoh scientists have given its location and speed, and I believe Radar system is the only possible way to find such details for a very far and fast moving object. That way I presume along with Mick's info that Radar system was used. I am not very sure if there is any other way to locate such a far object.  
> **username_3**: @Mick are you sure that radar signals have been intentionally sent to it? Can you add a link or reference for that? The distance was always quite far and the size relatively small, it's hard to imagine that there could have been a detectable reflection.  
> **username_4**: @uhoh I read that the Canadian Meteor Orbit Radar was involved, but apparently that was used to search for meteor showers from Oumuamua's radiant.  

---

## 1 answers

### Answer by username_3 on 2020-09-29. Score: 2 | Accepted answer

Answer posted under the wire during closure. Others may be able to write an additional and/or different answer as soon as two more reopen votes are cast.

> Would it have been possible to send a radio signal towards ʻOumuamua?

Yes. It is certainly true that at any time, a large radio telescope or even a small transmitter can always sent a signal *toward* something. [New Horizons](https://en.wikipedia.org/wiki/New_Horizons) can receive our signals now even though it's in the Kuiper belt because it knows when and where to listen and at what frequency.

> As we could not get any radio signals from asteroid [ʻOumuamua](https://en.wikipedia.org/wiki/%CA%BBOumuamua), couldn't we have sent a powerful radio signal to it and then check if we can get any radio signal in response?

The asteroid was quite close to us in the 2nd half of 2017, but now it's much farther than New Horizons. However if it had advanced technology it would know we were very active in the electromagnetic radio spectrum and it might be listening. If so, we certainly could have sent it a signal that it could receive.

Checking for a response is also hard because we'd need to know what frequency and when. We'd have to allocate resources, etc.

For the frequency, see also

* <https://scifi.stackexchange.com/q/197905/51174>
* <https://scifi.stackexchange.com/q/16348/51174>

---

**Below:** Plots of the distance of ʻOumuamua (bumpy, blue) and the New Horizons spacecraft (straight, green) from Earth as a function of years since 2017-01-01.

[![distance from earth after 2017-01-01 for New Horizions (straight green) and ʻOumuamua (bumpy blue)](https://i.sstatic.net/nxadL.png)](https://i.sstatic.net/nxadL.png)

[![JPL Horizons](https://i.sstatic.net/QvjWS.png)](https://i.sstatic.net/QvjWS.png)

[![JPL Horizons](https://i.sstatic.net/QEuIm.png)](https://i.sstatic.net/QEuIm.png)

> **Comments:**
> **username_4**: It takes some time for closures to propagate through the system. There may even be a grace period to allow in-progress answers to be posted.  
> **username_0**: @uhoh Thanks for the answer. I do agree with "Checking for a response is also hard because we'd need to know what frequency and when".  
> **username_3**: @Mahen for questions about radio communication and radar measurements within the solar system, Space Exploration SE might be a better site. They can be asked in both places perhaps, but maybe Space is better.  
> **username_3**: @Mahen *However* if you were to post a follow-up question here about all the kinds of measurements that contributed to our understanding of ʻOumuamua's distance and speed (visible, infrared, radar, etc.) that would be an excellent question in either site, and maybe best here in Astronomy since all of those are astronomical observations. You can ask how quickly each one gets weaker as it moves farther away, which is a really interesting question I think.  
> **username_3**: @Mahen have a look at [Why is Saturn invisible in this radar image of its rings?](https://astronomy.stackexchange.com/q/33060/7982) and [What is the farthest object we've been able to bounce signals off of to date?](https://astronomy.stackexchange.com/q/4749/7982) and [How did Arecibo detect methane lakes on Titan, and image Saturn's rings?](https://astronomy.stackexchange.com/q/33023/7982) and [What is the physical geometry of this apparent “eclipse” of a tiny moon of Asteroid Florence?](https://astronomy.stackexchange.com/q/22480/7982) for example  
> **username_0**: @uhoh Sure uhoh. Thanks for the interesting links.  
> **username_5**: @uhoh Mick is correct. There is a grace period for four hours after the question is closed where it still accepts answers that were started prior to closure: https://meta.stackexchange.com/a/316139/228367  

---

