Background & Introduction: On August 2, 2020 the brand new Where Is Roadster page Spacecraft launched to Mars in 2020 (announced here) said that it had been:

  • 13 days, 3 hours, 47 minutes and 13 seconds since the Emirates Mars Mission launched.
  • 9 days, 21 hours, 4 minutes and 27 seconds since Tianwen-1 launched.
  • 2 days, 13 hours, 55 minutes and 27 seconds since Mars 2020 launched.

The Solar System map extends out to pars of Ceres' orbit and at this scale the three spacecraft are on top of each other. Of course they also appear on top of Earth at the moment but that will change.

Question: Roughly how far apart from each other are they at the moment, and will they drift closer together or farther apart from each other during their interplanetary transfer between Earth's and Mars' orbit?

related: China, UAE and US all sending missions to Mars in 2020 (Summer of L̶o̶v̶e Mars); how far apart are their frequencies?

  • 1
    $\begingroup$ This is not an either-or question. The answer to your question is obviously yes. The spacecraft were rather close to one other (less than 13000 kilometers apart) prior to 19 July 2020, started diverging on that date with the launch of Mars Hope, and if all goes as planned, all three will once again be rather close to one another after February 2021. There's some point in time between launch and arrival where the distance between any two reaches a maximum. $\endgroup$ Commented Aug 2, 2020 at 10:06
  • $\begingroup$ @DavidHammen there is now way to completely pedantic-proof this question; my post has a timestamp and I quote information as of that time. My first sentence is written in the present tense; it is reasonable to assume "than they are now" and unreasonable to reach back in time to their pre-launch locales. Instead one could try to find their reference orbits or follow the procedure outlined at the end of the first posted answer and generate a reasonable prediction. $\endgroup$
    – uhoh
    Commented Aug 2, 2020 at 10:39
  • $\begingroup$ With the possibility of multiple edits to a post over a long period of time, I find that it's useful to add something like "as of August 2, 2020" in the body of the Q instead of "currently", "today", or the like, to avoid forcing someone into the edit history to be sure of what is meant. $\endgroup$ Commented Jan 6, 2021 at 2:40
  • $\begingroup$ @RussellBorogove that's a good point. I've adjusted the wording and changed to past tense to make it clearer that that is dated information, and that it only serves to motivate the question. In my answer I've used fried chicken designators to address this where just this kind of updating has in fact just happened. Thanks! $\endgroup$
    – uhoh
    Commented Jan 6, 2021 at 2:53

3 Answers 3


Only the mission operations team for each mission can answer that question.

During an interplanetary mission, spacecraft perform Trajectory Correction Maneuvers (TCMs). These are used to better target the B-Plane target (as explained in the documentation for FreeFlyer). The selection of the B-Plane depends on many factors, such as the thrust capability of the spacecraft, the attitude control authority, and more crucially the targeted injection orbit.

If a Mars mission adhered by the NASA Planetary Protection rules, then the spacecraft is not allowed to be on an orbit which intercepts Mars directly: the operations team must prove to NASA that the spacecraft is behaving well enough throughout the interplanetary leg.

Moreover, each mission design team has their reference trajectory which is built prior to launch. After launch, the navigation team will determine where exactly a spacecraft is for the mission design team to re-optimize the interplanetary trajectory: at that moment, the mission designers may decide whether to modify a planned TCM, or even remove it entirely.

Therefore, unless the mission designers publish this information, we cannot know whether EMM, Tianwen-1 and Mars 2020 will get closer or not for sure.

However, we can answer determine whether these spacecraft will get closer or not using their current orbit states if neither performs any TCMs. Another option would be to attempt to back-propagate their known Mars orbits and rebuild a trajectory to match their current states.

  • 1
    $\begingroup$ Surprisingly Tianwen-1 transmits its own state vectors! part 1 and part 2 $\endgroup$
    – uhoh
    Commented Jan 6, 2021 at 5:02
  • 1
    $\begingroup$ @uhoh WOW! That's an awesome set of blog posts, thanks for sharing. $\endgroup$
    – ChrisR
    Commented Jan 6, 2021 at 5:40

The hopelessness of my question expressed in this answer now seems "greatly exaggerated".

Only the mission operations team for each mission can answer that question.

Of course for detailed and reliable trajectories one will need TCM data, but to answer this question, I think we have enough now.

Will the Emirate Mars Mission, Tianwen-1 and Mars 2020 Mission get closer to each other or spread out on their way to Mars?

Answer: Yes! Both!

While EMM Hope and Tianwen-1 will continue to move further apart until late in their missions when they approach Earth, both of them will have a first minimum distance during their interplanetary travels before moving away from each other again.

All inter-spacecraft distances remain greater than about 1 million kilometers until they reach Mars.

JPL's Horizons now has nominal trajectories for all three spacecraft. These of course are preliminary trajectories as indicated by some of the spacecraft flying past Mars and continuing into deep space!

Below is a plot of 1) Earth/Mars distance, 2) Inter-Spacecraft distances, and 3) Planet-Spacecraft distances.

Original 18-Aug-2020

Python script that plotted Horizons data: https://pastebin.com/7evPAprS

Distances between the spacecraft on their way to Mars from Horizons on 18-Aug-2020

"Extra Crispy" 06-Jan-2021

Mars 2020 and EMM Hope calculated trajectories have been updated based on official tracking data as DSN is involved. Tianwen-1 calculated trajectories have been updated on an "as-is" bases based on tracking data from skilled but unofficial contributors.

  This trajectory is an UNOFFICIAL one based on optical and radio ground 
  observations collected by numerous volunteers. 

  It does not include FUTURE maneuvering events that will take place en-route 
  to Mars (no information on them has been made public), so will likely be 
  accurate only for days or weeks into the future past the "revision date" 
  noted above. 

  This as-is trajectory is being made available through Horizons by request, 
  to support volunteer ground-based radio tracking efforts described here:


  References for trajectory segment data:
   Jul23-Aug01: https://www.projectpluto.com/pluto/mpecs/tianwea.htm#o001
   Aug01-Sep20: https://destevez.net/2020/08/tianwen-1-tcm-1/
   Sep20-Oct09: https://destevez.net/2020/09/tianwen-1-tcm-2/
   Oct09-Oct28: https://destevez.net/2020/10/tianwen-1-dsm-final-trajectory/
   Oct28-Mar21: https://destevez.net/2020/11/tianwen-1-tcm3-final-trajectory/

  Trajectory (data from ground-based volunteers)       Start         End 
  -------------------------------------------------  -----------   -----------
  tianwen-1_p-mpec     (Bill Gray)                   2020-Jul-23   2020-Aug-01
  tianwen-1_post_tcm-1 (Amateur DSN)                 2020-Aug-01   2020-Sep-20
  tianwen-1_post_tcm-2 (Amateur DSN)                 2020-Sep-20   2021-Oct-09    
  tianwen-1_post_dsm   (Amateur DSN)                 2020-Oct-09   2021-Mar-21
  tianwen-1_post_tcm-3 (Amateur DSN)                 2020-Oct-28   2021-Mar-21

Distances between the spacecraft on their way to Mars from Horizons on 06-Jan-2021


Would not really make sense, because the were launched at different dates and will arrive at mars on different dates. That means the speed of each spacecraft is different. They drift away.

  • $\begingroup$ This seems to disagree with the answer I'd posted a few hours earlier. $\endgroup$
    – uhoh
    Commented Aug 18, 2020 at 8:12
  • 1
    $\begingroup$ -1 as a gentle reminder that this is unsupported and seems to disagrees with facts. Can you either develop your argument or consider deleting this or converting it to a comment? Thanks! $\endgroup$
    – uhoh
    Commented Mar 13, 2021 at 0:30

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