# Why did multiple nations decide to launch Mars projects at exactly the same time?

There seem to be multiple Mars projects all happening at around the same time:

• The Emirates Mars Mission was the first of the three to arrive at Mars on 9 February 2021.
• Tianwen-1, an interplanetary mission by the China National Space Administration (CNSA), entered orbit around Mars 10 February 2021.
• The American Mars 2020 mission landed the Perseverance rover on 18 February 2021.

Did everyone get together to decide to go to Mars at exactly the same time?

• Is your question about en.wikipedia.org/wiki/Launch_window or about the decision making in the last decade that had China, UAE and US all being in position to fly missions in 2020? Feb 20 at 7:36
• Most obviously because significant technological developments combined with a useful "launch window." What else might have been needed? Feb 21 at 2:16
• @RobbieGoodwin Martian permission. Feb 21 at 22:50
• I asked a question inspired by this one: Is there any coordination betwen the missions, officially or informally? Feb 23 at 10:40

Did everyone get together to decide to go to Mars at exactly the same time?

No. The Laws of Physics and the random fluctuations that created our solar system simply work out that way that every ~26 months Earth and Mars in just the right relative positions in their orbits that it costs the least propellant to reach Mars.

There is a single point in time where it is optimal to launch. The more you deviate from this point in time, either by launching earlier or by launching later, the more propellant you waste and the less useful payload mass to Mars you have.

In practical terms, it turns out that there is a window of a few weeks every 26 months where our most powerful, most expensive rockets have barely enough power to send a useful payload mass to Mars. (Don't forget: the Perseverance rover alone is the size of an SUV and weighs over a ton!) In fact, Mars 2020 was delayed by a couple of days, and they actually had to change the launch profile a bit to be able to make it.

• Quibble: The Atlas V isn't the most powerful rocket available. and useful payloads can be much less than the mass of Perserverance. Indeed, all previous missions were less. Feb 20 at 17:10
• @AtmosphericPrisonEscape Well, considering that the US (which I believe announced first) also had a launch (Curiosity) in the previous window, as well as at least a proposed mission for each of the next three, picking and unoccupied launch window was never an option.
– mlk
Feb 21 at 10:48
• @AtmosphericPrisonEscape Why would any space program wait 26 months if they had a spacecraft ready to go just for the sake of not "sharing" a launch window with another nation? And of course if they did that, someone else would announce a mission for the next launch window. Feb 22 at 10:05
• @AtmosphericPrisonEscape That seems a little like asking why there are other people on an island ferry with you - anyone who wants to go to the island has to get on a boat at certain times. The ferry is plenty big, and no one wants to wait for the next one just because someone else already has a ticket. Feb 22 at 14:46
• @NuclearHoagie: That is a perfect metaphor. They didn't "decide" to all launch at exactly the same time. They just decided they want to go to Mars and were all waiting at the bus stop for the only bus to Mars that only comes every 26 months. The fact they all ended up on the bus together is a consequence of the bus schedule, not a decision. Feb 22 at 16:26

Since the first and correct answer blames it on unnamed "laws of Physics", I'll try to name some names:

Nicolaus Copernicus, Isaac Newton, Johannes Kepler and the like give us basics for orbital mechanics and the nature of the solar system: Earth and Mars roughly orbit the Sun in roughly coplanar roughly circular orbits.

Walter Hohmann "was influenced in part by the German science fiction author Kurd Lasswitz and his 1897 book Two Planets" and gave us the Hohmann transfer orbit which for situations like Earth-to-Mars-like transfers is an elliptical orbit touching Earth's orbit at its perihelion (closest pass to the sun) and Mars' orbit at its aphelion (farthest pass from the Sun).

### But Earth and Mars have to be there at those times!

So timing is important.

For two periodic functions like the motion of two circular Kelperian oribts with periods $$T_<$$ and $$T_>$$ (indicates the shorter and longer periods) and assuming they orbit in the same direction, the period that they line up in some configuration with respect to each other but ignoring the stars is called the Synodic period

$$T_{syn} = \frac{1}{\frac{1}{T_<} - \frac{1}{T_>}} = \frac{T_< T_>}{T_> - T_<}$$

With $$T_>, T_<$$ of 1.881 and 1.0 years that's about 2.14 years. There's a short launch window and usually a predetermined arrival time.

note: Every synodic period the planets relationship to each other repeats, but if you were to plot their positions on paper, the Hohmann transfer points would be in different places, they'd move by 0.14 of a complete circle from one to the next.

• +1 already! But the way the parenthetical at the end of your third paragraph is written confuses me a bit; it makes it seem like you're defining perihelion and aphelion as nearest and closest passes, respectively. Maybe that should say "nearest and farthest"? Feb 20 at 16:17
• @BThompson ah, thanks for the edit!
– uhoh
Feb 20 at 23:56

The other answers have already addressed the logistics reasons for why three separate countries had such similarly scheduled missions, but that still leaves the question of why they all chose this transfer window. However, if we check Wikipedia's list of missions to Mars, it looks like there's been a mission or two most transfer windows since the early 90s. In fact, there were even another three missions from separate countries (USA, Russia, and China) as recently as 2011, though two of those failed. So it's probably coincidence, and the increasing level of global technology that lead to this conjunction.

• Cool! I wish I'd asked "Has there ever been three missions to Mars in one transfer window before 2020? Ever four?"
– uhoh
Feb 21 at 0:00

What they said, and an increase in a number of factors:

• Technology
• Better sensors.
• Better computers.
• Better software (AI/ML).
• Nationalism & economics
• National pride is a factor for countries with the resources.
• Local economic development is driven by talent.
• Fun projects retain/attract talent.
• Development of civil and military capacity.
• International cooperation
• At least some of the newest sensors augment those sent by other nations.
• Multi-national funding allows more probes.
• Getting the funding requires spreading the development around a bit.

The Hope, Emirates Mars Mission is a good example that encompasses all of the above points, that also goes beyond nationalism and economics, with it's Arabic name al-Amal, intended to send a "message of optimism to millions of young Arabs".

The projects don't have the same time scales. They didn't all start in the same year, and the small launch windows, spaced at just over two years apart, all combines to make the odds higher, that in any given window, there will likely be one or more probes heading towards mars. In my life-time, most of those windows were not used. Barring a world war or other world economy crippling disaster, over the next few decades, the rate will likely increase.

• "...at exactly the same time?" refers to the fact that they were all launched within eleven day window: Emirates Mars Mission 19 July 2020, Tianwen-1 23 July 2020, Perseverance 30 July 2020
– uhoh
Feb 21 at 2:48
• @uhoh, ya, it was more of a "and these other things also contributed" kind of answer. The rise of nationalism in particular, is going to motivate more "me too" missions to Mars. China's neighbors, probably don't want to be spectators, and China has an axe to grind wrt the US and Russia, so the next few decades should be interesting. Feb 21 at 5:03
• I think that space offers plenty of fascination and opportunity for any country capable of it to get involved in both local spaceflight and deep space missions. No need for any axes to understand this.
– uhoh
Feb 21 at 9:24
• And yet, some of the most aggressive progress was made by two super-powers during the cold-war. Ya, space is fascinating to most of us, but few nations could do much about it, until recently. Feb 21 at 19:43
• -1. This is an answer to "why are there so many Mars missions this year", not "why did the missions this year all launch in the same 11 day window". Feb 22 at 19:36