Mars and the Earth are nestling closer than they have for 15 years.
The phenomenon – known as a Mars approach – should allow people to see the Red Planet with the naked eye.
For Nasa, it is also a perfect opportunity to take advantage of the relatively short distance and send off missions to explore Mars. That is why Mars missions tend to launch roughly every two years – since that is how often the close approaches happen.
This time around, the agency launched its InSight mission, a robotic lander that will arrive on the planet later this year.
But people back down on Earth will get a good look at the planet, too. The close approach is coming close enough for exceptional viewing – allowing people to see details of the planet through a telescope, or even allowing it to be spotted with the naked eye.
It is worth noting, however, that Mars won't be quite as big as some people are claiming. Every time there is a close approach, a hoax spreads that the planet will be as big as the moon.
But that never happens. We would be in considerable trouble if it did, given that the gravitational pulls between Mars, the Earth and our Moon would be so strong.
In fact, it never gets closer than about 33.9 million miles away.
The phenomenon happens because neither Earth nor Mars have perfectly round orbits. Instead, they move round the Sun in an egg shape, and that orbit can sometimes be thrown out a little by the larger planets in our solar system.
Sometimes those egg shapes leave the planets further away, and at other times they will get closer. The current close approach is the consequence of that movement, bringing the two worlds relatively close together.
"All of these factors mean that not all close encounters are equal," notes Nasa. "In 2003, Mars made its closest approach to Earth in nearly 60,000 years! It won't be that close again until the year 2287."
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