
As I noted in my previous Space Sunday update, Mars is having one of its busiest period in the 50 years we have been sending probes to either orbit or land on that world, with no fewer than three new robotic missions either now in orbit or about to arrive.
The reason for this rapid-fire arrival is simple: Mars and Earth both orbit the Sun, but Earth, as the nearer of the two, completes a single orbit once every 365.25 days whilst Mars does the same once every 687 days. This means that Every so often, Earth “overtakes” Mars as they circle the Sun.
These periods of “overtaking” occur once every 26 terrestrial months, and are – slightly confusingly – called periods of “opposition”, so-called because Mars and the Sun appear to be on “opposite” sides of the Earth relative to one another in their orbits. However, where space missions are concerned, it’s not the point at which Earth “overtakes” Mars that is important, but the period of a couple of weeks beforehand, when Earth is in the final stages of “catching up”.
It is at this point that a mission to Mars can be most effectively launched. This is for a number of reasons: firstly, it marks the time when Earth and Mars are relatively close to one another in their respective orbits – perhaps as close as 50-60 million km when measured in a straight line. While spacecraft do not travel in a straight line between planets, it does mean the distance they do have to traverse is reduced to a few hundred million kilometres. Secondly, launching while Earth is still “catching up” with Mars means a spacecraft receives an added “boost”. Thirdly, it ensures the vehicle can enter a Hohmann Transfer orbit between the two planets.

Named for German engineer Walter Hohmann, who first calculated it in 1925, the Hohmann Transfer Orbit is the most fuel-efficient means for a spacecraft to move between the orbits of two different planets, further reducing the complexity of the journey by reducing the number of mid-course corrections that might otherwise be required. When taken as a whole, these three points mean that a mission to Mars can be launched with the minimum amount of time it needs to reach its destination and in a manner that maximises fuel efficiency.
Because the orbit of Mars is more elliptical than Earth’s, the actual time it takes to travel between the two during these periods can vary between six and seven months., with the distance this time meaning that the three missions launched in July 2020 have taken almost seven moths to reach Mars. They form an international flotilla, as I noted in my previous Space Sunday update, being from the United Arab Emirates by way of Japan, China and the United States.
All three are highly ambitious in nature, again as I noted last time around. The UAE’s Hope mission, the first to arrive, marks both the country’s first attempt to reach Mars and its very first interplanetary mission as a whole – no mean achievement for a country that has only recently committed itself to the goal of long-term space exploration and science.

The mission itself has been put together and is being run by a team of around 150 and at a cost of just US $200 million – which, as the saying goes, is just peanuts for space [missions]. It utilised a Japanese H-IIA launch vehicle to reach Mars, and in the face of understandable nervousness within the Hope mission team, the roughly cubic vehicle with a mass of around 1.4 tonnes, lipped into its initially orbit around Mars on Tuesday, February 9th following a 27-minute continuous burn of the vehicles main thrusters, a manoeuvre that used around half the craft’s available fuel load.
As it did so, the UAE staged a national celebration, with images of the Martian moons of Phobos and Deimos being projected into the night sky over the desert, while the skyline of Dubai saw buildings lit up with the mission name and images of the planet.

The aim of the mission is to further understand the Martian weather, atmosphere and climate, and to specifically close existing gaps in our knowledge of all three. It occupies what is called a high supersynchronous orbit, circling the planet once every 55 hours at a distance of between 20,000 km (periapse) and an apopapse of 43,000 km, altitudes that allow it to observe daily cycles across the entire visible hemisphere of the planet and witness season changes as they affect both the northern and southern hemispheres.
Continue reading “Space Sunday: orbits, landings, launches and a portrait”























