There are many things humanity must overcome before any return journey to Mars is launched.

The two major players are NASA and SpaceX, which work together intimately on missions to the International Space Station but have competing ideas of what a crewed Mars mission would look like.

Size matters

The biggest challenge (or constraint) is the mass of the payload (spacecraft, people, fuel, supplies etc) needed to make the journey.

We still talk about launching something into space being like launching its weight in gold.

The payload mass is usually just a small percentage of the total mass of the launch vehicle.

For example, the Saturn V rocket that launched Apollo 11 to the Moon weighed 3,000 tons.

But it could launch only 140 tons (5% of its initial launch mass) to low Earth orbit, and 50 tons (less than 2% of its initial launch mass) to the Moon.

Mass constrains the size of a Mars spacecraft and what it can do in space.

The solar system's two biggest planets, Jupiter and Saturn, came within planetary kissing range in Monday's evening sky, an intimacy that will not occur again until 2080.

This "great conjunction", as it is known to astronomers, occurred fortuitously on the winter solstice for those in the Northern Hemisphere and the beginning of summer in the global south.

The two planets were, in fact, more than 730 million kilometres (400 million miles) apart. But because of their alignment in relation to Earth, they appeared to be closer to each other than at any time in almost 400 years.

Optimal "conjunction" took place at 1822 GMT.

The best viewing conditions on Monday were in clear skies and close to the Equator, while people in Western Europe and along a vast swathe of Africa had to train their sight to the southwest.

But hundreds of space fans also gathered in Kolkata to watch—through a telescope at a technology museum in the city, or from surrounding rooftops and open areas.

A Southwest Research Institute-led team of scientists has identified a potentially new meteorite parent asteroid by studying a small shard of a meteorite that arrived on Earth a dozen years ago. The composition of a piece of the meteorite Almahata Sitta (AhS) indicates that its parent body was an asteroid roughly the size of Ceres, the largest object in the main asteroid belt, and formed in the presence of water under intermediate temperatures and pressures.

Another great photo opportunity occurs Monday after sundown: the Winter Solstice and the sighting of the "Christmas Star."

You can use a smartphone to capture what is promised to be the closest visible conjunction of Jupiter and Saturn in 800 years. The planets are expected to appear as one large star, lighting up the sky.

"Shooting a night sky is one of the most amazing things you can witness," says travel photographer Austin Mann.

You don't need a fancy DSLR or mirrorless camera to capture the light show. Mann and other photographers say you can get a great shot on a smartphone.

The 2020 and 2019 editions of the iPhone (11 and 12 series) offer "Night Mode," for making dark shots more possible for smartphone photographers, while the Google Pixel introduced "Night Sight," in the Pixel 4 and 5 series.

Samsung doesn't have an official name for night sky photos, but says it too can do awesome night photos on recent Galaxy S edition phones, and shows off examples on its website.

Mann recommends going to your location shortly after sunset, which is generally between 4:20 and 4:45 in New York, Chicago and Los Angeles.