In two decades orbiting the Earth the International Space Station has become a cutting-edge cosmic laboratory, with astronauts researching everything from black holes to disease and even gardening in microgravity.

The ISS, which orbits about 250 miles above Earth, is as large as a football field inside and divided up like a beehive into spaces where the crew can carry out experiments with guidance from researchers on the ground.

Often, the astronauts are also the guinea pigs.

More than 3,000 scientific tests have been carried out at the ISS since its manned missions began in 2000.

"From a science perspective, there have been some major discoveries," said Robert Pearlman, space historian and co-author of "Space Stations: The Art, Science, and Reality of Working in Space".

Detect harmful radiation, pilot a rover module, learn better sleep and body maintenance: astronauts aboard the International Space Station are preparing for future missions even further afield—from the Moon to, one day, Mars.

The latest arrival of four more astronauts to the ISS, due to blast off aboard a SpaceX rocket from Florida on Thursday, will open the door for new experiments aimed at priming humans for long-distance space travel.

"We're trying out technologies for exploration," said Remi Canton, director of Cadmos, the division of France's National Centre for Space Studies (CNES) undertaking 12 new experiments.

Whether it is humans revisiting the Moon for the first time since 1972 or eventually travelling as far as the Red Planet, the challenges are overwhelming.

Firstly, how can engineers ensure that astronauts and their equipment are protected from the flow of particles thrown out by solar storms and cosmic rays?

On Jan. 9, 2020, NASA's Lucy mission officially announced that it would be visiting not seven, but eight asteroids. As it turns out, Eurybates, one of the asteroids along Lucy's path, has a small satellite.

Though searching for satellites is one of the mission's central goals, finding these tiny worlds before Lucy is launched gives the team the opportunity to investigate their orbits and plan for more detailed follow-up observations with the spacecraft.

Matter in the hearts of neutron stars—dense remnants of exploded massive stars—takes the most extreme form we can measure. Now, thanks to data from NASA's Neutron star Interior Composition Explorer (NICER), an X-ray telescope on the International Space Station, scientists have discovered that this mysterious matter is less squeezable than some physicists predicted.

The finding is based on NICER's observations of PSR J0740+6620 (J0740 for short), the most massive known neutron star, which lies over 3,600 light-years away in the northern constellation Camelopardalis. J0740 is in a binary star system with a white dwarf, the cooling remnant of a Sun-like star, and rotates 346 times per second. Previous observations place the neutron star's mass at about 2.1 times the Sun's.

"We're surrounded by normal matter, the stuff of our everyday experience, but there's much we don't know about how matter behaves, and how it is transformed, under extreme conditions," said Zaven Arzoumanian, the NICER science lead at NASA's Goddard Space Flight Center in Greenbelt, Maryland.