SpaceX counted down Wednesday toward a nighttime launch of four astronauts who have been grounded for nearly two weeks by weather and medical delays.

The Falcon rocket was poised to blast off from NASA's Kennedy Space Center a few hours after sunset. That would put the one German and three U.S. astronauts at the International Space Station by Thursday night to begin a six-month stay.

On Dec. 18, NASA is set to launch its next flagship mission into space. The spacecraft, called the James Webb Space Telescope, brings a lot of risks: Its roughly 270-square-foot mirror, which will collect light streaming in from the far reaches of space, will launch folded up inside a rocket, then unfurl far from Earth.

Astronomers are betting that the challenges will be worth it. Many have dubbed James Webb a successor to the Hubble Space Telescope—an instrument that was launched in 1990 and has delivered startling images of the cosmos, including The Pillars of Creation in the Eagle Nebula. Among other goals, James Webb will search the atmospheres of alien planets for what scientists call "biosignatures," or gases that could, theoretically, indicate the presence of life on the surface.

After a series of delays, SpaceX is set to launch four astronauts to the International Space Station on Wednesday night on the "Crew-3" mission.

The orbital outpost is currently operating with just one NASA astronaut in the US segment to welcome the incoming crew, after the astronauts of the earlier Crew-2 mission splashed down in the Gulf of Mexico on Sunday night.

Crew-3's Raja Chari, Kayla Barron and Tom Marshburn of the United States and Matthias Maurer of Germany are now set to launch aboard a Crew Dragon capsule fixed to a Falcon 9 rocket at 9:03 pm local time (0203 GMT Thursday) from the Kennedy Space Center in Florida.

An international team of space researchers has learned more about the density of the Martian surface by analyzing data from the Mars InSight lander that was received during Perseverance's descent. In their paper published in the journal Nature Astronomy, the group describes their study of seismic data from InSight as Perseverance dropped heavy blocks during its descent.

One of the ways that planetary scientists learn more about the makeup of other planets is by studying seismic activity—waves from such activity can provide clues to the density of different parts of a planet. In this new effort, the researchers noted that gathering seismic data from extraterrestrial events such as asteroids striking the surface of a planet is difficult as they are so random. But they also noted that the Perseverance mission offered a unique opportunity—as part of its descent earlier this year, the rover's landing craft dropped two tungsten blocks—each weighing approximately 77.5kg to the surface below.

Short for Venus Mass Spectrometer, VMS is one of five instruments aboard the DAVINCI descent probe.