Thomas Zurbuchen, Nasa Associate Administrator for the Science Mission Directorate, says there are two tricky parts of Perseverance's journey.

The first is heading in the right direction from Earth to Mars.

"In February, when Mars comes, you wanna be right there," he says. "The second one that will make us nervous in February is entry, descent and landing."

Mars' atmosphere makes things difficult - and you can't just parachute in like on Earth.

He says Perseverance's mission is also unique because it will be humanity's first roundtrip to another planet - and will let scientists in the best labs around the world analyse Mars samples.

"For the first time in decades, [it's] the first astrobiology mission and we're ready for it," Zurbuchen says.

"It's an amazing first in that respect."

Perseverance will be packaging its most interesting rock and soil samples in small containers for later retrieval by missions that will leave Earth in 2026.

Image source, NASA

It took five years for scientists to determine a landing site, and they finally settled on Jezero Crater.

Project scientist Katie Stack Morgan says they believe the rover will be able to find a diverse set of samples at the crater, because "we know without a doubt there was an ancient lake and river delta" at Jezero.

"We know it filled up with water and overflowed. Life as we know it requires water to survive and thrive and we think Jezero has all the building blocks to support life."

The site also has one of the best-preserved river deltas on Mars - and some of the oldest rocks, between three-and-a-half and four billion years old.

Morgan notes that's around the same time as life was developing on Earth, which makes it a promising site to explore.

Image source, NASA

Astronaut Tim Peake sends his good wishes - and a map for the trip to Mars.

Bobby Braun, director for planetary science at Nasa's Jet Propulsion Laboratory (JPL), says: "It's a great day; we're all so excited to get started in this way and to be on our way after all this work."

But the journey's not fully under way just yet.

"We're almost on the path to Mars, if you will...We still have to have another burn of our upper-stage, we have to pass through the night side - or the shadow of the earth - come out on the other side, and find the Sun."

Once the spacecraft powers up, the team will be able to establish contact.

"Once we do, we'll truly be on our way to Mars. We'll have a spacecraft that is powersafe and we can communicate with - and our journey will truly begin."

Here's the latest tweet from Perseverance.

In case you missed the moment - or just want to rewatch it - Nasa's got you covered.

And, if you want to keep following Perseverance's journey through 2021, the rover has its own Twitter account (and personality - with hobbies listed as photography, collecting rocks, off-roading).

We're now moving into the first of two burns for this mission. The first is seven minutes in length.

The rocket for today's mission is an Atlas V 541 that features a main core stage, a Centaur upper-stage and four solid rocket motors.

The Centaur upper-stage will be involved in these burns, before Perseverance comes off the top.

Image source, Nasa

Engine response looks good, Nasa says, as the rocket is now supersonic.

We're just over a minute into the flight.

We have liftoff!

The rover Perseverance has begun its journey to Mars from Cape Canaveral, Florida.

The Atlas 5 rocket carrying the rover will now head east across the Atlantic for approximately an hour.

Nasa administrator Jim Bridenstine says that throughout this process, Covid-19 measures were in place to make sure people felt safe working on the project.

"Our highest priority was the safety of our people and we wanted them to know if they come to work, they'll be safer at work than they would be at home," he says.

Using personal protective equipment, social distancing and scheduling changes to minimise how many people were working on the vehicle at one time, were all precautions introduced by Nasa due to the pandemic.

"This is all about perseverance," Bridenstine says. "Going to Mars is all about persevering in general. Doing it now is more persevering than before."

The rover has a Covid-19 "perseverance" plate affixed to it as well, with a logo featuring the symbol of medicine.

Image source, NASA

Jim Bridenstine, Nasa administrator, says he's always nervous before a launch, but: "It's gonna be a good day for Nasa."

When asked about why the agency felt it was important to continue with this launch amid a global pandemic, Bridenstine says the US and its international partners have a history of "doing amazing things in the most challenging times and this is no different".

He says the path to Mars opens up only once every 26 months, when quite literally, the planets are aligned.

"If we miss this launch window, it would cost us half a billion dollars to store this vehicle for the next two years. There are a lot of reasons to go forward."

Down on the launch pad, weather conditions are still all clear for launch.

Image source, NASA

All systems are looking good so far, Nasa says.

With 28 minutes to go, a weather update says there are a few clouds to the east, and a small cell nearby has dissipated: So far, still clear to launch.

Image source, Nasa

The name Perseverance came from Alexander Mather, a 13-year-old student from Virginia who won Nasa's essay contest to name the rover.

"We are a species of explorers, and we will meet many setbacks on the way to Mars," Alexander - who hopes to work at Nasa one day - wrote.

"However, we can persevere. We, not as a nation but as humans, will not give up."

The UAE and China have both sent missions to Mars in recent weeks - in addition to Nasa's planned launch today.

Summer 2020 has been a popular time for missions to the Red Planet because of the way the planets align.

Laura Foster explains why all these missions are going at the same time.

Image source, ESA and MPS for Osiris

Jennifer Trosper, Perserverence deputy project manager, explains that Perseverance follows generations of rovers that have, over the decades, added to our knowledge about the red planet.

From Soujourner in 1997, which drove just 400ft on the planet, to Curiosity in 2012, which boasted a robotic arm and science lab, these rovers have helped scientists continue to understand Mars' ancient past.

"Understanding what happened to Mars will help us understand more about our solar system and also about Earth," Trosper says.

On a beautiful morning in Florida, the Perseverance Rover sits on top of an Atlas 5 rocket ready to be blasted into space.

Image source, NASA

The Atlas 5 rocket that will lift the Mars rover into space is being fuelled, with just under an hour to go before launch.

Image source, NASA

Paul Rincon
Science editor, BBC News website

Image source, NASA

Before the era of robotic spacecraft, scientists and authors alike speculated that Mars might be home to intelligent life with advanced civilisations.

But when the American Mariner 4 spacecraft flew by Mars in 1965, it returned pictures of a dry, cratered, seemingly barren world.

It was becoming clear that any life on the Red Planet must be a godawful small affair, as David Bowie once sang.

Thus, when Nasa launched its two Viking landers to Mars in 1975, the spacecraft carried experiments designed to detect signs of microbes in the Martian soil. The results from one of these tests, known as the labelled release (LR) experiment, remains a source of controversy to this day.

A sample of Martian soil was picked up and placed in a small tube to which a squirt of radioactive carbon was added. Were any Martian micro-organisms present, the hope was that they would recognise the nutrient, "eat" it and give off radioactive gas.

"No-one, including me, was very hopeful this would work on Mars," the experiment's chief scientist Gil Levin said in a 2014 interview with SPIE TV. , external

"Surprisingly, as soon as the first squirt of radioactive nutrient hit that Martian soil, we got a big bulge of gas coming out. And it came out for the entire seven days, or Martian sols, of the experiment. That was astounding."

Levin and colleagues also ran a "control", to exclude the possibility that chemicals in the Martian soil could react to produce the gas. The results again pointed to microbial production, according to Levin.

But another experiment on Viking had found no native organic matter in the Martian soil.

How could life exist without organic matter? This discrepancy led to the labelled release experiment being judged inconclusive. But a few advocates, such as Dr Levin, continue to believe in the results.

Not since its Viking landers in the 1970s has Nasa looked directly for life on Mars. Its recent missions have tried merely to describe past environments. But Perseverance, or Mars 2020 as it was called in development, has the tools to detect what researchers refer to as "biosignatures".

What are these and how would Perseverance recognise them? Katie Stack Morgan is a deputy project scientist on the mission. She spoke with our science correspondent Jonathan Amos.

The rocket for today's mission is a United Launch Alliance (ULA) Atlas V 541. It's a beast of a vehicle that incorporates a main core stage, a Centaur upper-stage and four solid rocket motors.

At launch, this rocket's engines will be producing something close to 9,000 kilonewtons, or 2 million pounds, of thrust. It should leap off the pad.

The Atlas will head east out of Cape Canaveral and across the Atlantic on what we expect to be about an hour's ride for Perseverance.

The rocket will go through its usual staging procedures which will see it dump the exhausted solid rocket motors after one minute and 49 seconds, and the core stage after four and a half minutes.

The Centaur upper-stage then conducts a couple of burns, before Perseverance comes off the top.

By then, the rover, tucked away inside its capsule and attached to its shepherding spacecraft, should be moving at 41,000km/h (nearly 26,000mph) on a trajectory to intercept Mars on Thursday 18 February next year.

If you've got a calendar to hand, you might want to jot this down. Landing is expected around 20:00 GMT. Where will you be?

Image source, ULA