Pluto probe New Horizons 'has phoned home'

Nasa's New Horizons spacecraft reached Pluto yesterday after crossing the solar system for nine and a half years. On its approach to Pluto, the spacecraft obtained the most arresting image yet of the dwarf planet. Join us for live updates and more pictures of this historic mission. All times NZ local.

3:00pm: Scientists are now expecting to receive the latest images from New Horizon in the next few hours.

Earlier today Randy Wesson, from Nasa's jet propulsion lab, said this was the first time we had had an up-close encounter with Pluto.

"We're finding all sorts of interesting features - a few craters, which tells us it's a relatively young surface. We have a mysterious ... icy region near the equator," Dr Wesson told Radio New Zealand.

"We're going to write a book about Pluto when before it was just a smudge and the best we had was from the Hubble Telescope."

New Horizons had now flown past Pluto and was looking back at the solar system.

Because of its 200-watt power, it would take months to transmit all its data back to Earth, although "big, juicy" information would be available soon.

2:34pm:

You ain't seen nothing yet, says Nasa

Nasa has just posted this video to instagram:

2:30pm: Locally, stargazers are lapping up the extraordinary images of Pluto beamed back to Earth.

2:23pm: "We have a healthy spacecraft". It appears New Horizons has got the green light from mission control.

"Just like we practiced. We did it."

2:00pm:

US President Barack Obama has just tweeted about the mission.

Pluto just had its first visitor! Thanks @NASA - it's a great day for discovery and American leadership. pic.twitter.com/FfztBSMbK0

— President Obama (@POTUS44) July 15, 2015

1.50pm Nasa administrator Charles Bolden sums up the team's achievement:

"You have inspired the next generation ... You have made Pluto almost human. We are going to be further mesmerised by this planet."

1.46pm: Mission Control should be renamed Whoop Central.

They just can't stop cheering in mission control.

It's a celebration! Watch @NASANewHorizons #PlutoFlyby success briefing live now: http://t.co/KX5g7zfYQe Q? #askNASA pic.twitter.com/qQSkuToskj

— NASA (@NASA) July 15, 2015

1.35pm: AP has filed this take on the mission:

Nasa's New Horizons spacecraft got humanity's first up-close look at Pluto on Tuesday, sending word of its triumph across 3 billion miles to scientists waiting breathlessly back home.

Confirmation of mission success came 13 hours after the actual flyby and, after a day of both jubilation and tension, allowed the New Horizons team to finally celebrate in full force.

Early indications had been encouraging, and a cheering, flag-waving celebration swept over the mission operations center in Maryland at the time of closest approach Tuesday morning. But until New Horizons phoned home Tuesday night, there was no guarantee the spacecraft had buzzed the small, icy, faraway - but no longer unknown - world.

The unprecedented encounter was the last stop on NASA's grand tour of the planets over the past half-century. New Horizons' journey began 9½ years ago, back when Pluto was still considered a full-fledged planet.

"This is truly a hallmark in human history," said John Grunsfeld, NASA's science mission chief. "It's been an incredible voyage."

According to NASA, the spacecraft the size of a baby grand piano swept to within 7,700 miles of Pluto at 31,000 mph. It was programmed to then go past the dwarf planet and begin studying its far side.

To commemorate the moment of closest approach, scientists released the best picture yet of Pluto, taken on the eve of the flyby.

Even better images will start "raining" down on Earth beginning Wednesday, promised principal scientist Alan Stern. But he had cautioned everyone to "stay tuned" until New Horizons contacted home.

It takes 4½ hours for signals to travel one-way between New Horizons and Earth. The message went out late in the afternoon during a brief break in the spacecraft's data-gathering frenzy. The New Horizons team kept up a confirmation countdown, noting via Twitter when the signal should have passed the halfway point, then Jupiter's orbit.

The uncertainty added to the drama. "This is true exploration," cautioned Stern, a Southwest Research Institute planetary scientist. "New Horizons is flying into the unknown."

1.31pm: The live briefing from Nasa has started. Watch it here:

1.30pm: New Horizons has already answered one basic question about Pluto, its precise size. Scientists used photos from the spacecraft's telescopic camera, the Long Range Reconnaissance Imager (Lorri), to determine that the dwarf planet is somewhat larger than previously thought, having a diameter of 1,473 miles.

The result confirms that Pluto is larger than any other known solar system object beyond the orbit of Neptune.

Mission scientist Dr Bill McKinnon, from the University of Washington, says: "The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest."

1.22pm: We've made contact - what next? Nasa's next job is processing all the data - and close-up photos - New Horizons has sent. Mission control is due to hold a live talk soon.

.@NASANewHorizons “phoned home" & is healthy! Q? Use #askNASA during our 9:30pm ET briefing: http://t.co/eapczW4esv pic.twitter.com/1XNvpHvA9m

— NASA (@NASA) July 15, 2015

1.18pm:Nasa is expecting big things. The picture released before clearly showed the dwarf planet's surprising Mars-like reddish hue, and the enigmatic heart-shaped feature on its surface that has already become Pluto's calling card on the internet.

Nasa administrator Charles Bloden said that his team was astounded when the first images came back revealing a red planet:

"It's not an icy planet. It's red. How can the surface be red? The geological world is excited about something they didn't even think they would be involved in. Geologists are marvelling at the colour and thinking how can that be so far away from the Sun. We're calling Pluto a planet, technically it's a dwarf planet. I call it a planet, but I'm not the rule maker. It's a big day for Nasa. The US today has become the first nation to visit every planet in our Solar System. We wanted to demonstrate that we could navigate the last known planet in our Solar System. That is an incredible tehcnological achievement.
I expected to see some cold, grey icy planet. It has reddish tint, not unlike Mars. That's facinating. We continue to be mesmerised by this incredible planet and its moons."

Experts believe the colour arises from the chemical action of sunlight generating red compounds in the atmosphere that then fall on the surface.

1.10pm: In the absence of more photos, the internet has filled the vacuum:

1.08pm: Here's that stunning shot that Nasa released yesterday. Hoping that this will be only the second most amazing photo of Pluto you'll see today:

1.05pm: This is why this is a significant moment. Before New Horizons made its fly-by on the left is all we could see of the dwarf planet:

On the left: our best image of Pluto in May. On the right: our current best picture of Pluto. #NewHorizons #Pluto pic.twitter.com/qxmW7EnAWZ

— Andras Toth 😷😷 (@tothur) July 13, 2015

1.03pm: They did it! New Horizons made contact.

LOCKED! We have confirmation of a successful #PlutoFlyby. pic.twitter.com/Krfo9qxxHw

— NASA New Horizons (@NASANewHorizons) July 15, 2015

1.00pm: It's tense in mission control room as Nasa scientists wait for New Horizons to phone home:

Waiting for the phone home signal from #NASANewHorizons in the control room at #jhuapl pic.twitter.com/L13hnVWSKv

— John Grunsfeld (@SciAstro) July 15, 2015