Cheers and applause greeted the news that NASA probe New Horizons had made radio contact with flight controllers following its fly past of Pluto. Pluto just had its first visitor! Thanks NASA – it’s a great day for discovery and American leadership. pic.twitter.com/FfztBSMbK0— President Obama (POTUS) July 15, 2015 About 13 hours after its closest approach… Continue reading →

If space is humankind’s ultimate challenge, then the first step starts close to home – we have still to explore much of our solar system that spans across enormous distances, never mind those galaxies and stars more distant still.

To learn more we must get closer, dispatching spacecraft such as New Horizons – which has just returned our first ever close-up images of Pluto after a nine year journey. Here are my top five missions that are chipping away at what we don’t know and building up a better sense of universe around us.

1. New Horizons

Rocketing past at 14km/s, the New Horizons spacecraft has provided our first close view of Pluto which will enormously improve our understanding of this distant body. Our facts regarding this icy dwarf are sketchy at best. Just knowing what Pluto looks like makes it much more real.

The probe’s brief two-hour visit captured images of parts of Pluto and its largest moon Charon at high resolution, improving our understanding of planet formation. This is an amazing feat, considering the signals take more than 4.5 hours to reach Earth and that the sun is so weak at this extreme distance that solar power is not an option.

But the mission doesn’t end here: in 2019 New Horizons will visit a small object in the Kuiper belt, a region of space filled with small rocky planetoids, giving us a chance to examine the make-up of these remnants of the early solar system. And by 2026, it will reach the outer edges of the solar system.

2. Rosetta

Conceived decades ago, Rosetta flew alone through space for years before it reached the tiny comet that was its target and swung into orbit around it. Given the comet’s highly irregular shape this is an outstanding feat by itself.

The lander Philae managed to touch down and collect surface data of the comet, and while it was forced to shut down from lack of sunlight falling on its solar panels, it has now awoken and is transmitting data again. The Rosetta orbiter mission has also been extended to 2016 when it will also attempt to land on the comet.

The missions have improved our understanding of comets which contain frozen, icy rocks, and have measured the gas composition of jets streaming off the comet before they are altered by solar radiation.

But more than just hard numbers, this mission has been capturing images that speak for themselves, showing an ambitious mission conceived by many nations working together. Images such as Rosetta’s pictures of Philae descending resonates with us more than just hard facts and figures.

3. Dawn

Dawn is another mission expanding our knowledge of dwarf planets, in this case Ceres. It is now orbiting this interesting object having spent 2011 conducting similar work around nearby Vesta. Both Vesta and Ceres in the asteroid belt are protoplanets but of quite different composition.

Dawn has illustrated how powerful imagery can be. The most intriguing image is a crater that contains a handful of bright white spots on a surface otherwise darker than coal – unexpected, unexplored, challenging terrain.

4. Messenger

Messenger is still in my list of impressive space probes even though the mission ended with its controlled crash on Mercury’s surface this April. Sent to explore a planet of which we had barely any imagery of its surface, in four years Messenger managed to not only give us high-resolution maps of the innermost planet, it discovered water in its dark polar craters. On a planet baked by the sun this could only arrive from comets and water-rich asteroids – objects currently under investigation by Rosetta and New Horizons.

5. Curiosity

The last is the Curiosity rover. For me it sums up the efforts to explore our neighbour, Mars. These missions went in search of life and traces of water, carrying a complex laboratory, drills, laser and high resolution cameras.

Curiosity particularly illustrates the challenges we are capable of mastering to land a probe on Mars – described by NASA themselves as “seven minutes of terror”.

These rovers have achieved an outstanding feat, where now those exploring beyond Earth are not astronomers but geologists, the rovers’ equipment replacing the hammer and microscope used during fieldwork. The missions have added Mars to the “territory” that humans have access to. It’s even on Google Maps – imagery so good that we can see its surface as if we were there and can look at rocks in such detail as if we were picking up pebbles at the beach.

It’s out there waiting for us

Probes have witnessed solar eclipses and comet fly-bys that provide an entirely different view than is possible from Earth – something that adds a feel of awe and wonder, like looking back on Earth from the moon.

The sort of incredible images provided by these probes connects us with the solar system, bringing it closer to home. Famous images of Earth from space, such as the Blue Marble and the Pale Blue Dot catalysed our ecological conscience, reminding us of the fragility of our world in comparison to the vast, cold emptiness of outer space.

Such images lead us on to explore the universe and ourselves, and the findings of these remarkable spacecraft that have travelled millions, sometimes billions of miles through space over many years remind us that it’s out there to be discovered. The challenge and rewards await, as J F Kennedy said: we choose to go to space not because it is easy, but because it is hard.

Daniel Brown is Lecturer in Astronomy at Nottingham Trent University.

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New Horizons’ fly-by of Pluto and its moons is the latest in a historic string of missions to objects in the solar system. But given that a fly-by lasts for just a short time, how much can we really get out of it? There’s no doubt that the mission will yield a great deal of interesting data, but surely more would be gained if the spacecraft could go into orbit for a number of days or actually land on the surface and take physical samples.

New Horizons is hugely important because it is giving us a first glimpse into the unseen world of a third class of objects in the Kuiper belt – the building blocks of the outer solar system, located beyond the terrestrial and gas-giant planets. Fly-bys such as this are very exciting as they provide just one chance for unique measurements at the target.

While we are only at the very first stage of exploring Pluto and its moons, the fly-by will provide the foundations for future missions. Indeed, a fly-by is the first in the classical four stages of solar-system exploration and is followed – in this order – by an orbiter, a lander and the return of a sample from a body (marked 1-4 in the table below).

Lessons from the past

The first fly-by was of our Moon, made in 1959 by the Russian Luna-1 spacecraft. And 50 years ago, nearly to the day (July 15), the US Mariner 4 made the first fly-by of Mars.

My generation was captivated by the historic fly-bys of the outer planets and some of their moons, and I’ve been lucky enough in my own career to have been involved in instrument teams for several historic fly-bys. These were the Giotto mission to comets Halley (1986) and Grigg-Skjellerup (1992), as well as several close “fly-by firsts” in the Saturn system with the Cassini mission (such as moons Titan, Enceladus, Rhea, Dione, Hyperion).

The Giotto fly-by of comet Halley only lasted a few days, but our knowledge of comets was revolutionised by this encounter. One of several probes to explore Halley in the mid-1980s, Giotto had the widest and most capable set of instruments and passed closer to its target than any of its companions.

It found cometary jet activity, a surprisingly dark surface, hydrocarbons in a crust and a complex bow shock and tail formation mechanism. These discoveries are now being followed up by the Rosetta mission and Philae lander at comet 67P.

But the fact that fly-bys happen so quickly can also make them very stressful and difficult to manage. When we were monitoring the Giotto spacecraft, flying past Halley at 68.4 km/s, it suddenly started spinning off its axis after encountering a dust particle near its closest approach. Fortunately it was possible to stop the wobble.

There are many other examples where data have been rescued – including with New Horizons during its worrying glitch (now fixed) on July 4.

New Horizons and beyond

After launch on an Atlas V in 2006, the 478kg spacecraft passed Jupiter only 13 months later, which was an express route. The main reason for the hurry was to reach Pluto before its tenuous atmosphere collapses by freezing as the planet moves further away from the Sun. The mission design of New Horizons gives a very fast fly-by at over 14 km/s (50,000 km/hour), with only a few hours and days for the highest resolution measurements.

Measured in “astronomical units” (one AU is about 149.6m kilometres), Pluto’s orbit takes it from its closest point to the Sun (29.7 AU, 1989), inside Neptune’s orbit (30.1 AU), through its current distance (nearly 33 AU) out to its furthest distance from the Sun (48.9 AU, 2113). Receding from the Sun, the surface temperature reduces from its current 40 Kelvin, leading to freezing of the atmosphere.

But why fly past, rather than going into orbit? The most simple answer is that a lot of energy, meaning a lot of fuel, would be needed to slow New Horizons enough to capture it into orbit. Instead, NASA opted to get to the Pluto-Charon system quickly with a relatively capable 30 kg payload, rather than taking a large amount of extra fuel using a different fly-by scheme, to get there before the atmosphere collapses.

New Horizons is already expanding the thin textbooks on the Pluto-Charon system with early images, and the data to be returned over the next 16 months from visible, infrared and ultraviolet spectrometers as well as plasma, dust and radio science instruments will broaden and rewrite them again.

But new questions will almost certainly arise, which can only be answered by a more detailed orbiter mission, following the usual exploration sequence. When that would happen is hard to say. The relative priority will need to be compared with missions to other objects, particularly those where the exploration stage is low, before possible implementation.

In the future, we can look forward to more detailed fly-by missions of objects where our knowledge is limited and later-stage missions such as Rosetta. Also we will visit new dimensions, such as ExoMars drilling underneath the Mars surface by up to 2 metres for the first time. The JUICE fly-bys of Jupiter’s moons Europa, Ganymede, and Callisto, before entering Ganymede orbit, will allow comparison of subsurface oceans, and Europa Clipper will fly past Europa 45 times to complete a detailed reconnaissance there.

These missions, and their follow-ons, will help us discover more about the humankind’s place in the universe – and whether we are alone. But it is clear that, while we have achieved a lot in solar-system exploration so far, there is still a large amount left to do.

Andrew Coates is Professor of Physics, Head of Planetary Science at the Mullard Space Science Laboratory at UCL.

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As I began typing this column, NASA’s New Horizon mission was on its final approach to its primary target, Pluto. By the time I finished composing my deathless prose, the main mission was over. And I’m not a slow writer.

Launched in January 2006, the spacecraft has travelled for nine and a half years for a flyby lasting only about 15 minutes. It doesn’t sound much of a reward for all the effort of designing and building the spacecraft – but for planetary scientists, the data coming back from the mission is pure gold.

For now all we can do is wait. Early in the morning on July 15, New Horizons is expected to phone home and confirm that the fly-by went well. Later that day the first high-resolution images should start trickle back to Earth – revealing what Pluto and its moon Charon actually look like up close. However, it will take nearly a year for all the data from the instruments aboard the spacecraft to come back.

But what is so exciting about Pluto? It isn’t even a planet anymore! When the New Horizons mission was conceived, Pluto stood (or rather, orbited) proudly as the ninth, and newest, planet in the Solar System. But eight months after New Horizons left Earth on its journey to Pluto, the International Astronomical Union downgraded Pluto from “planet” to “minor planet”.

Pluto’s change in status has, however, definitely not diminished the importance of the mission. Indeed, it has probably enhanced the scientific significance of the findings. Back when we thought Pluto was a planet, it was merely the last member of a series which represented a progression from the inner rocky and metallic bodies such as Mercury, through the gas and ice giants like Jupiter and Neptune, to Pluto – a small body of ice and rock.

But we now know that Pluto is not an isolated entity – it is the largest body in a huge family of primitive objects, many of which have their own moons. According to current models of how the solar system formed, there were once several hundred thousands of objects beyond Neptune, but Jupiter’s motion scattered most of them much further out from the Sun.

There are, however, still likely to be more such remaining bodies, known as Trans-Neptunian Objects, than the asteroids in the main belt between Mars and Jupiter. These objects are probably even more primitive in nature than some comets, which have been modified as they approach the Sun.

We already know that methane and ammonia ices are present on Pluto – but are there any higher hydrocarbons, or biologically interesting compounds such as amino acids? It will be interesting to see how analysis of the surface ices compare with results from the Rosetta mission or from the Dawn mission at Ceres. Can we draw any comparisons with the photo-chemistry on Saturn’s giant moon, Titan? Will Pluto demonstrate that trans-Neptunian objects are the most unchanged and unprocessed objects in the Solar System?

First look

Previous images of Pluto have been poorly resolved – the best view by the Hubble Space Telescope is of a fuzzy grey blob (see image below). But over the past few weeks, we have been able to enjoy increasingly more detailed images taken by the New Horizons spacecraft on its approach to Pluto. For example, we’ve learned that the planet is slightly bigger than we thought. We have also seen features on the surface, including what are probably ice-caps.

Although the closest approach to Pluto will be over in a matter of minutes, the amount of data captured will be immense. It could help us answer a number of questions about Pluto – such as the distribution of different ices (water, ammonia, methane), the relationship between rock and ice and the presence of a thin atmosphere. The fly-by could also shed light on whether there are indeed craters on the body and whether there is any evidence of resurfacing.

There is no expectation that cryovolcanism or ice geysers will be observed on Pluto, it doesn’t have the same gravitational heat source derived from a giant companion such as the case for Jupiter’s moon Europa. But it is in a binary system with its almost equally-sized moon, Charon – so it may surprise us yet.

For me, one of the highlights of the coming months will be synthesis of three sets of data – from New Horizons on Pluto, Dawn on Ceres and Rosetta-Philae on comet 67P/Churyumov-Gerasimenko. This will give us real insight to comet-asteroid interrelationships, and the primitive material from which the Solar System was built.

Whatever comes from the fly-by, we already have enough fresh information about our far-distant neighbour to ensure it is no longer seen as an underworld, the underdog of our planetary system. It is not the last planet to be visited but it is the first trans-Neptunian object to be seen – and so becomes a member of a very exclusive club.

15 July: Success confirmed

Following a long day of waiting, mission scientists got their reward just before 01:00 (UT), when the signal transmitted from the New Horizons spacecraft about four hours earlier was received at mission control. Cheers were heard from the team as Alice Bowman, the Mission Control Manager announced “we have telemetry lock”. This translates as “the spacecraft has called up, we have answered the phone, and we can hear what New Horizons is saying”. More cheers as each sub-system reported in: “thermal systems nominal”; “propulsion systems nominal”.

During the wait, we were tantalised with a fresh image of Pluto and its largest moon, Charon. New images will be released soon. For now, the waiting continues as New Horizons moves away from Pluto and its moons, and looks for its next target, a second TNO, which will be encountered in around 4 years time…

Monica Grady is Professor of Planetary and Space Sciences at The Open University.

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