After New Horizons, when will we next revisit the outer solar system?

“We have completed initial reconnaissance of the solar system,” said Alan Stern, the chief scientist of the New Horizons mission, shortly after the probe swung past Pluto – the most distant large object in our solar system. It’s a hell of a quote – combining triumph with a stark reminder that this is just the beginning of the next phase of our civilisation.

It means that we’ve now visited every world in the Sun’s sphere of influence. I use the term “world” to avoid arguments over the exact definition of a “planet”, but all nine of the objects that most people consider to constitute the solar system have now been visited at least once by a probe. How do we now best focus our efforts to deepen that knowledge?

We know a remarkable amount about Mars — our nearest interplanetary neighbour. There are two rovers on its surface (Opportunity and Curiosity), beaming signals back to Earth via three spacecraft (Mars Odyssey, Mars Express and the Mars Reconnaissance Orbiter) surveying the Red Planet from orbit, all connected up through an interplanetary internet. We also know a good amount about the Sun (SOHO and SDO), Mercury (MESSENGER), Venus (Venus Express), Jupiter (Galileo and the soon-to-arrive Juno) and Saturn (Cassini), thanks to the probes we’ve sent to orbit each one.

But scientists studying Uranus, Neptune and the other smaller planetoids beyond, like Pluto, Eris, Makemake and Haumea, have to make do with the data gathered during brief flybys and the scraps of information obtained from long-distance telescopes. Here’s what we know about them all, and what we’d love to find out.

New Horizons

You’ve probably been closely following the New Horizons mission, but what’s not so well known is how long scientists have been clamouring for data on Pluto.

New Horizons began as a letter-writing campaign from an alliance of scientists called the ‘Pluto Underground’ in 1989, aiming to pressure NASA into visiting what was then still considered the last planet in the solar system. Several low-cost, high-risk concepts were put together to satisfy budget requirements, but there was little appetite at the time for these kind of missions – NASA’s missions were either cheap and cheerful or big and impressive.usps_pluto_stamp_-_october_1991

Image: Wikipedia

In 1991, things changed when the US Postal Service released a set of stamps bearing the image of each planet in the solar system and the name of a spacecraft sent to it. Pluto’s stamp was an embarrassment – depicting a featureless sphere and the words ‘Not Yet Explored’. It was a call to arms for engineers at NASA’s Jet Propulsion Laboratory, who swiftly begun putting together more feasible proposals.

“Pluto’s stamp was an embarrassment – depicting a featureless sphere and the words ‘Not Yet Explored’.”

After the discovery of the Kuiper Belt in the late ’90s, one project in particular – the Pluto Fast Flyby mission – gained traction by adding a mission goal to visit the belt, rebranding itself as the Pluto Kuiper Express. The budget was expanded as a result, but the entire project was almost scuppered in 2000 when its cancellation was ordered by Edward J Weiler, then NASA’s science mission director.

There was an immediate outcry among much of the scientific community, and several internal NASA divisions. After extensive lobbying, a new class of mission was created – one that would fit between NASA’s low-budget Discovery Program and its high-end Flagship Program. In November 2011, most of the team from Pluto Kuiper Express were recruited into a mission called New Horizons, which was then officially selected for funding.

But just as it was finally looking like a Pluto probe would launch, disaster struck once again. NASA’s new administrator, Sean O’Keefe, was not supportive of the project and refused to include it in NASA’s 2003 budget – effectively cancelling it. Again, there was an immediate outcry, which prompted the scientific community to push the project to the top of the Planetary Science Decadal Survey – a highly influential “wish list” compiled by the US National Research Council of the projects considered the highest priority by scientists. O’Keefe and Weiler backed down, with the latter saying it was a result that the “administration was not going to fight”. The funding was finally secured.


New Horizons  launched from Cape Canaveral on 19 January 2006 (when Pluto was still considered a planet) and reached its destination on 14 July 2015. You might think that’s a remarkably brief travel time, considering how long it took the Voyagers to cover the same distance, and you’d be right — New Horizons hit a top speed of an astonishing 58,536km/h. That’s not even the fastest spacecraft we’ve built, though. The honour goes to Helios 2 – one of a pair of probes sent to the Sun in the late ’70s. During its closest approach to our star, it swung through the Sun’s gravity well at a whopping 252,792km/h.

For New Horizons, radio signals take approximately four hours to reach the craft from Earth, meaning that almost all of its operations need to be pre-programmed. As well as taking some holiday snaps, New Horizons will map the surface composition of Pluto and its moon Charon, analyse their atmospheres, and gather as much data as possible on the smaller moons in the Plutonian system.pluto_instagram

It’ll take about 16 months for the probe to send back all the data that it’s collected on Pluto and its moons, because it can only transmit at 2kb/s. As a result, most of the major scientific discoveries from our brief brush with Pluto will trickle out over the coming months. Meanwhile, New Horizons’ momentum will carry it further, past a trio of interesting objects in the icy Kuiper Belt that marks the edge of our solar system. The spacecraft is healthy and has plenty of fuel, so by 2016 the New Horizons team will have submitted a request for the mission (and its funding) to be extended. The odds are good that it’ll be approved, so it’s likely we’ll get our first sight of one of those objects in 2019.

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