At the edge of space

It's been a fascinating couple of weeks out at the furthest reaches of the solar system.

On New Years Day, the spacecraft New Horizons achieved flyby of the distant planetoid Ultima Thule (2014 MU69). You will recall that back in the summer of 2015, New Horizons made the most distant planetary flyby in history when it reached Pluto, taking shots in unprecedented detail. It's spent the last three-and-a-half years travelling ever outward, finally making it to Ultima Thule at a distance of 43 AU, or six light hours. The orbits of the two planetoids cross, and they are favourably positioned to one another during the spacecraft's window of opportunity.

Ultima Thule is a funny little planetoid. It's snowman-shaped, formed when two objects fused in a slow collision billions of years ago. It's nickname refers to a Latin phrase which became the name of the legendary northernmost point of the ancient world. NASA have also named the original planetesimals that came together Ultima and Thule (for the larger and smaller respectively) which is cute. Ultima Thule is only 31 km long at its greatest extent, and is a snapshot of the initial building blocks of the solar system, that came together to form larger planetoids and planets. NASA are now studying the data sent back for information on its composition, surface temperature and geology, and any evidence for rings, moonlets or a cometary coma.

The next destination for New Horizons remains to be seen, but it has power enough to remain operational until around 2030 and should be able to complete a final planetary flyby once a suitable, very distant target has been identified.

New Horizons is the fastest travelling spacecraft (in terms of initial speed) in NASA history, and is even catching up with the earliest space probes that were launched in the 1970s. Voyager 2 left the heliopause, officially entering interstellar space, in November (it's brother Voyager 1, although launched a little later, passed it and reached the threshold earlier). The Voyagers were be joined in interstellar space by New Horizons in the next few decades, as well as the now defunct Pioneer probes. NH will eventually pass the Pioneers (by the 23rd century, which is when Pioneer 10 is due to be destroyed by the Klingons), but will never catch up with the Voyagers.

Also in November, three astronomers (Sheppard, Tholen and Trujillo) observed the most distant trans-Neptunian planetary object ever. Officially designated 2018 VG18, but nicknamed Farout, the object is the first to be detected at a distance of over 100 AU. Currently at a distance of 120 AU, it beats Eris at an observed distance of 96 AU. In terms of average distance though, it is more than double that of Eris, although not nearly the farthest discovered. In 2014, the same team discovered FE72 which has a semi-major axis of a wahopping 1550 AU.

It's a fascinating time to be learning about the very fringes of our solar system.

It's Pluto Day!

Today, after nine-and-a-half years travelling through space, the spacecraft New Horizons made its closest approach on its flyby of Pluto. The icy dwarf planet is currently over three billion miles from Earth. The image above shows the planetoid in high resolution at New Horizon's closest approach, while below is a composite of Pluto and its largest moon, Charon, to scale by size of body but not distance (in reality, the Earth could just about fit between the two). Armed with an array of spectrometers, radiometers and telescopic cameras, the spacecraft has been relaying data not only about Pluto-Charon, but also Jupiter and its moons, and the space dust that swarms through the Solar System. It's a relief considering how much can go wrong on such a long journey, especially considering the ship's main systems went offline in a hairy moment ten days before closest approach

We've learned an enormous amount about Pluto, not all of it dry technical details that astronomers and physicists will spend years pouring over. Some facts just leap out. Pluto is a reddish colour, with a large, whitish patch in a rough heart-shape covering much of one hemisphere. It has a thin atmosphere which is losing gas, and this has previously distorted measurements of its size. Now that we can look closer, we can get an accurate measurement. Pluto has a diameter of 2370 km (+ or - 20 km), which means it is definitely larger than Eris after all (the diameter of Eris being a little less at 2336 km +/- 12 km). Which is a little ironic, considering that it was Eris' supposedly larger size that finally kicked off the (already simmering) debate concerning whether or not Pluto is a planet. 

Now that we know it's a little larger than previously thought, this means it's also of a lower density than astronomers had concluded earlier, which in turn means it is icier than previously suspected. More ice: less rock = a less dense overall body. Much of this ice forms a polar cap on Pluto, previously unknown, which is comprised of frozen methane and nitrogen.

Charon, on the other hand, is a whitish-grey colour, and has no appreciable atmosphere. The diamater of Charon is 1208 km, which was already calculated but has now been confirmed. With nitrogen gas escaping the Plutonian atmosphere and detectable at distance of over six million km, Pluto and Charon may share an atmosphere, making them even more like a double-planet than already thought.

This is just the start of it; New Horizons is still sending back data and scientists will be analysing it for a long time yet. Meanwhile, New Horizons will continue on its journey, with NASA set to choose a new target for it to travel towards on its long journey out of the Solar System. You can read all about the mission here.

The Plutonian System

NASA have released a cleaned-up composite of the latest images taken by spacecraft New Horizons. These images show all four of Pluto's faint, small moons, Stix, Nyx, Kerberos and Hydra. If you click here you can see the four moons' orbits around the central duo of Pluto and Charon.

Less than eighty days to Pluto!

Dwarf Planets

That's the first ever full colour photograph of dwarf planet Pluto and its major moon Charon. OK, it may not look like much, but it's a significant first. No spacecraft has ever been close enough to Pluto before to take a quality image, but now NASA's New Horizons space probe is closing in on the Plutonian system, snapping photos using its special long exposure colour imager (which is, charmingly, called Ralph). The resolution is low at the current distance, but even in this image we can see that Pluto has a brighter appearance than Charon, indicating different surface materials. The New Horizons team on Earth are now refining the image to make it clearer, and better, more detailed images will come as the probe gets closer to Pluto. As I write this, it's 89 days away from closest approach.

Meanwhile, NASA's Dawn spacecraft is in orbit around Ceres, the nearest of the dwarf planets, in the asteroid belt. Dawn is the first spacecraft to achieve orbit around a dwarf planet, and has been stable in orbit for around a month, since March 6th, although frustratingly much of that time it has been on the dark side of the orbit and unable to take photos. We do have some excellent images taken from its approach though. Above are two excellent monochrome shots of Ceres, half in shadow due to the position of the spacecraft relative to both the planetoid and the sun, while below is a composite colour image of the Cererean surface. This is not a true colour image, but is enhanced to show the diversity of surface features more clearly. From both images, we can see that the Cererean surface is heavily cratered and displays considerable variation. The bottom image shows several shots of the surface taken using an infrared mapping spectrometer, clearly showing the two mysterious bright spots on the surface of Ceres which are as yet unexplained and are provoking debate. It's an exciting time for space exploration. Makes up for that tragically disappointing solar eclipse.