This week has been a pretty significant one in the history of space exploration, as no less than three different space probes achieved important milestones in their missions.
At 5:33am GMT on 1st January, the New Horizons probe passed within 3500km of a 33km-long object named Ultima Thule (a Latin phrase used historically to describe places beyond the edge of the map) – at 6.5 billion km from Earth, this was the most distant object in the Solar System ever to be encountered by a probe. After New Horizons accomplished the first flyby of Pluto in 2015, astronomers were keen to attempt another encounter with an object in the Kuiper Belt, the area of the Solar System beyond the orbits of the eight planets. As described here by Alex Parker, the search for a suitable target had resulted in the discovery of Ultima Thule – or 2014 MU69 – in 2014. It is of particular interest for being a “Cold Classical” Kuiper Belt object; the fact that the plane of its orbit is similar to those of the planets suggests that it hasn’t been significantly disturbed since the Solar System was first formed, essentially making it a space fossil.
It will now take New Horizons almost two years to send all of its data back across billions of kilometres to Earth, including the best pictures of Ultima Thule. What we have seen so far reveals that Ultima Thule is actually two objects that have combined into one, in a snowman shape – though this had already been deduced by observing exactly how Ultima Thule eclipsed the light of a star that it passed in front of – and that it is a red colour. As for New Horizons, it has enough fuel left for an encounter with one more object as it continues its journey through the Kuiper Belt.
Then, on 3rd January, a Chinese probe named Chang’e 4 made a soft touchdown on the far side of the Moon – something that no lunar lander has ever done before. The obvious challenge of exploring the Moon’s non-Earth-facing side is that from there, a spacecraft cannot directly communicate with Earth. In the days of the Apollo program, geologist-astronaut Harrison Schmitt proposed a manned far-side landing using relay satellites for communication, but the idea was rejected for being both too risky and expensive. Now, China has achieved the feat (albeit with an unmanned spacecraft) using their own relay satellite in a precise orbit. Having touched down in the Von Karman crater in the Moon’s southern hemisphere, and deployed a rover, Chang’e 4 will now explore the geology of the far side; it is also carrying a spectrometer to pick up radio waves from the Sun and elsewhere (on the far side, there will be no interference from Earth), plus some insect and plant specimens.
These two accomplishments were pretty well publicised in the media, but there was also a third probe that made history on 31st December: OSIRIS-REx went into orbit around the asteroid Bennu, the smallest object that a probe has ever orbited. After being launched in September 2016, OSIRIS-REx caught up with Bennu – which follows an elliptical orbit, spending most of its time between Earth and Mars – on 3rd December; as they orbited the Sun together, the probe performed a number of flybys back and forth around the asteroid before making its orbital insertion. As Bennu only has a diameter of around 500m, its gravity is negligible; OSIRIS-REx must orbit about a mile from the asteroid’s centre, and so slowly that it takes 62 hours to complete one revolution. In 2020, the probe will fly down to try and collect a sample from Bennu’s surface, which it will then return to Earth in 2023.
Meanwhile, there’s more news: the Falcon 9 carrying SpaceX’s Crew Dragon spacecraft has been rolled out to Launch Pad 39A at Cape Canaveral, before its first unmanned test launch – if this mission is successful, SpaceX will be one step closer to sending astronauts to the International Space Station!