New Horizons Anniversary

Today marks the second anniversary of New Horizons’ closest approach to the dwarf planet Pluto, on July 14, 2015. To celebrate, we look towards the spacecraft’s past and future!

The New Horizons space probe launched from Earth in 2006 and spent nearly a decade traversing the Solar System en route to its primary target, Pluto. The dwarf planet had remained largely a mystery since its original discovery in 1930 by astronomer Clyde Tombaugh, due primarily to its tremendous distance from Earth—an average 6 billion kilometers (3.5 billion miles) away! Up until the New Horizons mission, photos of Pluto had looked like no more than a blurry smudge in the distance, even through the Hubble Space Telescope. (How blurry, you ask? Click here to see a comparison.)

New Horizons’ five billion kilometer (three billion mile) trip to the dwarf planet solved that problem by capturing the first detailed images of Pluto’s surface ever taken, providing breathtaking photos and scientific data that shattered scientists’ predictions. Pluto isn’t just a frozen ball of rock and ice as was previously thought, but a complex world filled with diverse terrain and active geology. New Horizons showed us massive flowing fields of ice, a tenuous atmosphere, and even signs of cryovolcanoes, amongst countless other discoveries. (For details on the big discoveries at Pluto, check out our interview with one of the mission scientists.)

New Horizons finished downloading its Pluto data in late 2016, but rather than concluding its mission, this instead marked the beginning of the probe’s next phase: exploring the distant Kuiper Belt beyond Pluto. While New Horizons was on the last leg of its journey to Pluto, roughly one year prior to its closest approach, the Hubble Space Telescope had been scouting ahead in preparation for this next objective. Carefully scanning farther down New Horizons’ trajectory, Hubble identified a handful of objects of interest before the team chose the probe’s next target: MU69.

2014 MU69 (its official designation) is a deep Kuiper Belt object more than a billion kilometers farther away than Pluto. Hubble’s observations indicated a size between 20–40 kilometers (12–24 miles) in diameter, but otherwise, the object remains a complete mystery. Investigating MU69 gives New Horizons an unprecedented opportunity to explore one of the most distant and uncharted parts of the Kuiper Belt, and to gain some insight into our solar system’s past.

That mysterious and uncharted nature also brings some danger along with it. If the orbit or size of MU69 haven’t been estimated correctly, the discrepancy could have an adverse effect on the mission. It’s also possible that unidentified debris could surround MU69, which could cause catastrophic damage to New Horizons as it barrels through the region. Astronomers are learning everything they can about the distant Kuiper Belt object before New Horizons arrives to give the spacecraft the best possible chance at success. Though investigating MU69 has proved to be a tremendous challenge up to this point, a rare opportunity coming up this week may help to fill in some of the gaps.

At certain points in its orbit, MU69 moves in front of background stars causing an event called an occultation. An astronomical occultation occurs when a star’s light is partially blocked and dimmed by a passing object (eclipses, for example, are a specific type of occultation). The occultation creates a shadow on the star when viewed from Earth, but the alignment needs to be so precise that the shadow is visible for just a few seconds, and can only be seen along a narrow band of Earth’s surface. By positioning themselves at just the right place and time, astronomers can measure how the starlight changes and learn more about MU69 before New Horizons begins its approach.

Three occultation events were identified for MU69: one visible from Argentina and South Africa on June 3, another visible over the pacific on July 10, and a final visible only from southern Argentina on July 17.

The first occultation was observed without any hitches from the ground, but MU69 couldn’t be identified in the any of the tens of thousands of images taken. This suggests a few possibilities: MU69 could be smaller (and more reflective) than expected, or it might not be a single object at all but multiple objects bound together (or even a pile of rubble). These results are perplexing, but the July 10 and 17 events should provide more answers. (The event on July 10 has already been captured by the SOFIA telescope, data is currently being analyzed.)

If all goes well, New Horizons will be making its closest approach to MU69 on New Year’s Day 2019, skimming even closer than it did to Pluto. Doing so will unveil incredibly detailed images from a part of the solar system never before seen, informing us what the Kuiper Belt may or may not have in common with the rest of the Solar System, how and when it formed, and giving us more perspective on our past and our place within our home system.