What you need to know about the Pluto flyby

The Wire
July 14, 2015

In under seven hours, the NASA New Horizons space probe will flyby Pluto at 49,900 km per hour, from a distance of 12,500 km. It’s what the probe set out to do when it was launched in January 2006. The flyby will allow it to capture high-resolution images of the dwarf planet’s surface and atmosphere as well as take a look at its biggest moon, Charon. For much of the rest of the day, it will not be communicating with mission control as it conducts observation. The probe’s Long-Range Reconnaissance Imager (LORRI) has already been sending better and better pictures of Pluto as it gets closer. During closest approach, Pluto will occupy the entire field of view of LORRI to reveal the surface in glorious detail.

Fourteen minutes into the Pluto flyby, New Horizons will make its closest approach to Charon, which is about 24,000 km away. Next: 47 minutes and 28 seconds after the Charon flyby, the probe will find itself in Pluto’s shadow where its high-gain antennae will make observations of how the dwarf planet’s atmosphere affects sunlight and radio signals from Earth as they pass through it. Then, 1 minute and 2 seconds after that, New Horizons will again be in sunlight. Finally, 1 hour and 25 minutes later, it will be in Charon’s shadow to look for its atmosphere.

That New Horizons survived the flyby will be known when, on early Wednesday morning (IST), it starts to send communication signals Earthward again. The timings of various events announced by NASA will have to be adjusted against the fact that New Horizons is 4.5 light-hours away from Earth. NASA has called for a press conference to release the first close-up images at 0030 hrs on July 16 (IST). The entire data snapped by the probe during the flyby will be downloaded over a longer period of time. According to Emily Lakdawalla,

Following closest approach, on Wednesday and Thursday, July 15 and 16, there will be a series of “First Look” downlinks containing a sampling of key science data. Another batch of data will arrive in the “Early High Priority” downlinks over the subsequent weekend, July 17-20. Then there will be a hiatus of 8 weeks before New Horizons turns to systematically downlinking all its data. Almost all image data returned during the week around closest approach will be lossily compressed — they will show JPEG compression artifacts. Only the optical navigation images are losslessly compressed. [All dates/times in EDT]

Downloading the entire science dataset including losslessly compressed observations will take until around November 2016 to complete. Until then, the best will always be yet to come. As always, all communications will be via the Deep Space Network – whose Goldstone base is currently all ears for the probe.

Incidentally, the ashes of the astronomer Clyde Tombaugh, who discovered Pluto in 1930, are onboard New Horizons.

What do we know about Pluto?

Among the last images taken by LORRI before the flyby revealed a strange geology on Pluto. Scientists noted dark and bright polygonal patches (in the shape of a whale and a <3, respectively) as well as what appeared to be ridges, cliffs and several impact craters. However, these features on the side of Pluto facing New Horizons as it flies in. During the flyby, it will image the other side of Pluto, where these features may not be present. The probe can’t hang around to wait to see the other side either because Pluto rotates once every 6.4 Earth-days.

During the flyby, images of Charon will also be taken. Already, the probe has revealed that, like Pluto, the moon also has several intriguing features – while until recently both bodies were thought to be frozen and featureless balls of ice and rock – like giant craters and chasms. In fact, NASA noted one crater near Charon’s south pole, almost 100 km wide and another on Pluto, some 97 km wide, both appearing to have been the result of recent impacts (in the last billion years). The particularly dark appearance of the Charon crater has two theories to explain it. Either the ice at its bottom is of a different kind than the usual and is less reflective or the ice melted during impact and then refroze into larger, less bright grains.

All these details will be thrown up in detail during New Horizons’ flyby. They will reveal how the two bodies evolved in the past, the structure and composition of their interiors, and if – for some astronomers – Charon might’ve harboured a subsurface ocean in its past. Complementarily, NASA will also be training the eyes of its Cassini, Spitzer and Keplerspace-borne instruments on Pluto. Cassini, from its orbit around Saturn, will take a picture of New Horizons just around the time of its flyby. From July 23 to July 30, the Spitzer Space Telescope will study Pluto in the infrared, mapping its surface ice. Then, in October, the exoplanet-hunting Kepler telescope, in its second avatar as K2, will start focusing on the changes in brightness off of and around Pluto to deduce the body’s orbital characteristics.

Then, there are also post-flyby missions whose results, when pieced together with the July 14 flyby and other observations, will expand our knowledge of Pluto in its larger environment: among the Kuiper Belt, at whose inner edge it resides.

Finally, as Dennis Overbye of The New York Times argued in a poignant essay, the Pluto flyby marks the last of the Solar System’s classical planets to explored, the last of the planets the people of our generation will get to see up close. The next frontiers in planetary exploration will be the exoplanets – the closest of which is 4.3 light-years away (orbiting Alpha Centauri B). But until then, be willing to consider the Solar System’s moons, missions to which are less than a decade away. Leaving you with Overbye’s words:

Beyond the hills are always more hills, and beyond the worlds are more worlds. So New Horizons will go on, if all goes well, to pass by one or more of the cosmic icebergs of the Kuiper belt, where leftovers from the dawn of the solar system have been preserved in a deep freeze extending five billion miles from the sun…

But the inventory of major planets — whether you count Pluto as one of those or not — is about to be done. None of us alive today will see a new planet up close for the first time again. In some sense, this is, as Alan Stern, the leader of the New Horizons mission, says, “the last picture show.”