New Horizons In The Home Stretch
Recent Images Show the First Visible Surface Features
Last week, NASA held a brief teleconference for updates on the New Horizons probe, still on track for its rendezvous with Pluto on 14 July. In broad strokes: the probe is operating perfectly, the New Horizons team is busy rehearsing what will be a relatively lightning-quick encounter, and a number of contingency plans are in place should anything go wrong.
But the biggest news was a fresh batch of imagery, taken in mid-April by the probe's Long Range Reconnaissance Imager (LORRI) instrument, that is both a testament to what the probe has accomplished so far, and the discoveries that await it just a few months from now.
LORRI is a sizable silicon carbide space telescope which relays the light it collects to a Charge-Coupled Device (CCD), the digital imager of choice for most space probes.
The LORRI instrument is installed on the New Horizons spacecraft in 2005. This occurred before the final paneling was installed; the instrument doesn't protrude as far from the spacecraft as it appears to here. Credit: NASA
After nine years of travel, covering over 3 billion miles of space, New Horizons is now close enough to its target that LORRI is able to send back images like this one, revealed during the teleconference:
Credit: NASA
Pluto is still far off here, and there's a lot of pixelation, but this is a momentous image that illustrates a few important features.
First, in the inset schematic, note the yellow gridlines superimposed over Pluto. These are intended to show longitude and latitude, and they convey that, like Uranus, Pluto is tipped over on its side; the axis of its rotation points much closer to what we would consider horizontal, while most other planets point closer to vertical, as shown in this regrettably comic sans annotated image:
Credit: NASA
This has been known for a long time via our observations with ground-based telescopes, but it's all the more evident now that we're able to see the faint disc of Pluto's largest moon, Charon (also tipped on its side), orbiting in roughly that same orientation.
Next, notice how Pluto appears to wobble around the center of the image, rather than remaining at a fixed point. This is another somewhat unique feature of the Pluto-Charon system, a product of a fairly basic principle of orbital mechanics. In any orbital system involving two bodies, both objects actually orbit around the system's center of mass, the point at which the masses of the two objects balance each other -- this is called the barycenter of the system.
For most systems we're familiar with, like the Earth orbiting the Sun, or the Moon orbiting the Earth, one body is so much more massive than the other that the system's barycenter lies within the larger body. Because of the Moon's mass, for example, the barycenter of the Earth-Moon system is not the center of the Earth, but a point that's "only" 1700 kilometers below the Earth's surface. This is the point that the Moon and Earth orbit as the whole system in turn orbits the Sun. But because the point is still within the Earth, we wouldn't notice the Earth's "orbit" around it. It would manifest as a (relatively) tiny wobble.
This is not the case for Pluto and Charon. Charon is the largest moon in the solar system relative to its host body. No other moon is as close to the mass of the planet it orbits. Because of this, the center of mass of the system, the barycenter, lies outside of Pluto, and we can observe Pluto orbiting it in the animation above (the barycenter is represented by the plus symbol in the inset diagram).
Finally, and most fascinating, is that, for the first time, we've observed discernible surface features on Pluto. We've known of Pluto's existence for 85 years, since its discovery by Clyde Tombaugh in 1930. For those 85 years its featured heavily in our imaginations, our science fiction, our elementary school science classes, our planetariums, and our surprisingly heated debates about its planethood. But never in that time can we be said to have laid eyes on it -- to know, even at a rudimentary level, what it looks like. Even if you know very little about our solar system, chances are that if someone said "Jupiter" or "Saturn" or "Venus" a reasonably faithful image would pop into your head: a big milky, pink and orange ball with the distinctive red spot, a yellow orb surrounded by rings, and a cloudy, featureless pearl.
We've never had that for Pluto, and will, for the first time, in July. The first clear, up close snapshot of Pluto will be, I think, an iconic image of the 21st century. For now, we can be content with the mystery of its wildly varying regions of brightness and darkness. In the animation above, these variations in albedo (the amount of light reflected by a given region of a body's surface), make Pluto look lumpy, but that's not necessarily the case. It's more likely that these represents regions of substantially different surface composition, perhaps contrasting deposits of dust and ice.
Alan Stern, the mission's Principal Investigator, speculated on the teleconference that the region of constant brightness, visible at about the 3 o'clock position in the LORRI image, may be evidence of a polar ice cap, which would be the first of many significant discoveries for New Horizons in the neighborhood of Pluto (if that seems like an odd place for a polar cap, recall from above how Pluto is tipped over on its side).
The team noted that, from this point forward, all LORRI images will be available on the mission's public gallery within 48 hours of downlink (with light now taking almost 9 hours to travel to New Horizons and back to Earth, image downlinking is no quick matter). Another series of images will be taken in mid-May, and we'll have a fresher batch of pixels to stare at and speculate over. At the time of writing, its 71 days, 14 hours, and 10 minutes to the historic encounter.