May 29, 2015
Alex Parker is a planetary astronomer at the Southwest Research Institute, Texas, and he posted his tweet just as I started writing this piece. And not just for Parker – it’s an exciting time for everyone, an exhilarating period in the history of space exploration. In just under 48 days – on July 14, 2015 – the NASA New Horizons space-probe will make its first fly-by of our favourite dwarf planet Pluto. Until then, it will be relaying less and less grainy pictures to Earth, each of more interest than the last, of a cold and distant world discovered by Clyde Tombaugh in 1930. One batch of images taken from May 8 to May 12 has already added to old evidence that Pluto hosts icy polar caps, and variations in surface brightness suggest a more uneven composition. On May 28, New Horizons restarted another phase of imaging – and as each day takes the probe 1.2 million km closer to its target, this is Pluto finally emerging out of the blur.
What more could we stand to find out? Quite a lot, as it turns out, from three points of view:
1. Toward the outer limits
The engineers operating the Voyager 1 space-probe (currently the farthest human-made object from Earth) had exciting news in September 2013: they claimed that about a year earlier, the probe had entered the interstellar medium – the space between stars, where the Sun’s influence was no longer the dominant one but had to contend with particulate emissaries from other stars in the galaxy. At the time, V1 was running on what little remained of its battery, a feeble ingot of blinking lights 19 billion km from Earth, and the occasion was replete with symbolism: humankind (or a representative) had set foot into the universe.
Actually, that moment could’ve transpired earlier. The engineers said that, in February 2012, the readings to indicate if V1 had entered the interstellar medium were spotted by the probe. However, they couldn’t be verified because the instrument that could do that had run out of juice. Luckily for them, a solar flare that erupted in March 2012 set the region of space around the probe thrumming with energy, which V1’s weak were able to pick up on and settle the matter.
Pluto, now, is much closer to the Sun than the threshold of the interstellar medium – in fact, the distance between Pluto and the Sun is 3.7 times smaller than the distance between Pluto and the medium. However, it is still quite far, and any space-probe sent to study it will either have to use up as little of its battery as it can until the rendezvous or be able to make only perfunctory observations of the dwarf planet. New Horizons is of the former kind – its primary mission is the farthest till date, and unlike the Voyager and Pioneer probes, will be able to respond to its environment agilely and be less susceptible to the vagaries of a dying battery.
2. Within the outer limits
Even if Pluto is among the outermost significant, planet-like bodies to orbit the Sun, it’s equally significant as being the largest body in the Kuiper Belt, a ring of asteroids like the one between the orbits of Mars and Jupiter. The belt starts from around the orbit of Neptune and extends to six AUs beyond the orbit of Pluto (AU is the astronomical unit, the distance between Earth and the Sun: 149.5 million km). It is also 200 times heavier than the Mars-Jupiter belt. Overall, both belts are important for two reasons in the context of New Horizons.
Before the Solar System took the form we now know – with a star at the centre, eight planets orbiting it, and two rings of asteroids – it comprised a young Sun at the centre of a massive disk of gases, dust and other materials called the protoplanetary disk. It is so named because it is out of this disk that the Solar System’s planets condensed, born as clumps of matter whose gravity accrued more matter, growing in size. And even as a planet formed, its gravitational pulls would ‘clean’ out a space in the protoplanetary disk, forming gaps. This phenomenon is visible among Saturn’s rings as well, with the space between rings having been cleaned out by the formation of small moons. The gaps in the disk survived to this day as the space between planets’ orbits. On the other hand, parts of the disk that didn’t get fully cleaned out formed the asteroid belts. So, they’re residues of the matter that the first planets were formed of, and studying them throws a lot of light on the history of the Solar System’s formation.
The second reason is that the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt are separated by 4.2 billion km – even on the cosmological scale, that’s a non-trivial gap. However, many objects in the two belts share chemical and physical properties as if they were once part of a common larger body. One logical explanation is that the belts were ‘mixed’ after they were formed. And to explain such mixing, astronomers have an awe-inspiring yet plausible explanation. According to them, as Jupiter was forming, its orbits moved closer to the Sun and then farther away, before shrinking down to place it between the inner asteroid belt and Saturn. The increase and decrease in the orbit’s size could’ve been due to the formation of other planets in the system, which would’ve disrupted the gravitational equilibrium. And while Jupiter moved, its prodigious gravity could’ve tugged a part of the inner asteroids out and vice versa, resulting in a mixed composition of asteroids in both belts. Since Pluto is the largest among Kuiper Belt objects, New Horizons studying it in detail could provide more clues about if such mixing could’ve happened.
3. Beyond the outer limits
Pluto is all of 2,300-km across – the distance between Kanyakumari in south Tamil Nadu and New Delhi – and it has five moons all to itself: Nix, Styx, Hydra, Charon and Kerberos. All of them are Kuiper Belt objects, too, and astronomers are curious to know if Pluto has a ring system as well, populated by smaller asteroids. The dwarf planet will also likely have smaller rocks orbiting it, and dust particles kicked up as a result of collisions between them. Such dust will be dangerous for New Horizons because they could impact the probe at some 50,000 km/hr and damage on-board systems. In January 2014, Simon Porter, one of the probe’s mission scientists, had told Wired that to protect against such collisions, his team had a contingency plan in mind: to turn the probe’s 2.1-metre-wide dish antenna into a shield.
If the probe does make it through the danger zone and get to within 12,500 km of the surface of Pluto, its observations of any rings as well as the dwarf planet’s surface, atmosphere and any craters/seismic activity will reveal more about the composition of Kuiper Belt objects, how they interact with each other, whether they sport any signs of violence from the past, and if at all they have atmospheres, what they’re composed of – information important to understand how and where the Solar System’s other planets could’ve formed. Astronomers also already know that Pluto’s surface has frozen methane and carbon monoxide.
This and other data gleaned from Pluto and its surroundings will take until late-2016 to be transmitted to Earth but the probe’s journey will continue – rather, has to continue because a probe that’s gone so far might as well just go farther because of the considerable time taken to travel such distances. Because the primary mission will almost exhaust its battery, the probe will subsequently become less manoeuvrable – like the Voyager and Pioneer probes did, yet still boast of a sophisticated suite of instruments. To take advantage, astronomers from the Southwest Research Institute, including Alex Parker, had spotted three other Kuiper Belt objects in New Horizons‘ path in late 2014 that would be interesting to study. All three objects are about 30-55 km across and located about 44 AU from the Sun, meaning the probe will reach them around 2020. This timeline is very interesting because NASA plans to launch the James Webb Space Telescope – successor to the Hubble and Spitzer space telescopes – in 2018. The JWST will be better equipped to study the Kuiper Belt objects than Hubble is, and its observations could be complemented by New Horizons‘.
It is probably from all these expectations that the probe draws its promising name. There are parallels to be drawn between its (impending) exploration of Pluto and the Kuiper Belt, and the space beyond, and how astronomers look into the older universe. The speed of light in vacuum is the highest possible speed in the universe, so when astronomers train their telescopes to look billions of lightyears in one direction, they’re simply looking billions of years into our past. The farther a part of the cosmos is from us, the older the light from it is – and the older the information it is carrying is. A parallel of this ingrained association between space and time can be drawn with the distance New Horizons is travelling and the more than four billions years into our past it will be able to see. Here’s waiting with bated breath…