Artist’s concept of the New Horizons spacecraft during its planned encounter with Pluto and its moon, Charon. Image: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
How many space-probes do we know that have been asleep for years on end before waking up in time for their scientific missions? Rosetta was the most recent, and still fresh in the public imagination. Some time before that was Voyager 1. These probes’ journeys provide interesting perspective for the times we live in, set to receive knowledge garnered from unexplored worlds billions of kilometers and many light-hours away1. These two probes will soon be joined by New Horizons, which on December 6 will beep back into life as it approaches the Solar System’s most controversial (dwarf) planet: Pluto.
NASA launched New Horizons in January 2006. In all this time, it has been cruising through the interplanetary realm, flying by Jupiter in 2007 and using the gas giant’s prodigious gravity to give itself a boost. By the time it encounters Pluto, it will be 4.6 billion kilometers from Earth. Even at its velocity of more than 10 km/s, it has taken 9.5 years to get where it is: about 250 days away.
To ensure it has enough juice to perform observations and dodge space-dust, mission engineers placed New Horizons in hibernation mode, not letting it perform any intense operations that could’ve induced wear and tear or battery consumption, It only sent a weekly signal home saying if everything was okay on-board. On December 6, this will change. New Horizons will be stoked back to life at 3 pm EST (4.30 am IST the next day). Ninety minutes later, the probe will send a signal to Earth reporting that it’s awake (but because it’s so far away, the signal will take more than four hours to reach mission control). Then, scientists and engineers will take six weeks to prepare the probe for its encounter with Pluto, which is expected to start January 15, 2015.
Fortune favors the prepared
New Horizons’ uniqueness lies with the fact that, together with all the probes that have traveled as far as it has or beyond, its primary scientific mission is the farthest to date. This means that, unlike Voyager 1 & 2 and Pioneer 10 & 11, New Horizons will be more active than passive, if only by virtue of having more juice leftover. It will be able to prioritize what to investigate instead of having to rely on good fortune.
For example, in September 2013, scientists at NASA announced that they had received signals in August 2012 that indicated Voyager 1 could have finally crossed the outer bounds of the Solar System and drifted into the interstellar medium. Apparently, however, there had been some confusion about the probe’s exact environment around February 2012 itself. Scientists were unsure from which direction certain charged particles were coming in, which would’ve indicated if Voyager 1 had entered the interstellar medium.
The only instrument on-board that could’ve quickly resolved the issue was a plasma sensor that could take but intermittent readings because of low battery. As a result, the scientists had to rely on good fortune, which came in the form of a massive burst of charged particles from the Sun in March. The particles set off a pattern of disturbances in the electromagnetic field around the probe that provided sufficient data to settle the matter: Voyager 1 was out and away.
The Kuiper Belt objects
In fact, New Horizons is already in an area of many unknowns. When it crossed the orbit of Neptune in August 2014, it entered the territory of a belt of rocks called the Kuiper Belt. The Kuiper Belt Objects (KBOs) have as much to say about the history of the Solar System as the Sun at its center has to say about its future. KBOs comprise rocky bodies and dust that didn’t clump together in the system’s younger days to form planets. They’re the system’s residues, and they have been pulled inward or pushed outward according to whichever planets’ paths they crossed.
This ‘path-crossing’ is evident in intriguing ways. For one, the distance between the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt is some 4.2 billion km. However, many of the bodies in the two belts have very similar physical and chemical characteristics, betraying a shared history. For another, Neptune’s moon Triton is the only moon with a retrograde orbit in the Solar System: it orbits Neptune in the direction opposite to its planet rotation. As a result, it is thought to have been captured by Neptune from among the KBOs.
These observations have encouraged a hypothetical model of the Solar System’s early years, when Jupiter didn’t form where it is right now – between the orbits of the asteroids and Saturn – but somewhere else. Then, it is thought to have moved outward, gathering up the KBOs, then inward dragging them along, depositing some of them in the asteroid belt, then moving back outward again, before ‘collecting’ some of them around itself, Saturn, Uranus and Neptune as some of their moons.
However, it’s not known for sure if this is what happened even if it helps explain strange similarities between bodies now in different parts of the Solar System. New Horizons could investigate the KBOs (Pluto’s declassification as not-a-planet was because astronomers think it’s just a large KBO) for signatures of these plausible inter-planetary migrations.
Whether or not it finds anything significant will also have important implications for why the Solar System has no planets that are heavier than Earth but lighter than Uranus. This feature has found to be increasingly anomalous because the NASA Kepler mission, as of December 2011, had discovered 680 of such so-called ‘super-Earths’, out of 2,326 planetary candidates. The chemical composition of KBOs could hold the clue because of the way the largest planets of their kind in the Solar System are composed. Both Uranus and Neptune are composed mainly of ice and rock; the largest planets are both gas giants; and the largest rocky planet is Earth. Of these, Uranus and Neptune could’ve been much heavier but aren’t for unknown reasons.
Dwarf Lord of the rings
Another curiosity that New Horizons can and will investigate is if Pluto has rings. Despite it just being a giant lump of rock about 2,300 km across, Pluto has five known moons (Charon, Nix, Hydra, Kerberos and Styx) of its own. Astronomers are curious if it also boasts a ring system, populated with smaller KBOs and dust particles kicked up as a result of collisions between themselves. This will be a tricky investigation because the dust particles could be moving around at almost 50,000 km/hr. If any of them hit New Horizons, the impact could be like a bullet and damage critical systems on-board.
According to Simon Porter, one of the New Horizons mission scientists, his colleagues will try to avoid a worst-case scenario like this by turning the probe’s 2.1-meter-wide dish antenna to face toward incoming projectiles and act like a shield. (At the same time, this dish antenna is a vital part of New Horizons because it enables effective communication from as much as 7.5 billion km away.)
Ultimately, if and when New Horizons manages to get within 27,000 km of Pluto, it will start mapping the dwarf planet’s surface, looking for signs of craters and tectonic activity, and study its atmosphere in detail. It’s known that the Pluto’s surface has frozen methane and carbon monoxide, frozen because the surface temperature is -230 degrees Celsius. Further investigation of these features will help scientists determine if Pluto is a KBO or a small planet, and the features will themselves throw light on how KBOs could’ve interacted among themselves when the Solar System was forming, and how that could’ve influenced how, where and when planets formed.
Away and beyond
New Horizons will accumulate enough data in the process that it will take until late 2016 to completely transmit it to Earth. In the meantime, the probe will continue onward in its journey to studying KBOs. Already, using the Hubble space telescope, astronomers have selected three prospective Kuiper Belt denizens for New Horizons to study, designated PT1, PT2 and PT3. They’re located about 44 AU from the Sun, which means the probe will be able to reach them by 20203. Each measures about 30-55 km across. After the Pluto mission, New Horizons’ maneuverability will decrease, and the probe will rely on good ol’ fortune to study the three candidates.
Pluto is now about 40 AU (5.98 billion km) from the Sun. The Solar System, however, extends to a distance of at least 121 AU (18.1 billion km), based on where Voyager 1 encountered the interstellar medium. This is a region of space called the heliopause, where the stream of charged particles being constantly emitted by the Sun encounters streams of particles from other stars. Here, the Solar System ends. Between the KBOs and the heliopause is a vast region of space populated by scattered rocks.
The farther outward the probe goes, the further it will be able to investigate the history of the Solar System. It’s not coincidental that the as yet unanswered questions about the Solar System concern its past and the least explored regions of the Solar System are its outer fringes – fringes that span a radial distance of 41-times the distance between the Sun and Pluto (as it is now). In this not-so-empty space, the Sun’s influence is weak and steadily tapering off, vulnerable to particulate encroachments from neighboring stars.
It is hard to imagine that the Solar System could’ve formed without any external influences. The Sun was formed after a part of a larger gas cloud collapsed under its own gravitational strength, and the remaining gas, dust and debris went on to form the planets, asteroids, KBOs and even some comets. Beyond Pluto, until the heliopause and into the interstellar medium – as New Horizons sails into these regions, it will join a veteran fleet of probes looking for any clue they can find that will tell us how we came to be, and if we’re special at all.
1Fitting that they happen now because the 2010s is when the NASA Deep Space Network turns 50. (“From rovers on the surface of Mars to Voyager 1 near the edge of the solar system, spacecraft regularly call home to Earth. For five decades, the Deep Space Network has been at the other end of the line.” Wow!)
2But ideas travel faster. When New Horizons was launched, Pluto was still a planet.
3By this time, the James Webb Space Telescope is expected to be launched. Planned as a successor to the Hubble and Spitzer space telescopes, the JWST will be specialized to observe in the long-wavelength visible to mid-infrared parts of the spectrum, giving it more resolution than the Hubble had to study the KBOs.
Featured image credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute