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ASTRONOMY

A voyage to Pluto

After successful observation of the dwarf planet, the New Horizons probe had to survive threats of mission cancellation

First image of the Plutonian atmosphere, obtained seven hours after the closest approach: hazier and higher than expected

NASA/JHUAPL/SwRIFirst image of the Plutonian atmosphere, obtained seven hours after the closest approach: hazier and higher than expectedNASA/JHUAPL/SwRI

The successful completion of the first mission to Pluto has received expansive news coverage, but almost nothing has been said about the work it took to get there—challenges both technological and political in nature. The New Horizons probe was one of the most enterprising—and endangered—projects ever carried out by NASA, the U.S. space agency. The observation of the planet (or ex-planet, by decision of the International Astronomical Union) of which we knew so little has now revealed features of surprising geography and composition, and promises much more to come. The findings are expected to keep scientists the world over very busy for at least a decade.

The technical challenge alone was extraordinary. In its journey from Earth, the probe had to be guided through space so as to pass through an imaginary 150 x 100-km rectangle nearly five billion km from here. In an illustrative comparison, Glen Fountain, the New Horizons project manager, said it was like hitting a golf ball from New York to Los Angeles and getting a hole in one—on the first try.

And what would happen if the probe failed to pass through that imaginary channel on its final approach to Pluto? Basically, it would be pointing its instruments into empty space, and the intended objects of study would not be in the expected places. The entire program of observations had to be automated and stored in the probe’s computers days before the closest approach, with no margin for last-minute corrections.

In a certain sense, New Horizons reproduced the successes of the Voyager 1 and 2 probes that visited the four largest planets of our Solar System—Jupiter, Saturn, Uranus and Neptune—in 1970 and 1980 before leaving the Solar System forever. But a mission to Pluto requires a greater degree of accuracy. Not only is the planet farther away than any of the targets visited by Voyager, but it also moves more slowly, which complicates the task of accurately calculating its orbit around the Sun, and therefore its position at any point in time. The Plutonian system was so uncharted that, during the early preparation phase of the probe in 2001, only its largest moon, Charon, was known. In 2005, while conducting reconnaissance observations with the Hubble Space Telescope, astronomers discovered two more moons, Nix and Hydra. And it was not until 2011 and 2012, late in the New Horizons mission, that the last two known moons, Kerberos and Styx, were found. It is quite possible that the images captured by New Horizons will reveal more objects in the vicinity. “I think that, with the images yet to be sent, chances are high that new satellites will be discovered,” says Silvia Giuliatti Winter, an orbital dynamics expert at São Paulo State University (Unesp) in Guaratinguetá.

Computer models developed by Giuliatti Winter helped the New Horizons team plan the safest passage during the flyby, as the probe passed just 12,500 km away from the surface of the dwarf planet (See Pesquisa FAPESP Issue nº 210).

During the years preceding the launch, however, the biggest threat to the mission was a much more prosaic one: U.S. government budget cuts at NASA. In 2000, NASA decided to cancel the then-ongoing project, known at the time as the Pluto Kuiper Express, which was being carried out by its Jet Propulsion Laboratory (JPL). But the U.S. Congress, which has historically rallied in favor of planetary science expeditions, reinstated the mission, and NASA issued an Announcement of Opportunity, in which it solicited lower-cost ideas. The new effort gave rise to New Horizons, operated by Johns Hopkins University’s Applied Physics Laboratory (APL). It gave Alan Stern, chief scientist for the mission, the chance to implement plans he had been developing since 1989.

Official portrait: heart-shaped area was given the name Tombaugh Regio

NASA/JHUAPL/SwRIOfficial portrait: heart-shaped area was given the name Tombaugh RegioNASA/JHUAPL/SwRI

Although the price tag for the new mission was lower than that of the old Pluto Kuiper Express, the outlay came to a respectable $720 million. But even after the mission was formally approved in 2001, it ran into trouble. In 2002, the White House—at the time occupied by George W. Bush—tried to cancel the mission again, but Congress once again prevailed. The scientific findings, not to mention the media frenzy that has reigned since July 2014, have been the direct consequence of the timely interventions of the U.S. Congress.

The third zone
The exploration of Pluto marks the first visit by a spacecraft to a previously unexplored part of the Solar System. The inner regions are inhabited by the rocky planets, of which Earth is the largest representative. Farther out are the gas giants, Jupiter being the largest. And beyond Neptune’s orbit, we have a cluster of hundreds of thousands of objects—known as the Kuiper Belt—where Pluto is the largest resident.

“The flyby by the space probe is a milestone in our knowledge of the so-called third zone of the Solar system,” says Giuliatti Winter. “I think the data sent by the New Horizons probe will bring surprises from Pluto and the Kuiper Belt. We’ll probably have to review and adapt the dynamic models of the formation and evolution of the objects in our Solar System.”

In a recent paper published in early 2015, the Unesp researcher and her colleagues suggested that Pluto may have been formed in a more internal region of the Solar System, and only later migrated to the Kuiper Belt, without losing its satellites. There is reason to doubt that the dwarf planet originated in the Kuiper Belt, because there would not have been enough mass there to produce an object of that kind 4.5 billion years ago. It remains a controversial issue.

There was no lack of controversy or surprise in the first images captured during the final approach. They revealed a rather unexpected geological scenario. Mountain ranges of water ice and geologically young terrain less than 100 million years old contrast with darker regions populated by craters, emblematic of terrain untouched for billions of years. All indications are that there are processes driven by internal heat on Pluto—which is not easily explained by the current geophysical models. The terrain observed on the surface of this small world measuring 2,372 km in diameter, which was thought to be geologically dead, could even signal the existence of an ocean of liquid water beneath the depths of its enormous ice crust. “It looks like the Solar System decided to save the best for last,” joked Alan Stern in one of the many collective interviews he gave to present the first scientific findings from New Horizons.

Clyde Tombaugh at Lowell Observatory, where in 1930 he discovered the planet...

Lowell Observatory archivesClyde Tombaugh at Lowell Observatory, where in 1930 he discovered the planet…Lowell Observatory archives

The surface appears to contain ices of various substances—water in the case of the mountains, but in particular methane and other organic compounds, nitrogen and carbon monoxide. The latter is apparently an important component of Pluto’s brightest, smoothest region, which the team has named Tombaugh Regio in honor of Clyde Tombaugh, the American astronomer who discovered the dwarf planet.

The researchers also found a few surprises in Pluto’s atmosphere. Some of the atmospheric models suggest that the Plutonian air may be just a temporary phenomenon, freezing and collapsing when the planet approaches its aphelion (the point farthest from the Sun), and then vaporizing again at its perihelion (the point closest to the Sun). Proving this hypothesis was one of the arguments that saved the mission from cancellation in 2002. Now, 25 years after the last perihelion, is a good time to analyze the Plutonian atmosphere and test the models. It has not been possible to determine if this really happens, based on the data collected so far. But we already know that the atmosphere is a little colder and thinner than scientists had envisioned.

One of the Brazilian scientists most interested in these findings in particular is Felipe Braga-Ribas of the Federal Technological University of Paraná (UTFPR) in Curitiba. He studies phenomena known as stellar occultations—points in time when objects in the Kuiper Belt, such as Pluto, pass in front of a more distant star, as seen from here on Earth. By observing the star’s obscuration pattern as it becomes hidden, first behind the atmosphere and then behind the surface of Pluto, atmospheric properties such as pressure or temperature can be inferred.

“The stellar occultation data complement the data obtained by New Horizons’ Alice ultraviolet instrument,” says Braga-Ribas. “Alice can measure the atmosphere to a height of some 170 km, and with occultations we can go the rest of the way, to near the surface.”

Braga-Ribas hopes that the extraordinary data source that New Horizons will provide will help in deciphering the present-day atmosphere of Pluto. In combination with new stellar occultations, the data will enable the researchers to investigate how it evolves over time, which will then make it possible to confirm processes such as the temporary atmospheric collapse hypothesis.

Charon, the largest moon, has also proven to be especially intriguing. Its surface is not as renewed as that of Pluto, but nevertheless it is younger than expected, and a dark region at the pole is a complete mystery to scientists.

The future
The scientists’ high level of enthusiasm about the data produced by the probe recalls that of fans at the final game of the championship—literally. “The week of the flyby, after the last talk at NASA, my team members and I went with the whole New Horizons team to a Washington Nationals baseball game,” says André Amarante, a researcher from the group at Unesp in Guaratinguetá, who is currently doing part of his doctoral research at the University of Maryland near Laurel, the city where APL is headquartered.

...whose name was suggested by 11-year-old Venetia Burney, relayed in a telegram from astronomer H. H. Turner

Lowell Observatory archives…whose name was suggested by 11-year-old Venetia Burney, relayed in a telegram from astronomer H. H. TurnerLowell Observatory archives

“The game had to be stopped a number of times due to power outages, and during those times, Alan Stern started showing us images of Pluto, right on his cell phone. It was great! Now we know where the New Horizons signal is going,” Amarante jokes.

His interest lies specifically in finding objects in stable regions around the known moons. “In the computer simulations we did, we discovered there’s a real possibility that some of these stable regions may contain a population of objects called Trojans, which share the orbit of a given moon,” he says. “For this reason, we are very anxious for the New Horizons data.”

The anxiousness is expected to continue for quite some time. The data transmission rate from beyond the confines of the Solar System is lower than that of the old dial-up internet connections. The researchers will have to wait 16 months to fully download the five gigabytes of data produced during the Pluto flyby. Judging from what has come in so far, it will be worth the wait.

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