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clyde tombaugh, New Horizons mission, Pluto

What Would Clyde Think?

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Five years ago, the world paused for a moment to admire the view of a distant world called Pluto. Its discoverer, Clyde Tombaugh wasn’t around to see the spectacular New Horizons views of the planet he discovered in 1930. But, in another sense, he WAS there with the rest of us.

Pluto's colorful surface. Pluto was discovered by Clyde Tombaugh in 1930 and explored by New Horizons in 2015.
Pluto’s colorful surface units show different compositions and terrains. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

A few grams of Clyde’s ashes fly on the New Horizons spacecraft that flew by his planet. So, for a collective moment, we explored Pluto with its discoverer and the team of scientists he inspired.

The late Clyde Tombaugh, discoverer of Pluto. courtesy NMSU.
The late Clyde Tombaugh in 1989. He died in 1997 at age 90,. Courtesy NMSU.

I’ve often wondered what Clyde would think of Pluto, up-close, and personal. I only met him a few times. He was a man with a great sense of humor and humility about his accomplishment. So, I imagine that he’d love it all. It would have been cool to have him on the science team as New Horizons flew by. His family members attended the flyby festivities in mid-July 2015 and celebrated the mission.


A Virtual Pluto Celebration

Today, we are in a very different world, one where a pandemic has closed us off to social gatherings like the one we all experienced at “mission control” at Johns Hopkins Applied Physics lab half a decade ago. Instead of meeting to talk over old times at Pluto, now we have to do it virtually.

So, instead, let’s imagine a circle of scientists sitting here with us. They’re reminiscing about the amazing mission to Clyde’s world. I asked a collection of New Horizons researchers to do exactly that. What did they learn about Pluto? If they could take Clyde on a guided tour, what would they show and tell him about Pluto?

Show and Tell for Clyde

Obviously, the first thing to show him would be heart-shaped Tombaugh Regio area of Pluto. It became THE ‘iconic’ view of the planet from the beginning of the flyby. That image changed forever how we saw that distant planet. Its icy terrain and lobe-shaped Sputnik Planitia section show glaciers butting up against mountains of water ice. Those features would likely excite Clyde. Those who knew him would be expecting him to crack a few puns to celebrate the occasion.

Mountains on Pluto
This image shows the inset in context next to a larger view that covers most of Pluto’s encounter hemisphere. The inset was obtained by the Multispectral Visible Imaging Camera (MVIC) instrument on New Horizons. North is up; illumination is from the top-left of the image. The image resolution is about 1050 feet (320 meters) per pixel. The image measures a little over 300 miles (almost 500 kilometers) long and about 210 miles (340 kilometers) wide. It was obtained at a range of approximately 9,950 miles (16,000 kilometers) from Pluto, about 12 minutes before New Horizons’ closest approach to Pluto on July 14, 2015. Courtesy NASA/JHU-APL, SWRI/New Horizons mission.

But, he might also be touched by other views. The most telling and effective image of Pluto is a view of its atmosphere and surface together. It encapsulated the planet, its atmosphere, and a feeling.

Pluto's mountains, plains, and layered hazy atmosphere.

For Anne Verbiscer, a member of the New Horizons team, that’s the view she would share with him. “I’d show him that image taken at twilight,” she said. “I was reduced to tears when I saw it. I’ve had the same reaction to only one other planetary image returned by a spacecraft…it was the first image of Enceladus taken by Cassini for which I had done the “commanding”. Enceladus was in eclipse (behind Saturn), so I had very little information to use to set the exposure time correctly. I was sure that the image would either be black or over-exposed. When that image arrived on the ground, I saw that it was perfect, and yes, I sobbed, just like I did when I saw Clyde’s amazing world in all its glory on that September weekend in 2015.”

That view reminds all of us of the strange beauty of planetary exploration. There’s always something new to be seen, and when the exploration ends, there’s much to learn from the data collected. “I would show Clyde that image of Pluto at twilight (the one with the bottom of Sputnik Planitia in the foreground, the mountains rising above the limb, and the hazes above the horizon)….. and just drop the mic,” said Anne.

Clyde’s Surprising World

Principal investigator Alan Stern, not often at a loss for words, finds it hard to say what surprised him most about Pluto. “Before the flyby I might have not been surprised to find one of these: an interior ocean, or blue skies, or bladed mountains, or vast glaciers, or cryovolcanoes, or a paleolake,” he said. “But we found them all! And that is the most surprising aspect to me. That is, just how incredibly complex and diverse and frankly amazing as a scientific wonderland that Pluto turned out to be.”

To give you an idea of what amazed Alan and the others, take a look back in time. Clyde’s view in 1930 was of a small dot. That’s about all any of us can see, even with the best ground-based telescopes Even a high-resolution Hubble Space Telescope view showed a bland surface with a few bright and dark areas. No details of what was to come.

A Surprising World

New Horizons showed that every region on Pluto has something different on it, with some features unique to that world alone. Five years later, after much study and analysis, all that geology tells us something we didn’t know about the planet before the spacecraft went by: it’s not a dead world. “It shows the action of a wide range of planetary processes,” said Hal Weaver, another team member and long time planetary scientist. “That includes some that happened long ago, and others indicating ongoing activity into the current epoch.”

A closeup of the highlands on the edge of Sputnik Planum. There are also ice pits and other features indicating some kind of geologic activity in the past and possibly in the present. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
A closeup of the highlands on the edge of Sputnik Planum. There are also ice pits and other features indicating some kind of geologic activity in the past and possibly in the present. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

In other words, Pluto is alive. That’s a surprise, even today. And, that’s what Hal would tell Clyde first. If he could go to Pluto on a guided tour, the first place Hal would land would be Sputnik Planitia. “It’s responsible for Pluto’s “mojo”,” he says. “Especially its periphery. And, it’s part of the region named for Clyde! And, I’d bring along his extended family, too!”

Pluto and the followup mission to Arrokoth have accomplished another amazing feat. Those flybys prove the outer solar system isn’t what people expected it to be. “Pluto and Arrokoth have showed us that the Kuiper Belt is so full of surprises and revelations about the formation and evolution of the solar system,” said Paul Schenk, of the Lunar and Planetary Institute in Houston. “Going back to the Belt should be a high priority, to look at small, medium, and large-sized bodies and find out what happened there in the early days of our solar neighborhood.”

Beyond Clyde’s World

Nowadays, New Horizons is far beyond the Plutonian system. A few years ago, it explored Arrokoth, which is another Kuiper Belt object. It has other tasks ahead of it, including possible visits to new worlds. Its story is really one of opening the exploration of the third regime of the solar system. We’ve explored the inner solar system, the “middle” solar system of the gas and ice giants, and now the distant regime is open to our view.

Indeed, the exploration of the outer solar system really began with this mission. It is the first spacecraft to give us a true “up close and personal” look at the Kuiper Belt neighborhood. It’s only fitting that another spacecraft should make its way out there, to follow up on what New Horizons started.

Team member Kelsi Singer pointed out that there would be much to explore. “If we could go back with a spacecraft that orbits Pluto [instead of flying by], we could learn if Pluto has an ocean beneath its icy shell,” she said. “We could also further investigate how all its unique features formed. Of course, a lander or rover would be fabulous, but we have much to learn from an orbiter, first.”

Celebrating a Milestone with Clyde

Pluto and its exploration five years ago sparked our imaginations. It opened our eyes to an alien world. And, it shows us that preconceptions about planets are meant to be shattered. If New Horizons does nothing else, it will have shown us to expect the unexpected at every turn.

Alan Stern, who waited many long years for that exploration, working with some of the best scientists in the solar system, would just as soon be out there in an orbiter right now. He and a continuing team of scientists want to follow the trail that their spacecraft blazed.

And, if he could take Clyde along with him on that trip, he would. “I’d tell Clyde, who I knew, that he discovered an amazing world that has literally revolutionized our knowledge of planetary science in multiple important ways, and that in my view, we now know that in Pluto, the solar system saved the best for last!”

The backlit view of Pluto as New Horizons ‘waved’ a last good bye. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
astronomy, New Horizons mission, parallax

parallax shift from new horizons

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Check out these sets of images. What you see the phenomenon called “parallax shift”. The first image in each set is the view of a star, either Proxima Centauri or Wolf 359, as seen from Earth. The second view is from a very unique perspective: the New Horizons spacecraft.

Earth-based and New Horizons views of the star Proxima Centauri.
A comparison of views of the star Proxima Centauri from Earth and the New Horizons spacecraft, illustrating the parallax shift in view between the two locations. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/University of Louisville/Harvard and Smithsonian Center for Astrophysics/Mt. Lemmon Observatory.
New Horizons and earth-based view of Wolf 359.
A comparison of views of the nearby star Wolf 359 from Earth and the New Horizons spacecraft, illustrating the parallax shift in view between the two locations. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/University of Louisville/Harvard and Smithsonian Center for Astrophysics/Mt. Lemmon Observatory.

New Horizons is currently about 6.6 billion kilometers away from Earth, traveling out through the Kuiper Belt. That distance gives it a very different view of the two stars.

Notice how the scene shifts in each view. That’s the effect of parallax—viewing an object from two different locations. Seeing it like this is reminiscent of Clyde Tombaugh’s view of Pluto through a blink comparator. In that case, Tombaugh took images of Pluto as it orbited the Sun. Over a period of time, the planet itself moved against the backdrop of stars. In the case of the two stars, however, we aren’t getting a view of two stars moving. What’s changing is OUR point of view between Earth and New Horizons.

You can do this same experiment by observing both stars from Earth. However, it would take half a year to see the two different views. That’s how astronomers have done simple parallax measurements for quite some time. But, this time is different. It’s the first time that a spacecraft has been used for this purpose. It has sent back pictures of the sky with stars appearing in different positions than the way they appear to us on Earth.

It’s a record-breaking achievement for a spacecraft that has already done quite a number of really cool things. It passed by dwarf planet Pluto in 2015. Then, it flew by the Kuiper Belt object Arrokoth in early 2019. On its way out, the spacecraft imaged Jupiter and measured dust in the solar system.

New Horizons and Parallax Shift

Caputuring the parallax shift from its current vantage point in the solar system was pretty straightforward for the spacecraft. It focused its long-range cameras in the LORRI instrument on Proxima Centauri and Wolf 359 in mid-April, 2020. It wasn’t the only camera to do so. At the same time, amateur and professional astronomers on Earth also imaged the two stars.

The resulting parallax measurement was something Stern suggested as a possible New Horizons project a while back. The result is a scene he described as an alien view. “It’s unlike what we would see from Earth,” he said. “It has allowed us to do something that has never been accomplished before, to see the nearest stars visibly displaced from the positions we see them on Earth.”

Why Measure Parallax Shift?

Parallax shifts like this one are the basic building block of distance measurements, particularly inside our own galaxy. Until now, the most common way was to look at a star and measure its position against the distant stars. Then, wait until Earth moves along in its orbit for six months, and measure the position of the star again. Its apparent position will have shifted a tiny bit against the distant stars. Using the measurements and some trigonometry, astronomers can get distances to stars within about 300 light-years of Earth.

The New Horizons method took advantage of the spacecraft being very far away from the Sun to image the stars. But, Alan pointed out an added twist: “Those same images and measurements allowed us to calculate exactly where the spacecraft is.”

The experiment on New Horizons is the largest parallax “baseline”—6.4 billion miles long—ever used to calculate distances to stars. Compare that to the two astronomical units that the traditional method uses.

Parallax Shift Provides a Stereo View

This achievement includes an interesting wrinkle: 3D stereoscopic viewing. Three team scientists put their heads together to do something special. They were astronomer Tod Lauer, New Horizons Deputy Project Scientist John Spencer, of SwRI, and astrophysicist, Queen guitarist and stereo imaging enthusiast Brian May. They created images that clearly show the effect of the vast distance between Earth and the two nearby stars. May, who posts and sells stereoscopy images on his own London Stereoscopic Company website, was enthused about the project.

“It could be argued that in astro-stereoscopy—3D images of astronomical objects—NASA’s New Horizons team already leads the field, having delivered astounding stereoscopic images of both Pluto and the remote Kuiper Belt object Arrokoth,” May said. “But the latest New Horizons stereoscopic experiment breaks all records. These photographs of Proxima Centauri and Wolf 359, stars that are well-known to amateur astronomers and science fiction aficionados alike, employ the largest distance between viewpoints ever achieved in 180 years of stereoscopy!”

You can check these images out here and do a little stereo viewing yourself. For those curious about the project, there’s a special “Ask Me Anything” on Reddit on June 12, 2020, at reddit.com/r.askscience. It starts at 1 p.m. EDT. Experts (including Brian May!) will be standing by!

Heralding the Earth-based Parallax Shift Team

To be sure, New Horizons had some pretty impressive help from the ground. That included Earth-based imaging from professional and amateur observers. They came from the Cumbres Observatory, through a remote telescope at Siding Spring Observatory in Australia, and from astronomers John Kielkopf, University of Louisville, and Karen Collins, Harvard and Smithsonian Center for Astrophysics, operating a remote telescope at Mt. Lemmon Observatory in Arizona. It was an extraordinary melding of amateur and professional astronomy along with spacecraft imaging. According to Lauer, the results were spectacular. “The images collected on Earth when New Horizons was observing Proxima Centauri and Wolf 359 really exceeded my expectations.”

Their sheer closeness to the Sun is one of the most important reasons the New Horizons/Earth-based parallax experiment worked so well.

About the Parallax Shifted Stars

Earlier, Brian May mentioned that the two stars, Proxima Centauri and Wolf 359, are dear to astronomers and science-fiction readers alike. Why is that? Well, let’s look at Proxima Centauri. First, it’s in the closest star system to Earth, at a distance of 4.24 light-years from us. It’s thought to have a planet, called Proxima Cen b. And, in the science fiction realm, it’s been in a number of works, including video games. The name “Proxima Centauri” just sounds alien and cool, and I’m sure that’s part of the allure for fiction fans.

Wolf 359 is also a close neighbor of ours, at a distance of 7.9 light-years. It’s a red dwarf, and is a fairly faint, low-mass star. It appears to have two planets, called c and d. Because it, too, is a neighbor, it also features in science fiction books, TVs, movies, and in games. Star Trek fans will recognize it as the site of an important war in Federation history. It took place between the cube-inhabiting Borg species and the Federation fleet in the Battle of Wolf 359.

Will New Horizons do another parallax measurement? According to Stern, probably not soon. “We have to reserve the cameras onboard for other planned observations,” he said. There’s still more to come from the spacecraft, particularly as it transits the Kuiper Belt over the next years.