(27 July 2015)
Photographs have the power to answer long-standing questions.
Pluto is one of the last objects in our Solar System to be explored. Pioneer 10 and 11 and Voyager 1 and 2 provided photographs, science, and insights about the gas giants: Jupiter, Saturn, Uranus, and Neptune. New Horizons is the latest spacecraft to encounter and fly past the outer reaches of the Solar System, and eventually wander into deep interstellar space.
- Long arduous travel and journey? Check.
- To a place where human technology will reach for the very first time? Check.
- Learn new things about a previously unexplored corner of our “backyard” known as the Solar System? Check.
New Horizons made its close-approach flyby at 1148h UTC on 14 July 2015 (0748h EDT in North America), passing within 12500 kilometres of Pluto’s surface and 28800 kilometres from Pluto’s primary moon Charon.
A few basics
- Distance between Earth & Sun about 150 million kilometres; known also as 1 “Astronomical Unit”, or 1 AU.
- One-way light travel-time, Sun to Earth: 8.3 minutes
- Distance to Earth when New Horizons flew past Pluto: 32 AU, shade under 5 billion kilometres
- One-way light travel-time, New Horizons to Earth: 4.5 hours
Over the years we’ve had limited insights to Pluto, even though its status was switched from “planet” to “dwarf planet” by the International Astronomical Union. Images immediately before the flyby encounter hinted tantalizingly of ice and terrestrial-like surface features.
Does Pluto have more than one moon or satellite?
Pre-encounter imaging showed the existence of four additional satellites, bringing thus far a total of five moons for Pluto: Charon, Styx, Nix, Kerberos, and Hydra.
How does Charon look like?
Pluto’s primary moon, Charon, is 1200 km across, or about one-half the size of Pluto. This satellite-to-primary size-ratio is one of the largest in the Solar System. By comparison, the Moon-to-Earth size-ratio is between one-quarter and one-third (0.27).
Is there evidence of (recent) geological processes on Pluto?
The presence of frozen hydrocarbon ice-plains, nitrogen ice-flow patterns, and high mountains suggest ongoing dynamic processes on the planet’s surface, despite the very low temperatures.
Is there an atmosphere on Pluto?
What happens when a spacecraft flies past a (dwarf) planet and looks back into the backlit shadow? What the following shows is an answer to that question: Pluto has an atmosphere with two hydrocarbon layers between the surface and a height of 80 kilometres. If there was no atmosphere, there would be no ring of scattered light around the disk.
Imagine a picture of your region, your city from a geostationary satellite from a height of over 35 thousand kilometres above the Earth’s surface. Reduce the distance by a factor of about 30 to a height of 1000 kilometres. Reduce the distance by a factor of 1000 to a height of 1 kilometre. Reduce the distance by another factor of 1000 to a height of 1 metre. You’re not going to see the roses in your garden from the geostationary satellite, but from 1 metre, not only will you see the roses, you’ll also see the bees. And that’s a reduction of distance by a factor of 35 million (i.e., 35 thousand kilometres divided by 0.001 kilometre).
So, it’s remarkable how distance to an object changes the amount of detail you see on the object and the way you view that very same object. To Pluto a spacecraft was sent, specifically to “check in” and have a look at the planet and its surroundings. From a distance of 5 billion kilometres to a flyby closeup encounter of 13 thousand kilometres above the planet’s surface, Pluto’s appearance goes from a faint fuzzy blob to being able to discern surface features for the first time, with a reduction of distance by a factor of almost 400 thousand.
All this appeals to my inner physicist/astronomer who’s been fascinated with the planets from a young age, and to my traveler/photographer for the pictures coming from a place at the outer reaches of the Solar System. Given bandwidth limitations imposed by onboard hardware and the amount of data recorded by New Horizons, there are months of downloads for NASA scientists and undoubtedly more surprises to come.