Just a few days ago, the New Horizons probe made its flyby of Pluto after almost a decade in flight. Launched in January 2006 when Pluto was still a planet, New Horizons was halfway to Jupiter when the IAU declared that Pluto was now a “dwarf planet,” a decision that people still argue about to this day. This “demotion” had no effect on the mission of New Horizons, which continued to zip across the Solar System, faster than any probe ever launched. Now, New Horizons is transmitting the data back home to Earth and journeying on into the unknown. As we await more pictures, I’d like to take this time to address some misconceptions and common questions about Pluto and the mission.

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Source:
Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Source:

Regarding Pluto’s demotion…

When Pluto was discovered in 1930, it was thought to be much larger than it actually is – more around the size of Mercury. This remained the case until the 1970s, when Pluto’s largest moon Charon was discovered. Charon was responsible for the apparent larger size of Pluto, and the estimates were downgraded to being smaller than our Moon. The status quo was changed further in the 1990s and 2000s, when the Hubble Space Telescope and other tools began to discover other large icy bodies beyond the orbit of Pluto in the Kuiper belt. The largest of these, Eris, was first called Planet X and commonly nicknamed Xena, after the warrior princess. Size estimates for Eris were larger than Pluto, and other large bodies such as Makemake and Sedna were discovered, which led to a quandary. Would the solar system be expanded to include these new planets, putting the count at around 13-14? Or would these planets be demoted to a new category, and take Pluto and Charon with them? The eventual decision to create the category of “Dwarf planet” imposed some order (some astronomers called Pluto and Charon “double planets” and put the count at 10), but scientists still debate the ruling to this day.

Why doesn’t New Horizons orbit Pluto?

To paraphrase Douglas Adams, Pluto is far. Very far. You might think a trip down to the chemist’s is long, but that’s peanuts compared to Pluto.

It is over 30 times farther away from the Sun than the Earth is, and the Sun is just a very bright star in the sky. New Horizons left Earth faster than any probe we have ever launched, and made it to Jupiter in about a year to get a gravity assist, all so that it would arrive at Pluto in less than a few decades. The Voyager 2 probe, launched in 1977, did not arrive at Neptune until 1989, and Pluto is almost 5 billion miles further. When you have that much velocity, it’s very hard to slow down and orbit Pluto. One would either need to take a route that essentially matches Pluto’s orbit at around 18 km/s, which would take decades, or take a direct route with a massive rocket to slow it down. That rocket would have to be about the size of the Atlas V that launched New Horizons, and would in turn require a launcher about the size of a Saturn V moon rocket. There are some proposals out there that use a medium-lift rocket, such as the Ariane V (a European booster), as well as nuclear generators and a large number of ion thrusters to get a mission there in about 15 years, but such a mission would be costly and require plutonium reserves we do not have. So, for now, a flyby is the best we can get.

Why does New Horizons take so long to communicate?

Part of it is that it takes light around 5 hours to reach Pluto. The Moon has a delay of just a second or so, and Mars is anywhere from 10 minutes to an hour, but Pluto is way out there. But this is compounded by the fact that the communications array can only sent data at about 1 KB/S, which is dozens of times slower than even the slowest Internet connection. Even the Voyager probes, which were made 30 years before New Horizons, have faster download speeds. This is because New Horizons has a very small battery. The probe uses a nuclear RTG battery, which provides power from the decay of Plutonium-238. During the 1970s and ’80s, this isotope was in abundance, because it was produced when making nuclear weapons. But with the end of the Cold War, no new isotopes were being manufactured, and NASA had to be very sparing with what they could use it for. Luckily, NASA and the Department of Energy have begun to produce more plutonium, and NASA has enough in reserve for about one more mission to cover the gap. Even so, New Horizons had a fairly small battery, so small that it couldn’t use every instrument at once, and it does not have enough power to send a strong signal back to Earth, hence the slow speed.

Why can’t we just point Hubble at Pluto to get pictures? 

We have done that before, but those pictures aren’t very good. It makes a small pale dot turn into a slightly larger circle with the vaguest ideas of surface detail, due to changes in light and contrast. Hubble isn’t actually more powerful than Earth-based telescopes – if you pointed Hubble at Earth, the Statue of Liberty would be about one pixel – but Hubble has the advantage of being able to see much clearer without an atmosphere in the way. New Horizons has a much weaker telescope than Hubble, but has the advantage of passing very close to Pluto.

So is that it for New Horizons?

Maybe not. NASA planned for New Horizons to visit a second body in the Kuiper belt after Pluto. It still has a good amount of maneuvering fuel, so it could visit a body that lies sort of in the path of New Horizons. The problem is that NASA has not found such a body yet. They’re trying to get more time on Hubble in order to try to pinpoint a new place to fly by, but have had no such luck yet. Over the next few years, the plutonium battery will grow fainter and fainter, and eventually will not provide enough energy for New Horizons to take pictures or do science, so there is a deadline on trying to find a new planetary candidate.

Cover Photo: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Source:


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