What if you could ride a comet?

Comets zoom around the Sun. Some grow long, glowing tails.

What if you could climb on one and go for a ride? What would it be like?

Comets are icy travelers that loop around the Sun, sometimes growing long, glowing tails. What if you could hitch a ride on one? What would you stand on? How fast would you go? Could you jump? And where would the tail be?

Nobody has ridden a comet, but a real spacecraft has landed on one. This thought experiment uses what scientists learned to imagine the ride.

A comet nucleus is a small, icy body on an elongated orbit; near the Sun it becomes active, building a coma and tails. ESA's Rosetta mission orbited comet 67P/Churyumov–Gerasimenko from 2014 to 2016, and its lander, Philae, made the first touchdown on a comet.

This thought experiment combines those measurements with NASA's comet facts to imagine what riding a nucleus would involve. Measured results are facts; how a human rider would fare is labeled as an idea.

The middle of a comet is a big lump of ice and dust. People call it a dirty snowball.

It is only a few kilometers wide. That's tiny for something in space!

The solid heart of a comet, its nucleus, is often a few miles across, NASA says. It's made mostly of ice coated with dark dust and other material, which is why comets are nicknamed 'dirty snowballs'. Comets are leftovers from when the solar system formed, about 4.6 billion years ago.

Rosetta's comet, 67P, is about 4 kilometers across. Its surface is covered in dust and rubble, from tiny grains to boulders a meter wide.

NASA describes comets as leftovers from the solar system's formation about 4.6 billion years ago, mostly ice coated with dark organic material, with a nucleus often a few miles across. ESA estimated 67P at about 4 km before arrival; Rosetta then revealed its double-lobed shape.

Philae's descent camera showed a surface covered in dust and debris from millimeters to meters in size. Rosetta also detected molecular oxygen and nitrogen, water with a different 'flavor' from that in Earth's oceans, and the amino acid glycine (ESA).

If you stood on a comet, you would weigh almost nothing.

A tiny jump might send you floating off into space! You would need to hold on tight.

Imagine climbing onto comet 67P. It's so small that its gravity is several hundred thousand times weaker than Earth's, ESA says. You'd weigh almost nothing, and a small hop might carry you a long way before you drifted down, or not come down at all.

You'd also be moving fast: 67P travels at up to 135,000 kilometers per hour around the Sun. And a day there, one turn of the comet, lasts 12.4 hours.

ESA gives 67P's surface gravity as several hundred thousand times weaker than Earth's; landing was planned at no more than walking pace, with a harpoon to anchor Philae and keep it from bouncing back into space. The nucleus travels at up to 135,000 km/h along an orbit of about 6.5 years, between roughly 186 million and 800 million km from the Sun, and rotates once every 12.4 hours.

How a human would move there is an idea: with so little weight, ordinary steps could lift a rider off the surface, much as Philae's touchdown did.

Far from the Sun, the comet is frozen and quiet.

Near the Sun, it warms up. Ice turns into gas and puffs out with dust. That makes a glowing cloud and long tails!

Far from the Sun, a comet is frozen and quiet. As it gets closer, the Sun's heat turns some of its ice straight into gas, which carries dust out with it. That builds a fuzzy cloud around the nucleus, the coma, which can stretch hundreds of thousands of kilometers.

Sunlight and the solar wind push the gas and dust away into two tails, one of dust and one of gas, both pointing away from the Sun. So your ride would get busier, dustier and gassier as you neared the Sun.

Near the Sun, solar heating sublimates ices; outflowing gas drags dust into the coma, which may extend hundreds of thousands of km (NASA). Radiation pressure and the solar wind push material into a dust tail and an ion tail directed away from the Sun.

ESA notes that 67P showed minimal activity at Rosetta's August 2014 arrival, 539 million km from the Sun, and became more active toward perihelion on 13 August 2015 at 186 million km. Rosetta watched the coma develop at close range, as no spacecraft had before.

In 2014, a robot lander named Philae landed on a comet. Its harpoons did not work.

So it bounced! It floated up and came down again. Then it hopped once more and stopped.

ESA's Rosetta spacecraft launched in 2004 and traveled ten years to reach comet 67P. On November 12, 2014, it sent its lander, Philae, down to the surface. Philae's harpoons didn't fire, and in the weak gravity it bounced: it floated for 1 hour and 50 minutes, traveling about 1 kilometer, then made a smaller hop before settling.

Rosetta followed the comet around the Sun and ended its mission on September 30, 2016, by descending onto the comet.

Launched on 2 March 2004, Rosetta needed four planetary gravity assists and over 6.4 billion km of travel to reach 67P in August 2014. Philae touched the surface three times on 12 November 2014, at 15:34, 17:25 and 17:32 GMT: with its harpoons unfired, it rebounded for 1 hour 50 minutes, covering about 1 km at 38 cm/s, then made a second hop at about 3 cm/s (ESA).

Signals took over 28 minutes to reach Earth. Rosetta passed perihelion with the comet in August 2015 and ended its mission with a descent to the surface on 30 September 2016.