Philae's On a Comet, Hayabusa2 is Off To an Asteroid — What's the Difference?

Robotic space science missions to comets and asteroids are in the news right now because of Europe’s Rosetta/Philae mission to Comet 67P and Japan’s imminent launch of Hayabusa2 to an asteroid.   Many may wonder what the difference is between comets and asteroids and what other spacecraft have investigated them.

Nancy Atkinson at Universe Today explains that the biggest difference is their composition: “While asteroids consist of metals and rocky material, comets are made up of ice, dust, rocky materials and organic compounds.  When comets get closer to the Sun, they lose material with each orbit because some of their ice melts and vaporizes.  Asteroids typically remain solid, even when near the Sun.”  Another difference is the population, she adds, with millions of asteroids, but only about 4,000 comets, having been discovered so far.  There may be many more of each, but that is the count to date.

Several robotic space missions have been sent to study both asteroids and comets already.  Rosetta and its Philae lander are particularly newsworthy because they are the first to orbit and land on a comet and will accompany the comet as it travels in toward the Sun, observing how it reacts and its tail forms.   Hayabusa2 is of particular interest because it is Japan’s second mission to return a sample of an asteroid after its first mission, Hayabusa, overcame long odds to successfully return a small amount of material from a different type of asteroid in 2010.

There have been a number of other robotic missions whose primary purpose was studying asteroids and comets, though, and more are planned. 

• NASA’s Interplanetary Comet Explorer (ICE) was the first spacecraft to fly past a comet, Giacobini-Zinner, in 1985.
• The Soviet Union’s Vega-1 and Vega-2 flew past Halley’s Comet in 1986.
• Europe’s Giotto flew past Halley’s Comet in 1986 (the closest flyby of a comet at that time).
• Japan’s Sakigake and Suisei flew past Halley’s Comet in 1986.
• Europe’s Giotto continued its journey after the Halley’s Comet encounter and flew past comet Grigg-Skjellerup in 1992.
• NASA was first to orbit (beginning in 2000) and land (2001) on an asteroid, Eros, with the Near Earth Asteroid Rendezvous (NEAR) mission (the landing was not originally part of the mission plan, but improvised at the end).
• NASA’s Deep Space 1 made a successful flyby of Comet Borrelly in 2001.
• NASA’s Stardust flew through the tail of Comet Wild-2 in 2004 and a canister returned a sample of the comet tail’s material to Earth in 2006.
• NASA’s Deep Impact spacecraft flew past Comet Tempel 1 in 2005, sending an impactor to collide with the comet and studying the after-effects.
• Japan’s Hayabusa orbited and landed on asteroid Itokawa in 2005 and returned a sample to Earth in 2010.
• NASA’s Deep Impact spacecraft, after its successful impact experiment with Comet Tempel 1, was renamed EPOXI and redirected to fly past Comet Hartely 2, which it did in 2010.
• NASA’s Stardust spacecraft, after obtaining the sample from Comet Wild-2 and sending it back to Earth, was renamed Stardust-NEXT and redirected to flyby Comet Tempel 1 in 2011 to provide further observations of that comet after the Deep Impact experiment.
• NASA’s DAWN spacecraft orbited an asteroid, Vesta, from 2011-2012.
• China’s Chang’e-2 flew past asteroid Toutatis in 2012 (after completing its primary mission of orbiting Earth’s Moon).
• Europe’s Rosetta became the first spacecraft to orbit and land (with the Philae lander) on a comet, 67P/Churyumov-Gerasimenko, in 2014.
• Japan’s Hayabusa2 is currently scheduled for launch on December 2, 2014 EST, to orbit and return a sample of asteroid 1999JU3.
• NASA is developing an asteroid sample return mission, OSIRIS-REx, for launch to the asteroid Bennu in 2016.

For completeness:

• NASA’s DAWN spacecraft is on course to rendezvous with and orbit Ceres in 2015, but Ceres is currently categorized as a dwarf planet, not an asteroid, so is not included in the list.
• Other robotic space missions, including NASA’s Galileo, Cassini, and New Horizons, have flown past asteroids, but it was not their primary mission, so they are not included in the list.
• NASA is planning to send a robotic probe to nudge an asteroid from its native orbit into a new orbit around the Moon so astronauts can be sent to study it.  This Asteroid Redirect Mission (ARM) is very controversial and whether or not it comes to fruition is an open question, but in any case it is primarily a human spaceflight endeavor, so is not included in the above list of robotic missions.

Editor’s note:  the list was complied by searching a number of Internet sites.  Any errors or omissions are entirely our responsibility.

Update:  The launch of Hayabusa2 has slipped from November 30 to December 2 EST (December 1 to December 3 Japan Standard Time).  The list was updated accordingly.