Harpooning a Comet

Europe is set to try to do something no-one has ever done before – to chase and land on a comet. The Lander science will focus on the in situ study of the composition and structure of the nucleus material.

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This artist’s impression shows the Rosetta Lander anchored to the comet’s surface with instruments, legs and solar panels. Credit: ESA 2001. Illustration by Medialab

Comet-chasing mission Rosetta has refocused its sights on Comet Churyumov-Gerasimenko. During its meeting on May 13-14th 2003, ESA’s Science Program Committee decided Rosetta’s new mission baseline.

The spacecraft will be launched in February 2004 from Kourou, French Guiana, using an Ariane-5 G+ launcher. The rendezvous with the new target comet is expected in November 2014.

Delayed indefinitely earlier this year to troubleshoot launch issues, ESA’s Rosetta lander, is now back on track to be the first man-made object to land on a comet.

Hampered by rocketry concerns, the landing phase presented planners with another set of challenges altogether. "Firstly, we don’t know anything about how rough the surface is," said Rosetta Project Scientist Gerhard Schwehm. "It could be covered with fluffy snow like the Alps or it could be hard rocks and craters. We can, however, be sure that it will not be smooth and flat resembling parking lots."

Scientists designed Rosetta’s landing gear to cope with most nasty surprises as soon as it is to touch down on its selected target.

Halley’s Comet. Credit: NASA

Two harpoons will anchor the probe to the surface. The self-adjusting landing gear will ensure that it stays upright, even on a slope. The lander’s feet will drill into the ground. These devices will help counteract the fact that there is no appreciable gravity on a comet.

The choice of a new comet has required intensive efforts, including observations by telescopes such as the Hubble Space Telescope and the ESO Very Large Telescope to ensure scientists know as much as possible about the new target.

Scientists will now investigate an alternative launch to this comet, in February 2005, as a back-up plan. Rendezvous with the comet, Churyumov-Gerasimenko, is now expected in November 2014. En route to the comet it will inspect two asteroids (Otawara and Siwa) at close quarters.

Measurement goals on Comet Churyumov-Gerasimenko include the determination of the elemental, molecular, mineralogical, and isotopic composition of the cometary surface and subsurface material. Highest priority is given to the elemental and molecular determinations as it is believed that some mineralogical and isotopic measurements can be carried out adequately by orbiter science investigations. In addition properties like near-surface strength, density, texture, porosity, ice phases and thermal properties will be derived. Texture characterization will include microscopic studies of individual grains.

What’s Next

close-up of Eros as it rotates.
Video: NEAR Shoemaker flyover of Eros. Credit: Johns Hopkins Univ. APL

On Valentine’s Day, 2001, the Near-Shoemaker spacecraft successfully landed on the asteroid, Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid about 176 million kilometers (109 million miles) from Earth, prompted Andrew Cheng, NEAR Project Scientist, to note: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."

On Jan. 2, 2004, another NASA spacecraft called Stardust will fly within 75 miles of a cometary main body (called Wild-2)–close enough to trap small particles from the coma, the gas-and-dust envelope surrounding the comet’s nucleus. Stardust will be traveling at about 13,400 miles per hour (mph) and will capture comet particles traveling at the speed of a bullet fired from a rifle. Its main camera, built for NASA’s Voyager program, will transmit the closest-ever comet pictures back to Earth. Launched in February 1999, Stardust was designed to capture particles from Wild 2 and return them to Earth for analysis. The spacecraft already has collected grains of interstellar dust. It is the first U.S. sample-return mission since the last moon landing in 1972.

In the next 5 or so years, there will be several encounters of spacecraft with comets and asteroids. All the following missions are funded, though not all have been launched yet

2004 Jan. 1 Comet Wild 2 Stardust (coma sample return)
2005 July 3 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. Asteroid 1998 SF36 Muses-C (sample return)
2014 Nov Comet Churyumov-Gerasimenko Rosetta (simple flyby)