Encyclopedia of the Solar System 2nd ed

558 Encyclopedia of the Solar System

FIGURE 1 Comet Hale–Bopp on 8 April 1997, showing the
whitish dust tail and the blue plasma tail. (Courtesy of H. Mikuz,
Crni Vrh Observatory, Slovenia.)

that appeared to be valleys, hills, and craters. The average

albedo or reflectivity of the surface was only 0.04; the sur-

face was very dark. Thejetscontaining the dust and gas

emission from the nucleus came from approximately 10%

of the entire surface and were active when their location

was in sunlight. The direction of emission was generally

sunward.

The National Aeronautics and Space Administration

(NASA) sent theDeep Space 1spacecraft to within about

2171 km of the nucleus of comet Borrelly on 22 September

2001 (Fig. 2B shows a close-up view of the nucleus). The

long axis of the nucleus is approximately 8 km and the short

axes are about 3.2 km. The surface showed features and

was also very dark. The albedo varied between 0.01 and 0.03

over the surface. The jets with dust and gas emission, which

were clearly seen, occupied 10% or less of the surface area.

NASA’sStardustspacecraft passed within approximately

236 km of comet Wild 2 on 2 January 2004. Excellent im-

ages were obtained, and an example, taken just after closest

approach, is shown in Fig. 2C. The nucleus is roughly a

rounded body with a diameter of 4 km. Jets of dust and gas

emission were seen, and the albedo determined was 0.03±

0.015. Features with steep slopes have been identified, pro-

viding clues to the history of the surface. The main goal of

theStardustmission, which is to return to Earth dust sam-

ples collected in the comet’scoma, has been achieved with

the return of samples that parachuted to the Utah desert

on 15 January 2006 (see the discussion in Section 4).

The Deep Impactimpactor spacecraft collided with

comet Tempel 1 on 4 July 2005. The impactor spacecraft

separated from the flyby spacecraft 24 hours before impact,

and the flyby spacecraft passed the nucleus at a distance of

500 km. An image of the nucleus taken from the impactor

is shown in Fig. 2D. The average diameter of the nucleus

is close to 6.0 km, the longest dimension is 7.6 km, and

TABLE 1 Missions to Comets

Spacecraft Comet Encounter Date Imaging

International Cometary Explorer (ICE) Giacobini–Zinner 11 September 1985 No

VEGA 1 Halley 6 March 1986 Yes

Suisei Halley 8 March 1986 No

VEGA-2 Halley 9 March 1986 Yes

Sakigake Halley 11 March 1986 No

Giotto Halley 14 March 1986 Yes

ICE Halley 25 March 1986 No

Giotto Extended Mission (GEM) Grigg–Skjellerup 10 July 1992 No

Deep Space 1 Borrelly 22 September 2001 Yes

Stardust Wild 2 2 January 2004 Yes

Deep Impact Tempel 1 4 July 2005 Yes