Asteroids Close Up

Excerpted from Chaisson McMillan

The first close-up views of asteroids were provided by the Jupiter probe Galileo which, passed twice through the asteroid belt, making close encounters with asteroid Gaspra in October 1991 and asteroid Ida in August 1993 (Figure 14.2). Both Gaspra and Ida are S-type asteroids.

Gaspra and Ida (a) The S-type asteroid Gaspra as seen from a distance of 1600 km by Galileo on its way to Jupiter. (b) The S-type asteroid Ida, by Galileo from 3400 km. (Ida’s moon, Dactyl, is visible at the right of the photo.) The resolution is on the order of 100 m.

Gaspra and Ida are irregularly shaped bodies with maximum diameters of about 20 km and 60 km, respectively. They are pitted with craters ranging in size from a few hundred meters to 2 km across and are covered with a layer of dust of variable thickness. Ida is much more heavily cratered than Gaspra, in part because it resides in a denser part of the asteroid belt. Ida has suffered more from the ravages of time. Ida is about a billion years old, far older than Gaspra, which is estimated to have an age of just 200 million years, based on the extent of cratering. Both asteroids are thought to be fragments of much larger objects that broke up into many smaller pieces following violent collisions long ago.

Dactyl, just 1.5 km across, at a distance of about 90 km. Collisions between asteroids may be quite common, providing a source of both interplanetary dust and smaller asteroids and possibly deflecting one or both of the bodies involved onto eccentric, Earth-crossing orbits. The less violent collisions may be responsible for the binary systems we see.

Ida’s mass at about 5-10 10¹⁶ kg. This information in turn allowed them to measure Ida’s density as 2200-2900 kg/m³, a range consistent with its rocky, S-type classification.

	Asteroid Mathilde The C-type asteroid Mathilde, en route to the near-earth asteroid Eros. It measures some 60 x 50 km, and rotates every 17.5 days. The largest craters visible are about 20 km across. The reason may be the asteroid’s low density (approximately 1400 kg/m^3) and rather soft composition. (NASA)

NEAR

In June 1997 the Near Earth Asteroid Rendezvous (NEAR) spacecraft visited the C-type asteroid Mathilde on its way to the mission’s main target, the S-type asteroid Eros. Shown in Figure 14.3, Mathilde is some 60 km across. By sensing its gravitational pull, NEAR measured Mathilde’s mass to be about 10¹⁷ kg, implying a density of just 1400 kg/m³. To account for this low density, scientists speculate that the asteroid’s interior must be quite porous. Indeed, many smaller asteroids seem to be more like loosely bound "rubble piles" than pieces of solid rock. The interior’s relatively soft consistency may also explain the unexpectedly large size of many of the craters observed on Mathilde’s surface. A solid object would probably have shattered after an impact violent enough to cause such large craters. However, like crumple zones in a car, Mathilde’s porous interior could have absorbed and dissipated the impactor’s energy, allowing the asteroid to survive the event.

On arrival at Eros on February 14, 2000, NEAR-Shoemaker went into orbit around the asteroid, changing its trajectory several times and coming as close as 5.5 km to the surface (Figure 14.4a). For one year, the spacecraft sent back high-resolution images of Eros (Figure 14.4b), and made detailed measurements of its size, shape, gravitational and magnetic fields, composition, and structure. The craft’s various sensors revealed Eros to be a heavily cratered body of mass 7 10¹⁵ kg and roughly uniform density around 2700 kg/m³. The asteroid’s interior seems to be solid rock—not rubble, as in the case of Mathilde—although it is extensively fractured due to innumerable impacts in the past. Eros a primitive, un-evolved sample of material from the early solar system. On February 12, 2001, NEAR-Shoemaker landed on Eros, sending back a series of close-up images as it descended to the surface. Remarkably, despite the lack of landing gear, the spacecraft survived the low-velocity impact. While no further images were obtained, the probe maintained radio contact with Earth for 16 more days before communication finally ceased.

NEAR at Eros (a) The NEAR-Shoemaker entered orbit around asteroid Eros in February 2000, making a series of orbit corrections during April to bring it closer and closer to the surface before landing on the asteroid on February 12, 2001. (b) A mosaic of detailed images showing the entire asteroid. Craters, ranging from 50 m to 5 km, pit the surface. Part (c) shows a close-up image of a "young" section of the surface, where loose material from recent impacts has apparently filled in and erased all trace of older craters. (JHU/NASA)

Last, Closest Image of Eros

This is the last image of asteroid 433 Eros received from NEAR Shoemaker from a range of 120 meters (394 feet). It measures 6 meters (20 feet) across. What we can see of the rock at the top of image measures 4 meters (12 feet) across. The streaky lines at the bottom indicate loss of signal as the spacecraft touched down on the asteroid during transmission of this image.

Apart from Ida, Mathilde, and Eros, most asteroid masses are unknown. However, a few of the largest asteroids do have strong enough gravitational fields for their effects on their neighbors to be measured and their masses thereby determined to reasonable accuracy. Their computed densities are generally compatible with the rocky or carbonaceous compositions just described.