PerspectivePlanetary Science

New Views of Asteroids

Science  19 Dec 1997:
Vol. 278, Issue 5346, pp. 2070-2071
DOI: 10.1126/science.278.5346.2070

Imagine exploring the surface of a near-Earth asteroid. You must be delicate in your traversal of this warped and mountainous world, an irregular agglomeration several kilometers across that pulls with a mere ten-thousandth the gravity of Earth (1)—a body so small you might jump off [HN1], never to return. The asteroid spins beneath a brilliant sun, sweeping out constellations and cycling the landscape into night and day more rapidly than you are used to (2). Promontories loom at improbable angles, and a stark horizon drops abruptly a hundred meters from your feet. In this precipitous world, your progress is further hindered by microgravity and the extraordinarily loose soil: Each gentle step raises volumes of dust and sends you floating for minutes. Global voyages may in principle be achieved with measured steps, but it takes an hour or more to complete the slow, all-but-unpredictable trajectories governed by the weird gravity and Coriolis forces (3) [HN2]. For the most part you just relax and enjoy the view of your home planet, appearing the size of a marble at arm's length, into which this asteroid may someday collide if left alone.

This microgravity fantasy is rooted in spacecraft and radar imaging of several near-Earth asteroids [HN3] and other minor planets, and some application of routine physics. We can hope to witness this scenario in our lifetimes. More detailed speculation remains imprudent until we find out whether asteroids are intact, or fractured and cavernous; whether they sequester volatile ices in their not-so-deep interiors; and whether they harbor rich deposits of metals, exotic minerals, or prebiotic compounds. Almost certainly asteroids are stranger than we assume, and the enchantment of their discovery—spurred on by wide public access to recent images and reflected in the popularity of comets and asteroids in contemporary doomsday cinema [HN4]—is spreading to a wide forum as we begin to learn the answers to these questions.

Near Earth Asteroid Rendezvous (NEAR) [HN5], the first-launched NASA Discovery spacecraft, is a spearhead for asteroid science. NEAR will maneuver in early 1999 into the first orbit about a low-gravity planet, the ~14 km × 40 km Earth-approaching asteroid 433 Eros [HN6], to circle some 30 to 100 km above the surface, revolving at a stately ~5 m s1 for a year or more. Multispectral mineralogy, altimetry, magnetometry, orbital gravimetry, and unprecedented color images (with a resolution of 3 m per pixel) will transform little-known Eros into one of the most exhaustively explored members of our solar system, and the first body in that size range to be (we hope) approximately understood.

Last June, as an en route preview to that encounter, NEAR flew by the main-belt asteroid 233 Mathilde [HN7] for the first look at a primitive C-type object (4) [HN8], as reported by Veverka et al. and Yeomans et al. in this issue on pages 2106 and 2109 (5). Although the resolution was 50 times as coarse as expected at Eros, the images of Mathilde reveal some surprises and provoke an overdue reevaluation of asteroid geophysics. Mathilde has survived blow upon blow with almost farcical impunity, accommodating five great craters [HN9] with diameters from 3/4 to 5/4 the asteroid's mean radius, and none leaving any hint of global devastation. Given that one of these great craters was the last to form, preexisting craters ought to bear major scars of seismic degradation, which they do not. Furthermore, asteroids Gaspra [HN10] and Ida [HN11] (encountered by Galileo en route to Jupiter) and the small martian satellite Phobos [HN12] all exhibit fracture grooves related to impact, yet fracture grooves are absent on the larger, more-battered Mathilde. Perhaps fractures are hidden beneath deep regolith [HN13], or are so pervasive that Mathilde is nothing but regolith: a “rubble pile.” In any event, Mathilde demonstrates that the formation of large craters can be quite local, and locally energetic: Ejecta was accelerated to escaping speeds (~20 m s1) without greatly disturbing the remainder of the asteroid. Interstitial voids greatly limit an impact shock wave's propagation but also enhance particle speeds within a smaller shocked region; porosity may thus explain Mathilde's strange craters, given its very low (~1.3 g cm3) density.

The view from asteroid 4179 Toutatis on 29 September 2004, when it comes within 0.01 astronomical unit of Earth. The view is from an observer in close orbit; the stars and the appearance of Earth are exact, with Earth about the size of a full moon. [Composite image created by E. De Jong and S. Suzuki of DIAL/JPL]

Until NEAR succeeds at Eros, the most detailed information about Earth-approachers derives from radar echo experiments [HN14]. Powerful polarized signals are beamed from (and echoes received at) either of two antennas—one in Arecibo, Puerto Rico, and the other in Goldstone, California [HN15]. Unlike optical imaging, this technique additionally constrains surface roughness, electrical properties, density, position, velocity, rotation, and shape (6). Less than 2 weeks after the discovery of Earth-crossing asteroid 1989PB, radar echo experiments revealed the first detailed images (7) of an asteroid—a probable contact binary later named 4769 Castalia [HN16]. A more favorable apparition was provided by 4179 Toutatis [HN17], yielding the reconstructed view (8) shown in the figure. Extensive upgrades to the Arecibo antenna will be completed this spring, providing dozens of Toutatis-quality detections per year, spacecraft-quality images of the closest approachers, and hundred-pixel images of dozens of main-belt asteroids (9).

These irregular bodies (~1 to ~50 km in diameter) may hardly seem like planets in their own right, yet the distinction is becoming vague. Consider the third largest asteroid, 4 Vesta [HN18], a basalt-covered volcanic body 530 km in diameter that resembles the moon as much as it does Mathilde or Toutatis. Recent views (36 km per pixel) by the Hubble Space Telescope (10) show a 460-km crater, with raised rim and central peak, covering the entire southern hemisphere—an impact scar surpassing (in relative diameter, but not relative depth) the great chasms of Mathilde. Such craters greatly challenge our understanding of impact processes on asteroids, and on planets in general; evidently, our science must adapt. The study of asteroids is therefore particularly exciting, as small planets provide the fulcrum for the growth of planetology, and for an evolution of geophysics in general. Complex and poorly understood solar system processes—such as impact cratering, accretion and catastrophic disruption, the evolution of volcanic structures, and the triggering of differentiation—may reveal themselves only in a study across the gamut of planets, from the least significant house-sized rock to the most stately terrestrial world. Like clockwork miniatures, asteroids demonstrate primary principles governing planetary evolution at an accessible scale, and thousands await discovery and exploration in near-Earth space alone.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

General information and extensive astronomy links are available at the Astronomical Society of the Pacific Web page and at S. Odenwald's Astronomy Café Website. Extensive information is also available in Views of the Solar System created by C. Hamilton of the Los Alamos National Laboratory.

Background information on asteroids is available from several sites, including anasteroid page created by S. Hudson, About Asteroids from Bill Arnett's The Nine Planets page, the Asteroid page of University of Texas McDonald Observatory StarDate Online, a Web page of EARN — the European Asteroid Research Node Web site (Institute of Planetary Exploration, Berlin), and University of Michigan's Windows to the Universe which features an Asteroid section. S. Ostro's Asteroid Radar Research page also has a large collection of links.

Data about asteroids can be found at the Planetary Data System, Small Bodies Node, and at the Lists and Plots page of the Minor Planet Center.

The NASA Ames Research Center has an Asteroid and Comet Impact Hazards site with information about impact risk. The center also has made available Spaceguard Survey, a report of the NASA International Near-Earth Object Detection Workshop, 25 January 1992.

Glossaries of astronomical terms are provided by Views of the Solar System, Nine Planets, and the Jet Propulsion Laboratory's Welcome to the Planets.

Numbered Hypernotes

1. The relation between gravity fields and planetary interiors is discussed by R. S. Nerem in an article in Reviews of Geophysics.

On a lighter note, the Exploratorium has a calculator for determining Your Weight on Other Worlds, and S. Odenwald's “Ask the Astronomer” considers the question, From which asteroids could you jump into orbit?

Hamilton's Views of the Solar System offers a Planetary Data Browser for retrieving physical facts about objects in the solar system.

2. To learn about Coriolis forces, see the article by D. Van Domelen of Ohio State University.

See also the Java applet on Coriolis and centrifugal forces provided by the physics department of the University of Toronto.

3. The National Space Sciences Data Center has a page with images of asteroids and comets.

Hamilton's Views of the Solar System also has an Index of Asteroid Images.

A technical report on the structure of 433 Eros, including pictures, tables, statistics, and mpeg movies is available from the Jet Propulsion Laboratory.

Johns Hopkins University has information on and a picture of 433 Eros.

4. The entertainment industry's view of asteroids can be gleaned from the The Sky is Falling Web page about NBC's made-for-TV movie Asteroid.

P. Plait's critique of the science of this movie, with links to reviews of other recent “asteroid impact” shows, is available from the Bad Astronomy Web site.

The “Ask a Space Scientist” section of the Astronomy Café Web site has questions from the public about asteroids, meteors, and comets answered by S. Odenwald. There is an archive of earlier questions.

5. Johns Hopkins University Applied Physics Laboratory hosts the NEAR Hompage. They also produce an Educator's Guide to NEAR.

6. The NEAR mission Web page has a section on the mission to Eros.

An article on the Near-Earth Asteroid Mission to 433 Eros was published in Earth in Space by the American Geophysical Union.

L. McFadden's page about the NEAR Mission to Eros includes diagrams and an Eros bibliography.

7. Johns Hopkins University has provided theseMathilde images from the June 1997 encounter.

See also the Mathilde encounter page.

Information on Mathilde is available from Hamilton's Views of the Solar System, Nine Planets, and L. McFadden's page.

8. Classification of asteroids, including type descriptions, is provided by the Johns Hopkins University Applied Physics Laboratory and the Nine Planets site.

9. An introduction to impact cratering is provided by the Experimental Impact Laboratory at NASA's Johnson Space Center.

Several publications by C. R. Chapman of the Southwest Research Institute on moons, asteroids, and cratering are available.

In September of 1997, the Space Telescope Science Institute issued a press release with links to the Hubble Space Telescope (HST) observation of a giant crater on Vesta.

NASA has released images of craters on Ganymede from the Galileo orbiter.

10. Hamilton's Views of the Solar System has a page of information aboutGaspra, as does the Nine Planets Web site.

NASA's Planetary Photojournal page has seven images of Gaspra.

The Galileo mission also had a successful encounter with Gaspra.

11. The Jet Propulsion Laboratory's Welcome to the Planets offers a page of images of Ida.

NASA's Photojournal site has these10 images of Ida.

The Nine Planets site has images of Ida, as does Hamilton's View of the Solar System.

The journal Icarus published a Special Issue: Galileo at Ida.

12. Hamilton's Views of the Solar System has information about the martian moon Phobos. More can be found at the Nine Planets site, and the U.S. Geological Survey provides Phobos data from the USGS Solar System Browser.

13. Regolith is the layer of dust and debris formed by meteorite impact on many planetary bodies.

14. S. Ostro's Asteroid Radar Research page includes this introduction to radar imaging of asteroids.

15. Both theArecibo Observatory and the Deep Space Network observatories (one of which is Goldstone) have Web pages. Goldstone antennae photos and an asteroid observing schedule are also available.

16. Hamilton's Views of the Solar System has this item on Castalia.

17. The Official Toutatis Homepage is hosted by the Observatoire de la Côte d'Azur.

Information on Toutatis is also available in Hamilton's Views of the Solar System and S. Hudson's 4179 Toutatis page.

NASA's Photojournal site has a computer-generated view of Earth as seen from the asteroid Toutatis, and a radar image of Toutatis can be seen at the University of Michigan's Windows to the Universe site.

18. Information on Vesta is available from Hamilton's Views of the Solar System and HSTpress releases. The Space Telescope Science Institute also has released HST images of Vesta.

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