The asteroid 253 Mathilde was photographed by the Near Earth Asteroid Rendezvous (NEAR) spacecraft on June 27, 1997. Mathilde is about 60 km across. NEAR will get a gravity assist from the Earth in January 1998 that will point it for its rendezvous with asteroid 433 Eros on January 10, 1999. It will orbit Eros for about a year.
For the latest information on Near-Earth Asteroids, look at the NASA Asteroid and Comet Impact Hazard homepage .
Information about Toutatis, which reached perihelion in November, and other asteroids is available at the following websites:
Jane X. Luu and David C. Jewitt, discoverers of objects in the Kuiper Belt, have reviewed the topic in "The Kuiper Belt," Scientific American, May 1996, pp. 46-52. Jewitt maintains a Kuiper Belt homepage.
The Near Earth Asteroid Rendezvous mission (NEAR) was launched in February 1996. It will visit a main-belt asteroid, 253 Mathilde, en route to the asteroid Eros. It is to fly by Mathilde on June 27, 1997, and reach Eros on February 6, 1999. It will then orbit Eros at a speed of 5 m/s for one year.
Clementine 2, a relatively inexpensive Department of Defense probe (where "relatively inexpensive" is defined as "only" about 100 million dollars) is to make closeup photographs of Earth-orbit-crossing asteroids in 1998. It is hoped that three such can be observed on the same mission. The spacecraft will also carry one impacting probe per asteroid. Each will photograph the asteroid from close up and then impact it. The main Clementine 2 will study the resultant dust and fragments. To minimize the cost, the whole mission must contain less than 180 kg, which leaves only 10 kg per probe.
Chiron and 1992QB are trans-Neptunian objects that are 200-300 km across. Though first thought to be asteroids, we now think they were the first members to be discovered of the Kuiper Belt, from which short-period comets (those with periods less than 20 years) come.
As reported in June 1995, the Hubble Space Telescope has detected about 30 new comet-like objects 6 to 15 km in diameter, undoubtedly members of the Kuiper Belt, providing convincing evidence that this belt really does exist. Pluto may even merely be the largest object in the Kuiper Belt, which extends from about the orbit of Neptune to a diameter of abou 500 A.U., far greater than that of the solar system. There may be a billion to 10 billion objects in it. (Gerard Kuiper was an astronomer who worked for many years in Tucson.)
The Kuiper Belt, as the word "belt" implies, is flattened, while the even larger Oort Comet Cloud is spherical.
The Hubble Space Telescope's WFPC-2 was used in late 1994 to take a series of 24 images showing the 5.34 rotation of the asteroid (4) Vesta. We can see features as small as 80 km across on Vesta's 525-km-diameter surface. Ancient lava flows and a huge, deep impact basin can be seen. A few meteorites from Vesta that landed on the Earth are known, resulting from a collision of another asteroid with Vesta. The Moon, Mars, and Vesta are the only three solar-system objects from which we on Earth have samples.
Seventeen objects now known to orbit the Sun outside Neptune's orbit represent the discovery of the Kuiper Belt of comets. Since such an object was discovered in 1992 by David Jewitt of the University of Hawaii and Jane Luu, now at the Harvard-Smithsonian Center for Astrophysics, an additional sixteen objects have been found. Based on the rate such objects have been found given the small amount of sky searched for them, Jewitt and Luu estimate that there may be 35,000 objects larger than 100 km in diameter outside Neptune's orbit. Thus these objects may make the Kuiper belt more significant than the asteroid belt, which contains only about 200 objects larger than 100 km in diameter. It is thought that the short-period comets come from the Kuiper belt.
An interesting question is whether Pluto is merely the largest of these Kuiper-belt objects rather than being a true major planet. At a diameter of 2320 km, it is larger than the other objects known and is much brighter, reflecting 60 per cent of the light that hits it compared with about 4 per cent for the other objects. Perhaps its larger size, which allowed it to retain an atmosphere, gave it this higher albedo through deposits of frost on its surface.
Please refer to Jewitt and Luu in the Astronomical Journal for March 1995.
If you see a fireball, please report it to
USA and Canada: Robert Hawkes, Meteorites and Impacts Advisory Centre, e-mail: rhawkes@mta.ca
or by mail to him at Department of Physics, Mount Allison University, Sackfille, New Brunswick E0A 3C0, Canada.
Fireballs and how to report them are discussed on the Web site of the International Meteor Organization's Fireball Data Center. Reports can be filed at starex@tron.gun.de.