HST News & Updates Archive

September 4, 1997:
HST Update:
Hubble Reveals Huge Crater On The Surface Of The Asteroid Vesta

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Astronomers have used NASA's Hubble Space Telescope to discover a giant impact crater on the asteroid Vesta. The crater is a link in a chain of events thought responsible for forming a distinctive class of tiny asteroids as well as some meteorites that have reached the Earth.

The giant crater is 285 miles across, which is nearly equal to Vesta's 330 mile diameter. If Earth had a crater of proportional size, it would fill the Pacific Ocean basin. Astronomers had predicted the existence of one or more large craters, reasoning that if Vesta is the true "parent body" of some smaller asteroids then it should have the wound of a major impact that was catastrophic enough to knock off big chunks. The observations are described in the September 5 issue of Science Magazine.

"In hindsight we should have expected finding such a large crater on Vesta," says Peter Thomas of Cornell University, Ithaca, NY. "But it's still a surprise when it's staring you in the face." Another surprising finding is that such a large crater, relative to Vesta's size, might have been expected to cause more damage to the rest of the minor planet.

"This is a unique opportunity to study the effects of a large impact on a small object," says Michael Gaffey of Rensselaer Polytechnic Institute, Troy, New York. "This suggests that more asteroids from the early days of the solar system may still be intact."

The collision gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space. This tore out an eight-mile deep hole that may go almost all the way through the crust to expose the asteroid's mantle (Vesta is large enough to be differentiated like Earth - with a volcanic crust, core and mantle, making it a sort of "mini-planet".)

Because of the asteroid's small diameter and low gravity, the crater resembles smaller craters on the Moon that have a distinctive central peak. Towering eight miles, this cone-shaped feature formed when molten rock "sloshed" back to the bull's-eye center after the impact.

One clue for a giant crater came in 1994 when Hubble pictures showed that one side of Vesta's football shape appeared flattened. "We knew then there was something on Vesta that was unusual," says Thomas.

The astronomers had to wait for a better view from Hubble when Vesta made it closest approach to Earth in a decade, in May 1996, when the asteroid was 110 million miles away.

A total of 78 Wide Field Planetary Camera 2 pictures were taken. The team then created a topographic model of the asteroid's surface by noting surface irregularities along the limb and at the terminator (day/night boundary) where shadows are enhanced by the low Sun angle.

The immense crater lies near the asteroid's south pole. This is probably more than coincidental, say researchers. The excavation of so much material from one side of the asteroid would have shifted its rotation axis so that it settled with the crater near one pole.

Unlike some other large asteroids that have jumbled surfaces due to the asteroids breakup and recollapse, the rest of Vesta's surface is largely intact, despite the cataclysm. This is based on previous measurements showing it has a surface of basaltic rock - frozen lava - which oozed out of the asteroid's presumably hot interior shortly after its formation 4.5 billion years ago, and has remained largely intact ever since.

Approximately six percent of the meteorites that fall to Earth are similar to Vesta's mineralogical signature, as indicated by their spectral characteristics. Vesta's spectrum is unique among all the larger asteroids. The crater may be the ultimate source of many of these meteorites.

Most meteorites are believed to come from other asteroids, but their specific objects of origin cannot be determined in most cases. Thus the distinctive mineralogical makeup of these meteorites means that Vesta is the only world other than the Earth, the Moon and Mars for which scientists have samples of specifically known origin.

A mystery has been that the meteorites could not have traveled directly from Vesta because at Vesta's location in the asteroid belt, there are no perturbing gravitational forces which would cause pieces to fall into orbits intersecting the inner planets like apples shaken out of a tree. However, Vesta's "daughter" asteroids -- literally "chips off the block" which have color characteristics similar to Vesta, are near a "chaotic zone" in the asteroid belt where Jupiter's gravitational tug can redirect fragments into orbits which intersect Earth's orbit.

A good determination of the shape of Vesta was necessary for the next step in interpretation, which will use multi-color images of Vesta obtained with HST to study the detailed mineralogy of surface regions including the region of the giant crater. Also, a team led by Don McCarthy of The University of Arizona plans to obtain additional images of Vesta at longer wavelengths this fall using the new Near Infrared and Multi-Object Spectrometer (NICMOS) science instrument on board Hubble.

Members of the Vesta research team are Principal Investigator Ben Zellner of Georgia Southern University; the Co-Investigators are Richard Binzel, MIT, Michael Gaffey, Rensselaer Polytechnic Institute, Alex Storrs, Space Telescope Science Institute, Peter Thomas, Cornell University and Dr. Ed Wells, Computer Sciences Corporation.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

July 1, 1997
HST Update:
World's Most Powerful Telescopes Team Up With A Lens In Nature To Discover Farthest Galaxy In The Universe

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Star Cluster Used as Lens

An international team of astronomers has discovered the most distant galaxy found in the universe to date, by combining the unique sharpness of the images from NASA's Hubble Space Telescope with the light-collecting power of the W. M. Keck Telescopes -- with an added boost from a gravitational lens in space.

The results show the young galaxy is as far as 13 billion light-years from us, based on an estimated age for the universe of approximately 14 billion years. This would place the galaxy far back in time during the "formative years" of galaxy birth and evolution, less than a billion years after the birth of the universe in the Big Bang.

The detailed image shows that bright dense knots of massive stars power this object. Due to the firestorm of starbirth within it, the galaxy is intrinsically one of the brightest young galaxies in the universe, blazing with the brilliance of more than ten times our own Milky Way.

Predicted by Einstein's theory of general relativity, gravitational lenses are collections of matter (such as clusters of galaxies) that are so massive they warp space in their vicinity, allowing the light of even more-distant objects to curve around the central lens-mass and be seen from Earth as greatly magnified.

If dust rises to the elevations where the water-ice clouds form, ice condenses on dust grains and the heavier ice/dust particles quickly fall back out of the atmosphere. Though the dust could extend at low altitudes over the landing site, researchers say current prevailing winds should take the dust northward.

"If dust diffuses to the landing site, the sky could turn out to be pink like that seen by Viking," says Philip James of the University of Toledo. Otherwise, Pathfinder will likely show blue sky with bright clouds."

The imaging team includes Steve Lee of the University of Colorado at Boulder's Laboratory for Atmospheric and Space Physics, Todd Clancy of Boulder's Space Science Institute, Phillip James of the University of Toledo, Mike Wolff of the Space Science Institute and Jim Bell of Cornell University.

Images and text reproduced for educational use with permission from The Space Telescope Science Institute. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

June 19, 1997
HST Update:
Pele Erupts on Io

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Eruption on limb of Io

The Hubble Space Telescope has snapped a picture of a 400-km-high (250-mile-high) plume of gas and dust from a volcanic eruption on Io, Jupiter Io was passing in front of Jupiter when this image was taken by the Wide Field and Planetary Camera 2 in July 1996.

The plume appears as an orange patch just off the edge of Io in the eight o'clock position, against the blue background of Jupiter's clouds. Io's volcanic eruptions blasts material hundreds of kilometers into space in giant plumes of gas and dust. In this image, material must have been blown out of the volcano at more than 2,000 mph to form aplume of this size, which is the largest yet seen on Io.

Until now, these plumes have only been seen by spacecraft near Jupiter, and their detection from the Earth-orbiting Hubble Space Telescope opens up new opportunities for long-term studies of these remarkable phenomena.

The plume seen here is from Pele, one of Io's most powerful volcanos. Pele's eruptions have been seen before. In March 1979, the Voyager 1 spacecraft recorded a 300-km-high eruption cloud from Pele. But the volcano was inactive when the Voyager 2 spacecraft flew by Jupiter in July 1979. This Hubble observation is the first glimpse of a Pele eruption plume since the Voyager expeditions.

Io's volcanic plumes are much taller than those produced by terrestrial volcanos because of a combination of factors. The moon's thin atmosphere offers no resistance to the expanding volcanic gases; its weak gravity (one-sixth that of Earth) allows material to climb higher before falling; and its biggest volcanos are more powerful than most of Earth's volcanos.

This image is a contrast-enhanced composite of an ultraviolet image (2600 Angstrom wavelength), shown in blue, and a violet image (4100 Angstrom wavelength), shown in orange. The orange color probably occurs because of the absorption and/or scattering of ultraviolet light in the plume. This light from Jupiter passes through the plume and is absorbed by sulfur dioxide gas or is scattered by fine dust, or both, while violet light passes through unimpeded. Future HST observations may be able to distinguish between the gas and dust explanations.

Images and text reproduced for educational use with permission from The Space Telescope Science Institute. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

May 20, 1997
HST Update:
Hubble Finds Cloudy, Cold Weather For Mars Spacecraft

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Global Weather Map of Mars

As two NASA spacecraft speed toward a mid-year rendezvous with Mars, astronomers using the Hubble Space Telescope are providing updated planetary weather reports to help plan the missions.

Hubble's new images show that the "martian invasion" of spacecraft will experience considerably different weather conditions than seen by the last U.S. spacecraft to land on Mars 21 years ago.

Martian atmospheric conditions will affect the operation of both the Mars Pathfinder landing on July 4, and the September 11 arrival of the Mars Global Surveyor which will map the planet from orbit. Hubble images taken barely three weeks apart, on March 10 and March 30, reveal dramatic changes in some local conditions, and show overall cloudier and colder conditions than Viking encountered two decades ago.

(con't) "Because Pathfinder uses the atmosphere to decrease its velocity for landing, and because the lander and rover are solar-powered, understanding the state of the atmosphere prior to landing is important," said Dr. Matthew Golombek, Pathfinder project scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA.

"On July 4, Mars Pathfinder will enter the atmosphere directly from approach and slow itself behind an aeroshell with a parachute, small solid rockets and giant airbags. The lander carries a small rover to explore the surface and investigate the kinds of materials present. Hubble images of Mars are helping us to adjust our flight path for landing and effectively plan surface operations," said Golombek.

"It's not the dusty Mars of the Project Viking days (mid 1970s to early 1980s) or the habitable oasis of science fiction stories," says Todd Clancy of the Space Science Institute in Boulder, CO. "We're finding a Mars that's colder, clearer, cloudier. Hubble is rapidly changing our view of Mars' environment. The planet's weather apparently has a flip-side to it."

Hubble's findings also offer new insights into the differences and similarities of weather on the other terrestrial planets. "The planets are similar in many important ways, so the very major differences between them are interesting from a viewpoint of understanding meteorology better," said team leader Phil James of the University of Toledo in Ohio. "Hubble is allowing us to look at Mars in ways never before seen."

In September, NASA's Mars Global Surveyor will skim across the upper martian atmosphere to slow down by friction and enter orbit around the red planet. Atmospheric density is a key factor in precisely executing this complex and delicate aerobraking maneuver. Hubble is ideal for tracking regional dust storms which could pose a threat to Surveyor by drastically changing the planet's air density. Such storms can cause a tenfold increase in the martian atmosphere's drag at 60 miles above the surface.

Comparing the appearance of Mars to that in earlier spacecraft observations, Hubble has found some areas of the martian surface that have been changed dramatically by wind-blown dust. The most prominent example is the "classic dark feature" called Cerberus, which is roughly the size of California (800 by 250 miles). This feature has been seen as a low albedo (dark) region by ground-based telescopic observers since early this century, and was studied in detail by the Mariner 9 and Viking orbiters in the 1970s.

In Hubble's view, only three dark splotches remain, probably related to dark sand being carried out of craters by the wind. The astronomers think that dust storms in the region have covered the formerly dark surface with bright dust, effectively erasing Cerberus from the map.

Hubble is ideally suited for long-term study of Mars. When Mars has been closest to Earth, Hubble has resolved surface details as small as 25 miles across. This allows astronomers to track subtleties in the shifting cloud patterns and periodic dust storms. This planet-wide "weather satellite" view is complementary to the close-up views which will be provided by Mars Pathfinder and Mars Global Surveyor.

Images and text reproduced for educational use with permission from The Space Telescope Science Institute. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).