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Q: About how large
does an asteroid need to be before its gravity makes it round?
A: The largest known asteroid, 1 Ceres, is about 1,000 km (600 mi) in
diameter. Only the six largest asteroids are larger than 300 km (200 mi)
in diameter, and they are all round. The dividing line below which
asteroids do not become round is somewhere between 100 and 300 km (60 and
200 mi), depending on the circumstances of their formation and cooling.
The most studied asteroid is 433 Eros. The Near-Earth Asteroid Rendezvous
(NEAR) Shoemaker spacecraft, created and run by the National Aeronautics
and Space Administration (NASA) and the Johns Hopkins University Applied
Physics Laboratory, orbited Eros for about a year, starting on February
14, 2000. Eros is oblong, about 33 by 13 by 13 km (21 by 8 by 8 mi), and
images of it are posted on the Web.
Several space missions are now discovering asteroids at an astonishing
rate. During the year 2000, 10,000 new asteroids were discovered-�as many
as had been discovered during the preceding 200 years.
Q: If a giant asteroid were headed toward Earth, could people do anything
to stop it?
A: We need more information about asteroids before we can tackle this
task. If we were to put a bomb on an asteroid, we don�t know whether the
asteroid would swerve slightly, missing Earth, or just break into bits,
with each of the bits still coming straight at us. For this reason,
astronomers would like to make a survey of the sky to detect all the
asteroids that might intersect Earth�s orbit, and to understand the
composition of asteroids well enough to know how solid they are.
Our current understanding of these near-Earth objects indicates that we
can expect a big collision every few hundred thousand years, and a really
devastating one every few million years. Astronomers think that over 1,000
of these near-Earth objects are more than 1 km (0.6 mi) across. More and
more scientists agree that a collision with an asteroid or comet 65
million years ago killed off the dinosaurs and many other species. In
fact, mammals like us benefited from that event, since mammals survived
while the dinosaurs didn�t. Statistically, scientists think there is a 1
percent chance that a much smaller object, around 300 m (1,000 ft) across,
will hit Earth sometime in the next century. An asteroid of that size
would still make a devastating crater on land or create a tidal wave if it
fell in the ocean.
The spacecraft NEAR Shoemaker (Near-Earth Asteroid Rendezvous, also named
for planetary geologist Eugene Shoemaker) went into orbit around the
asteroid Eros on February 14, 2000, and it has sent back incredibly
detailed images. NEAR Shoemaker will get closer and closer to Eros,
eventually landing on the asteroid. Eros seems to be solid, while the
asteroid Mathilde, which NEAR Shoemaker passed in 1997, seems to be more
of a rubble pile, with only half the density of Eros.
Q: I�ve heard that astronomy absorbs tremendous amounts of money, but I
cannot find exact figures. Can you tell me how much is spent yearly? And
is it too much for just pondering the mysteries of the universe? Wouldn�t
it be wiser to spend a fraction of this money for more mundane purposes? I
like seeing the pretty pictures from the Hubble telescope, but I wonder if
it is worthwhile.
A: Actually, astronomy absorbs relatively little money compared with other
government functions. Over the years, it has been demonstrated that
investing money in basic research such as astronomy has a return of
several times the investment. Astronomy attracts some of the world�s
brightest people to scientific problems and leads to discoveries of
physical laws that are important to everybody, although the results of
such discoveries are not usually immediately apparent.
Astronomic research that could have important impacts in the near future
includes studies that may improve our understanding of Earth�s atmosphere
and studies of the runaway greenhouse effect on Venus.
In the United States the National Science Foundation (NSF) and the
National Aeronautics and Space Administration (NASA) are the primary
sources of funding for astronomy. The NSF astronomy budget is about $100
million per year. The population of the United States is more than 250
million, so that is 40 cents per person per year.
NASA�s budget is harder to categorize since about one-third of it is
devoted to crewed space flight�largely for political reasons or for
general exploration rather than for scientific research. Indeed, many
scientists decry the current emphasis on the International Space Station
because of the limited amount of science that will be carried out on it.
However, NASA conducts resource mapping and other studies of Earth that
have proved very valuable.
Still, even if we say that about $2.5 billion of NASA�s $15 billion per
year budget is related to space sciences, with a smaller fraction devoted
to astronomy, that is only around $10 per person. The astronomy part is
about $500 million, roughly $2 per person.
Note that for $10 per person, you aren�t going to solve major problems on
Earth. You aren�t going to solve poverty or make medical advances that
will revolutionize the world, or even provide health insurance for
individuals. It seems more worthwhile to invest that level of money in
basic scientific research that has the promise of making breakthroughs
that will bring new health and prosperity to people in the future. Our
country spends about 40 times as much on social programs as it does on
space.
Incidentally, the Hubble Space Telescope puts out pretty pictures to show
the people that it is working, but its major scientific work isn�t in
those pretty pictures. It has spectrographs and special filters that allow
details to be investigated. Most people like the pictures, but please
don�t be misled to think that pictures are Hubble�s main scientific work.
Q: How rare is the aurora borealis? What causes it?
A: The aurora borealis isn�t rare if you live near one of the Earth�s
magnetic poles. The north magnetic pole currently is in the Queen
Elizabeth Islands of Canada�s Northwest Territories. Views from space show
an auroral oval most of the time.
The aurora borealis and its sister in the Southern Hemisphere, the aurora
australis, are caused by particles from the Sun hitting the air molecules
in the Earth�s atmosphere. These particles give energy to the air
molecules and make them glow. Different molecules glow in different
colors.
When a coronal mass ejection or a solar flare sends a large number of
high-energy particles into the auroral oval, the oval expands. Such events
take place more often near the maximum of the solar-activity cycle (which
is most commonly regarded as the sunspot cycle). This 11-year cycle is
already or will soon be past its peak, which will probably be shown to
have occurred in 2000 and 2001.
Solar coronal mass ejections and flares occur frequently in the declining
phase of the cycle, so we have hope of seeing auroras at latitudes closer
to the equator than is usual. Such an event can cause an aurora that is
visible throughout the whole continental United States. If you live in
Alaska, you can see an aurora most of the time when the night sky is
clear. Phenomena that result from the interaction of the Sun and
Earth�like the aurora�are called space weather.
NASA has several satellites in space that image the aurora regularly,
including Polar and IMAGE (Imager for Magnetopause-to-Aurora Global
Exploration). The IMAX movie SolarMax (2000) shows auroras and other
Earth-Sun phenomena on a huge screen.
Q: What was there in the universe before the big bang?
A: Astronomers� observations indicate that the universe is expanding, with
every cluster of galaxies moving away from every other cluster. Projecting
their motion backward in time, the clusters of galaxies would be closer
and closer together, and the universe�s density would be very, very high.
We can project back in time only as far as this original explosion, which
we call the big bang. (The name was given derisively in the 1940s, but it
caught on and is now used as the formal name.)
Astronomers measure time from the big bang. As far as we know, time
started and space originated in the big bang. So there was no �before the
big bang.� The universe simply did not exist.
This explanation may become clearer if we examine a similar example: On
the Earth�s surface, the North Pole marks latitude 90 degrees. If you go
north along any line of longitude toward the North Pole and then keep
going, you don�t go above latitude 90 degrees. Once you reach 90 degrees
(the North Pole), you start going south. So the question �what is north of
the North Pole� doesn�t have a meaningful answer in the same way that
�what was there in the universe before the big bang� doesn�t have a
meaningful answer.
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