A reader wrote, after last month’s landing of the NEAR spacecraft on the asteroid Eros, asking for more information on these wanderers of the solar system. Asteroids are the rocky debris left over from the formation of the solar system and orbit mainly between Mars and Jupiter.

The largest, Ceres with a diameter of 933 kilometers, was also the first spotted in 1801. Most are much smaller, less than a kilometer in diameter and have been likened to “orbiting ballparks.” Today, around 6,600 asteroids have been catalogued, and estimates of their numbers range as high as 100,000.

Some asteroids have orbits that intersect with that of the Earth. This means that it is possible for them to arrive at the intersection at the same time and – like two cars speeding into a crossroad – collide.

On Dec. 9, 1994, an asteroid about the size of a school bus passed within 105,000 kilometers of Earth as it intersected our planet’s orbit. This may seem like a large distance but, given the Earth’s orbital velocity of about 29 kilometers per second, we missed a collision by only a little more than an hour. An asteroid that struck Earth about 65 million years ago is credited with causing the extinction of much of the planet’s life forms, including dinosaurs.

Focus on the Planets

Mercury is very low in the east-southeast about a half-hour before sunrise as the month opens. Your best chance to spot the innermost planet is on the 11th, when it is at greatest western elongation.

Venus is prominent on the western evening horizon, where it is engaged in taking the steepest dive into the setting sun in eight years. Each night reveals a dramatic change in altitude for the crescent-shaped planet until it disappears into the sun’s glare. Venus reappears in the morning sky at the end of the month.

Mars is well up on the southern horizon at dawn. Look for orange-golden Antares of Scorpius to Mars’ lower right.

Jupiter is high in the west-southwest at dusk. Aldebaran, the “red eye of Taurus,” is to the immediate left of the giant planet.

Saturn lies to the lower right of Jupiter and, while dimmer than either Jupiter or Venus, provides its own viewing spectacle with a favorable tilt to its ring system.

Uranus and Neptune are in the constellation of Capricornus, but you’ll likely have to wait until summer for a good chance of spotting them.

Pluto, never visible without a powerful telescope, is three months away from escaping the predawn glare.

March Events

1 Sunrise, 6:14 a.m.; sunset, 5:22 p.m.

3 Moon in first quarter, 9:03 p.m.

5 The two bright stars above the moon tonight are Pollux and Castor of Gemini.

8 The moon is at perigee, or closest approach to the earth, tonight.

9 Full Moon, 12:23 p.m. The full moon of March is called the Crow Moon, Sap Moon, or Lenten Moon.

11 Look just above the southeast horizon about 40 minutes before sunrise for your best chance of spotting Mercury this month. The sun enters Pisces on the ecliptic.

15 Mars, the waning moon, and Antares form a close grouping in the southeast predawn sky.

16 Moon in last quarter, 3:47 p.m.

17 St. Patrick’s Day and time to plant your peas, according to some optimistic gardeners.

20 The spring or vernal equinox occurs at 8:28 a.m. This is the point at which the sun crosses the equator returning to the Northern Hemisphere. The sun enters the astrological sign of Aries but, astronomically, is still in Pisces. The moon is at apogee, or most distant from the earth, today.

25 New Moon, 8:23 p.m.

28 Look to the west around 8:00 p.m. for a cluster consisting of the thin crescent moon, Saturn, the Pleiades, Jupiter and Aldebaran.

31 Sunrise, 5:19 a.m.; sunset, 6:01 p.m.