Asteroids have Seasons, Too

Asteroids have Seasons, Too

Later this week, the Sun will rise over the south pole of asteroid Eros, revealing unexplored terrain to the instruments on NASA's NEAR-Shoemaker spacecraft.

June 21, 2000 -- When NASA's NEAR spacecraft entered orbit around 433 Eros in February 2000, the asteroid was in the middle of northern summer. Eros's north pole was constantly bathed in sunlight while southern regions were in total darkness. Three of NEAR's scientific instruments depend on reflected sunlight to do their jobs, so many of the mission's early results have focused on observations of the space rock's northern regions.

Scientists have been waiting expectantly to see more of the asteroid and later this week they will get their wish. On June 25, 2000, the subsolar point on Eros will cross the asteroid's equator heading south. As the Sun rises over Eros's south pole, sunlight will illuminate terrain that's been hidden from view since NEAR went into orbit four months ago.

"We're looking forward to seeing the south polar region of Eros for the first time," says Andrew Cheng, the lead scientist for the NEAR mission at the Johns Hopkins University Applied Physics Laboratory. "Once the Sun rises over the south pole, the oblique lighting there will highlight features on the surface, which is ideal for taking pictures."

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Above: This dramatic horizon view of asteroid Eros recorded by NEAR on May 18 spans about 0.8 miles and reveals features as small as 13 feet across. The rolling surface of Eros is strewn with boulders and craters. The jagged-looking boulder near the picture center is over 190 feet tall! Scientists hope to capture more scenes like these when the Sun rises over southern areas of Eros later this week. Credit: NEAR

On Earth, sunrise at the south pole means that southern spring has arrived. We don't often think of asteroids as having seasons, but they do. Like Earth, Eros passes through two solstices (when the Sun shines down over the poles) and two equinoxes (when day and night are of equal length) during its 1.76 year circuit around the Sun. The names of the seasons on Eros are the same as the ones on our planet -- fall, winter, summer, and spring -- but that's where the similarities end. Seasons on Eros last different lengths of time (northern spring is only half as long as autumn) while the apparent size of the Sun nearly doubles between fall and spring. The difference in polar surface temperatures from summer to winter may be as great as the difference between liquid nitrogen and boiling water. Seasons on Eros are truly alien.

The Orbits of Eros and Earth

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Above: The orbits of Earth (blue) and Eros (brown). The blue circled cross and the brown "E" denote the locations of Earth and Eros on June 22, 2000. The vertical and horizontal gray axes describe a system of heliocentric coordinates in which Earth's northern autumnal equinox occurs at 0 deg., the winter solstice at 90 deg., etc. Eros's solstices and equinoxes occur when the asteroid passes through the following heliocentric longitudes: vernal equinox 107 deg.; summer solstice 196 deg.; autumnal equinox 288 deg.; winter solstice 17 deg. Orbital data are courtesy Jim McAdams of the NEAR project team at Johns Hopkins University's Applied Physics Lab. Editor's Note.

Earth's seasons are caused by the 23.5-degree tilt of our planet's axis with respect to its orbital plane. Contrary to a widespread misconception, our planet is not closer to the Sun during summer -- Earth's orbit is almost perfectly circular. The orbit of Eros, on the other hand, is highly elliptical and its spin axis is tilted 89 degrees! In this respect, Eros is similar to the planet Uranus whose spin vector is also nearly parallel to its orbital plane. Uranus's 82-degree tilt is by far the greatest of the nine planets. This triggers extreme seasons including gigantic storms that are comparable in size to North America with temperatures of 300 degrees below zero.

Although an airless world like Eros doesn't have "weather" in the same sense as a planet like Uranus, it does exhibit its own seasonal extremes. The primary change from season to season involves the surface temperature.

"As far as we know, the surface of Eros is a regolith -- a mixture of dust and broken rocks of many different sizes" says Cheng. "If we assume that the surface of Eros is similar to the surface of the Moon, then it must have similar temperature swings between night and day."

Above: Eros is elongated like a peanut. It rotates every 5.27 hours around an axis that goes through the narrow part of the asteroid. Thus, the "north pole" is near the middle and the "equator" traces Eros's long, irregular circumference. The images that make up this animated gif show the northern "hemisphere" of Eros during its summer.

On the Moon, the temperature at night drops to minus 173 degrees Celsius, which is colder than liquid nitrogen. At noon, it rises to more than 127 degrees Celsius, far hotter than boiling water. As the Sun rises this week on southern regions of Eros that have been in darkness for months, the temperature is likely to skyrocket hundreds of degrees.

"Unfortunately, we don't have a way of directly measuring the surface temperature of Eros," notes Cheng. "The thermal infrared spectrum of the asteroid peaks at a wavelength around 13 microns. We have an infrared spectrometer on board, but we deliberately chose its range of operation to be between 0.8 and 2.6 microns because that's the best for mineralogical studies."

Seasons On Asteroid 433 Eros
ORBIT DATA COURTESY JIM McADAMS, NEAR PROJECT TEAM

Season Date Physical Description Eros-Sun distance Apparent size of the Sun
(as seen from Eros) Solar Flux
(relative to the autumnal equinox)

Vernal Equinox Sept. 2, '99 The subsolar point crosses the asteroid's equator heading north.
1.14 AU

0.44 deg.

2.4

Summer Solstice Dec. 19, '99 The sun shines down directly over Eros's north pole
1.30 AU

0.34 deg.

1.8

Autumnal Equinox June 25, '00 The subsolar point crosses the asteroid's equator heading south.
1.76 AU

0.28 deg.

1.0

Winter Solstice Feb. 1, '01 The sun shines directly over Eros's south pole
1.48 AU

0.34 deg.

1.4

Notes: The names of the seasons refer to Eros's northern hemisphere. The seasons are reversed in the south. The last column shows the seasonal intensity of sunlight relative to incident solar radiation during Eros's autumnal equinox. These values simply assume that solar radiation declines as the inverse square of Eros's distance form the Sun.

For an observer on Eros, one of the most striking features of the asteroid's seasons would be the constantly changing Sun. Eros's elliptical orbit brings it within 1.13 AU of the Sun and also carries it nearly 1.8 AU away. (An AU, or astronomical unit, is the distance between the Earth and Sun. It equals 149,597,871 km). During northern autumn, the Sun would appear to be 0.28 degrees across; in the spring, when Eros is closer to the Sun, it would swell to 0.44 degrees.

Facts about Eros

Eros circles the Sun once every 1.76 Earth years. It spins on its axis once every 5.27 hours.[more]

Eros is about 21 by 8 by 8 miles (33 by 13 by 13 kilometers) in size. Its shape has been compared to a shoe, a battered boat, or a peanut. [more]

The gravity on Eros is very weak but enough to hold a spacecraft. A 100-pound (45-kilogram) object on Earth would weigh about 1 ounce on Eros. [more]

Eros is "Near-Earth Asteroid" or NEA. Its next close approach to Earth will come in January 2012, when it will pass 0.178 AU from our planet. Although Eros is a NEA, there is no chance that it will
collide with Earth.[More]

A bigger Sun means a brighter Sun, too. During northern spring sunlight falling on Eros is 240% more intense, on average, than it is during the fall. This difference would further amplify seasonal changes in surface temperature.

Eros's elliptical orbit also affects the length of its seasons because the asteroid travels faster when its closer to the Sun than it does when it's farther away. For this reason northern spring on Eros is three and a half Earth-months long (about the same as the length of springtime on our planet) while autumn persists for seven Earth-months. These values are reversed in the asteroid's southern hemisphere. The planet Mars also has seasons of unequal length because it moves in an elliptical orbit, but the effect is much greater on Eros. [Learn more about seasons on other planets.]

NEAR is currently in a 50-km orbit around Eros. It's the first time a spacecraft has ever circled an asteroid. On July 7, 2000, the probe will begin moving even closer as it descends to an orbit just 35 kilometers from the asteroid's center. The car-sized spacecraft will come within
several km of the surface before the mission ends in February 2001, days after the beginning of southern winter on Eros.

The first in NASA's Discovery Program of low-cost planetary missions, NEAR was launched from Cape Canaveral Air Station, FL., on Feb. 17, 1996. The car-sized spacecraft is observing Eros from various distances and will come within several miles of its surface before the mission ends in February 2001. The Johns Hopkins University Applied Physics Laboratory in Laurel, MD., designed and built the NEAR spacecraft and manages the mission for NASA.

Web Links

Near Earth Asteroid Rendezvous mission - NEAR home page from Johns Hopkins University

Eros or Bust - February 8, 2000. SpaceScience.com. NASA's Near-Earth Asteroid Rendezvous Mission is nearing 433 Eros. It is scheduled to go into orbit around the space rock on Valentines Day, 2000. University

Guess Who's Coming to Breakfast? - February 13, 2000. SpaceScience.com. Critical science observations of Eros are scheduled to begin 11 hours before NEAR's orbit insertion on Valentines Day, 2000.

First Orbit Around an Asteroid - February 14, 2000. SpaceScience.com. NEAR successfully entered orbit around 433 Eros on Valentines Day, 2000

Highlights from Asteroid Eros - February 19, 2000. SpaceScience.com. Scientists review exciting results from the first few days in orbit.

NEARer to Eros - February 25, 2000. SpaceScience.com. NEAR moves closer to Eros.

Wanted: a few good solar flares - March 3, 2000. SpaceScience.com. Solar radiation could reveal new details about Eros

NEAR Shoemaker -- Mar. 14, 2000. SpaceScience.com. NASA has renamed the Near Earth Asteroid Rendezvous (NEAR) spacecraft for planetary science pioneer Gene Shoemaker.

Earth's Seasons -- A table of solstices and equinoxes from the US Naval Observatory