June 20, 2014
Comet Siding Spring Won’t Disrupt Martian Orbiters As It Makes Close Approach Of The Red Planet
April Flowers for redOrbit.com - Your Universe Online
Later this year, comet Siding Spring will have a very close brush with Mars. That brush will be so close, in fact, that it lead scientists to worry about the safety of the three spacecraft currently orbiting the planet. Some experts from the University of Maryland (UMD) have used a satellite mounted telescope to observe the "fresh" comet, however, and they find that it poses minimal danger to the Martian spacecraft.
Fresh comets are those that have never before approached the sun and therefore have not gone through the process of sublimation — or the transformation of frozen material from solid ice to gas. As such, fresh comets have some of the most ancient material scientists can study. The nucleus of the comet, or its solid center, is a lumpy, non-spherical clump of frozen gases mixed with dust, much like a dirty snowball.
"Comet Siding Spring is making its first passage through the inner solar system and is experiencing its first strong heating from the sun," said UMD assistant research scientist Dennis Bodewits, lead researcher on the UMD astronomy team that used NASA’s Swift satellite to estimate the comet’s size and activity. “Comets like this one, which formed long ago and remained for billions of years in the icy regions beyond Pluto, still contain the primeval building materials of our solar system in their original state.”
When a comet gets too near the sun, sublimation begins and different gases are released. These gases carry large quantities of dust from the nucleus that reflect sunlight and brighten the comet. When the comet reaches a distance of around two and a half times Earth's distance from the sun (2.5 astronomical units, or AU) it has become warm enough that the majority of gas being released is from water.
The researchers used Swift's Ultraviolet/Optical Telescope (UVOT) to capture a sequence of images between May 27 and 29. These images depicted Siding Spring cruising through the constellation Eridanus at a distance of about 2.46 AU (229 million miles) from the sun. UVOT is unable to detect water molecules directly, however it can detect light emitted by fragments that form when ultraviolet sunlight breaks up water — specifically, hydrogen atoms and hydroxyl (OH) molecules.
"Based on our observations, we calculate that at the time of the observations the comet was producing about 2 billion billion billion water molecules, equivalent to about 13 gallons or 49 liters, each second," said team member Tony Farnham, a senior research scientist at University of Maryland College Park (UMCP). An Olympic-sized swimming pool could be filled in about 14 hours at this rate, but the scientists say this is a modest output compared to other comets observed by Swift.
They have estimated the size of Siding Spring to be approximately 2,300 feet across. This places the comet at the lower end of an estimated size range created from earlier observations by other spacecraft.
On October 19, comet Siding Spring will make its closest approach to Mars. It will pass just 86,000 miles above the planet's surface. This is close enough to allow the gas and dust in the outermost reaches of the comet's atmosphere, or coma, to interact with the atmosphere of Mars.
The closest recorded comet approach to Earth occurred on July 1, 1770, when the now-defunct comet Lexell passed within 1.4 million miles from our surface. This distance is about six times farther out than the moon. The approach of Siding Spring to Mars will be 16 times closer than this.
The spacecraft orbiting Mars — the Mars Reconnaissance Orbiter (MRO), Mars Express, and the Mars Atmosphere and Volatile EvolutioN (MAVEN) — will be in no danger from Siding Spring, but they will be used to observe the comet during this unprecedented opportunity. The team hopes to learn more about the Martian atmosphere, which is thinner than Earth's, from this event.
The Swift spacecraft is used to single out and observe comets at distances where the comets are emitting mostly gases other than water vapor. They follow these new comets as they course through the inner solar system to learn how comets' activity changes during repeated orbits of the sun. The hope is that these observations will lead to a better understanding of the evolution of our solar system and the comets that formed some five billion years ago.
Image 2 (below): This composite of C/2013 A1 (Siding Spring) merges Swift UVOT images taken between May 27 and 29, 2014. Sunlight reflected from the comet's dust, which produces most of the light in this image, appears yellow; violet shows ultraviolet light produced by hydroxyl (OH), a molecular fragment of water. Credit: NASA/Swift/D. Bodewits (UMD), DSS