Closing in on Mars

Seth Jarvis

As I write this (12 noon on Sunday, August 5th) the Mars Science Laboratory (MSL) spacecraft is just 90,000 miles from Mars and falling towards the Red Planet at over 8,000 MPH. MSL is in the final few hours of its “cruise” phase, having spent almost nine months in space on a 350 million mile journey from Earth.

Assuming everything goes as planned, here’s a timeline (Mountain Daylight Time) of tonight’s exciting events.  

The illustrations accompanying this timeline were created with the Jet Propulsion Laboratory’s excellent “Eyes on The Solar System” web application, which I highly recommend you play with.

11:14:38 PM      The 15-foot diameter “aeroshell” containing the Curiosity Rover separates from the Cruise Stage, the portion of the spacecraft that has provided power, communications and propulsion since it launched last November. MSL is now about 1,160 miles above the surface of Mars and approaching the planet at more than 11,400 miles per hour.

Separating the Cruise State from the fifteen-foot diameter Aeroshell. It's big!


11:15:35 PM      The spacecraft briefly fires small attitude control rockets to cancel the 2 RPM spin it has used since launch. This maneuver also orients the spacecraft to the correct angle needed for safely entering Mars’ atmosphere. MSL is now 1,030 miles above the surface and the tug of Mars’ gravity has accelerated MSL to 11,585 MPH.

11:16:33 PM      Small weights that have been used to provide precise balance for the spacecraft when it was slowly spinning during its cruise phase are jettisoned. It is now important that the spacecraft not be perfectly balanced in order to maintain the correct orientation for atmospheric entry. MSL is now about 900 miles above the surface and has accelerated to more than 11,700 MPH. The spacecraft now spends the next eight minutes plunging ever-closer towards Mars.

11:24:35 PM      MSL is now at the top of Mars’ thin but significant atmosphere. The spacecraft is now less than 85 miles above the surface but has accelerated due to Mars’ gravity to over 13,000 MPH. Interesting things are about to start happening, and happening fast. At this point Mars’ atmosphere begins to come significantly in to play. The tremendous speed of the MSL severely compresses the thin Martian atmosphere in front of the spacecraft, sending temperatures at the forward surfaces of MSL skyrocketing to 3,800° (F). An ablative heat shield protects the spacecraft from these extreme temperatures during atmospheric entry.

A heat shield is necessary if you hope to survive entering Mars' thin atmosphere.


11:26:52 PM      Aerodynamic drag created by MSL’s passage through Mars’ atmosphere creates a powerful deceleration, and during the roughly 150 seconds of initial atmospheric entry the spacecraft has descended to an altitude of about 11 miles and slowed to about 2,500 MPH. MSL now uses attitude control thrusters to make small adjustments to its trajectory to refine its aim on the target landing site.

11:28:37 PM      By now atmospheric drag as slowed MSL to just over 1,000 MPH, and it is less than 9 miles above the surface of Mars. MSL spends the next ten seconds jettisoning more ballast weights and performing a series of “Straighten Up and Fly Right” maneuvers to keep the spacecraft oriented correctly and flying on target for the northern portion of Gale Crater, its ultimate destination.

11:28:56 PM      MSL is now at an altitude of 38,000 feet and falling at nearly 900 MPH. It’s time to slow down – a lot. Time for a parachute! MSL pops open a 52 foot diameter parachute that produces 65,000 pounds of deceleration force against the decent of the spacecraft. In the next 20 seconds this parachute will slow the vehicle down to about 200 MPH.

11:29:15 PM      Midway through the three minute parachute decent the heat shield is jettisoned and the Curiosity rover gets its first good look at the ground.

The heat shield that protected you is now blocking your view of the ground. It's time to get rid of it.


11:30:49 PM      Now at an altitude of 5,400 feet and a speed of about 200 MPH, the rocket-powered landing vehicle separates from the backshell and parachute and quickly maneuvers away sideways to avoid a collision with the parachute.

At 200 MPH, the parachute has slowed you all it can. It's time to cut loose and fire your rockets! (And dodge sideways to avoid hitting your own parachute!)


Once away from the parachute, the eight rocket thrusters on the lander spend the next 90 seconds guiding the Curiosity rover even more closely to the chosen landing site, and to an altitude of just 60 feet and a downward velocity of 1.7 MPH.


The lander hovers using rockets as it lowers Curiosity on cables to a soft touchdown.


11:31:26 PM      Now we have a problem: Rockets powerful enough to safely deliver a one-ton car-sized robot to the surface of Mars are also powerful enough to create a huge cloud of dust and small rocks if that much rocket exhaust ever got close to the ground. That would be very, very bad. To avoid this, the decent stage hovers about 60 feet above the surface and lowers the Curiosity rover down to the surface on three cables, using what is referred to as the “SkyCrane” maneuver. Clever!

11:31:45 PM      Touchdown!  Whew!

But there is still one important bit of housekeeping that needs to be done. What do you do with the rocket-powered decent vehicle that got you there? Do you let it just hover above you until it runs out of fuel and falls on you? Nope, it is cut free from Curiosity and the rocket-powered lander zooms away and crashes to the ground to to become several hundred pounds of Earthling litter, a good safe distance from Curiosity.

The rocket-powered lander needs to leave the seen. It separates from Curiosity and flies away to crash land elsewhere.


And now, if all goes well, Curiosity will undertake a years-long mission to study the Martian atmosphere and climate, Martian geology, and in general look for signs that Mars at one point in its past might have been hospitable to some type of alien life.

That’s the plan.  That’s what’s supposed to happen late tonight.  Will everything go as planned?  We’ll have to wait and see and trust that the smart, hardworking folks at JPL have solved about a zillion difficult technical challenges without error.  If anyone can safely land a one-ton nuclear powered six-whee-drive robot explorer on Mars, they can.

And btw, didn’t I mention in my previous post that Getting to Mars is Hard?

I’ll be up late tonight, glued to NASA-TV, and I’ll post what we know about the landing as the news comes in.

And BTW, there will not be images from Mars tonight as soon as Curiousity lands.  Even if everything goes according to plan it will be at least another day, or more, before hi-res images begin to be sent from Curiosity to Earth.  The only thing we’ll know at 11:32 PM tonight is whether Curiosity made it safely onto Mars or not.

Here at Clark Planetarium, we’ll keep you posted on this site as news comes in, and we’ll be featuring Mars news throughout the month in the Hansen Dome Theatre.  Stay tuned!









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