Microsoft JScript runtime error: Object expected

[ERUM]

i m getting an error when i click on java script calender ,here is the code snippet and image where its showing an error

	<div>
										<asp:UpdatePanel ID="UpdatePane5" runat="server" UpdateMode=conditional>

                                    <ContentTemplate>
											<table width="100%" border="0" cellspacing="0" cellpadding="0">
											  <tr>
												<td width="50%" align="left"><strong> Do you need Pick and Drop Service </strong></td>
													
												<td width="50%" align="left"><strong></strong>   
                                                    <asp:RadioButtonList EnableViewState=true  RepeatDirection=Horizontal  ID="rdbtn_pickDrop"  
                                                        runat="server" AutoPostBack="True">
                                                        <asp:ListItem Value="0">Yes</asp:ListItem>
                                                        <asp:ListItem Selected="True" Value="1">No</asp:ListItem>
                                                    </asp:RadioButtonList>
                                                   
                                                  </td>
											  </tr>
											</table>
											<div runat=server visible=false id="pick_drop_div" style="border:dotted 1px black;"> 
										<asp:UpdatePanel ID="UpdatePanel5" runat="server">   
										 <ContentTemplate>
											<asp:PlaceHolder ID=placeholder4 runat=server ><strong>Arrival Information</strong><br /><br />
											Flight No.<asp:TextBox ID="txt_flightno" runat="server"></asp:TextBox>Airline
                                                <asp:TextBox ID="txt_Airline" runat="server"></asp:TextBox>
											Arrival Date: 
                                                <asp:TextBox ID="txt_arrival_date" Enabled =false runat="server"></asp:TextBox>
                                                <a href="javascript:calendar_window=window.open('calendar.aspx?form1=form1.txt_arrival_date','calendar_window','width=554,height=488');calendar_window.focus()">

                                                <img alt=""  src="images/calendar.png"   />  
                                                </a> 
                                                <br />                                                
                                                <br />
											<strong>Departure Information:</strong><br /><br />
											Flight No.<asp:TextBox ID="txt_dept_flight_no" runat="server"></asp:TextBox>Airline
                                                <asp:TextBox ID="txt_departure_airline" runat="server"></asp:TextBox>
											Departure Date: 
											
											  <asp:TextBox ID="txt_departure_date" Enabled =false runat="server"></asp:TextBox>
                                                <a href="javascript:calendar_window=window.open('calendar.aspx?form1=form1.txt_departure_date','calendar_window','width=554,height=488');calendar_window.focus()">

                                                <img alt=""  src="images/calendar.png"   />  
                                                </a> 

											
										</asp:PlaceHolder>
										</ContentTemplate>
										
										</asp:UpdatePanel>
										</div> 
											</ContentTemplate>
                                            <Triggers>
                                            <asp:AsyncPostBackTrigger ControlID="spous_atend_Conference" EventName="SelectedIndexChanged" />
                                                               

                </Triggers>
                                            
                                </asp:UpdatePanel>
										</div>